- Basic Assumptions
     - Basic Receiver Mixer Operation
     - Basic PLL synthesizer Operation
     - Syntor X VCO Operation
     - Syntor X Bandwidth
     - VCO Frequency Adjustment Theory
     - Actual VCO Frequency Adjustments
     - VCO Lock Test
     - Beyond the Basic Assumptions
     - Wrap Up
   - Syntor X Home
   - HOME





Basic assumptions:

In order to make it a little easier to cover this section the examples will use a VHF range 2 (150 to 174 MHz) Syntor X radio as the basis for all the examples.

Some of the sections on this page cover basic theory. They are worth reading for background information and were written to help people understand the reasons behind the process of modifying the radio's frequency coverage. Understanding this material can help you decide if retuning is the right thing for you to do and just how much you need to do. The most common modification is covered in the Actual VCO Frequency Adjustments section.

If you only intend to use the radio's original factory frequency range then no modifications are required.

This information also applies to the Syntor X 9000 with the differences noted on the Syntor X 9000 Extended Frequency Modifications page.

Basic receiver mixer operation:

Mixer diagram

The Preselector Filter only allows a specific range of frequencies to pass through it. The VHF range 2 radio will allow from 150 to 174 MHz to pass. No filter has a perfect cutoff so some frequencies below 150 MHz and above 174 MHz will also get through. The further away from the allowed frequencies it gets, the less signal will make it though the Preselector Filter. If you want to receive outside the factory preset frequency range, the Preselector Filter can kill the received signal unless you retune it (more on this later).

The Injection Filter is used to clean up noise from the VCO signal and it will only allow a specific range of frequencies to pass through it. The VHF range 2 radio will allow 203.9 to 227.9 MHz to pass. It also does not have a perfect cutoff. If this filter is operated too far outside the original factory preset range it can kill the mixer performance and also help kill the received signal. It can also be retuned (more on this later).

The 53.9 MHz IF Filter is used to remove unwanted products from the mixer (see below).

The example 1st Mixer will perform in the following way. The output will contain the frequencies from input A (Preselector Filter), the frequency from input B (Injection Filter) and the sum of frequencies A + B and the difference frequencies B - A.

Our example Preselector Filter only allows 150 to 174 MHz to pass, so these frequencies will be present at the mixer output. Whatever frequency the VCO is generating, lets say it is set to 203.9 MHz, will pass through the Injection Filter and will be present at the mixer output. Now the SUM of the frequencies will be there (i.e. 150 MHz + 203.9 MHz = 353.9 MHz to 174 MHz + 203.9 MHz = 377.9 MHz). The DIFFERENCE frequencies will also be there (i.e. 203.9 MHz - 150 MHz = 53.9 MHz to 203.9 - 174 MHz = 29.9 MHz). Out of all these frequencies, the 150 MHz difference signal from the antenna is the only one that will pass the 53.9 MHz IF filter. The receiver is tuned to 150 MHz. You can do the same math for a VCO frequency of 227.9 MHz and you will find the receiver is tuned to 174 MHz.

Mixer chart

The above chart has 3 different graphs. The first graph is for a receiver tuned for 150 MHz., the second graph is for a receiver tuned to 162 MHz and the third graph is for a receiver tuned to 174 MHz. Please refer to the 1st Mixer in the previous drawing for the A, B and OUT connections. The tuning is selected by the injection frequency (B). This shows how the entire range of frequencies from the Preselector Filter (A) are subtracted from the single Injection Filter frequency to produce the B-A OUT range of frequencies. As mentioned in the paragraph above, all the frequencies in an above graph will appear at the 1st Mixer output (the A+B frequencies 353.9 to 377.9 MHz are not shown in the graphs), but only the one frequency that is 53.9 MHz will pass the IF Filter. You may have also noticed that the highest Preselector Filter (A) frequency becomes the lowest B-A OUT frequency as a result of the frequency subtraction performed by the 1st Mixer (this inversion does not really matter, it was just something interesting to point out).

Please take note, that without the Preselector Filter, the receiver would always receive signals at 53.9 MHz. This is prevented because the Preselector Filter will not pass a 53.9 MHz frequency signal from the antenna. Also, if you do this interesting bit of math using the Injection Filter frequency of 203.9 MHz (for 150 MHz receive), 257.8 MHz - 203.9 MHz = 53.9 MHz you will see the 150 MHz receive example would have been simultaneously receiving 150 MHz and 257.8 MHz if it was not for the Preselector Filter blocking these unwanted frequencies from getting to the 1st Mixer. This effect is called the mixer image frequency. The higher the IF frequency (i.e. 53.9 MHz for the VHF Syntor X) the further away the image frequency is and the easier it is for the preselector to filter out. If you subtract the IF Filter frequency (53.9 MHz) from the Injection Filter frequency (203.9 MHz in this example) you will get the receive frequency of 150 MHz. If you add the IF Filter frequency (53.9 MHz) to the Injection Filter frequency (203.9 MHz in this example) you will get the receive image frequency of 257.8 MHz. This simple bit of math shows how the image frequency is related to the IF frequency.

The 53.9 MHz IF frequency is why the Syntor X can have a Preselector Filter that allows frequencies from 150 MHz to 174 MHz to pass. Take a quick look at what would happen if the Syntor X IF frequency was only 10.7 MHz like the original Syntor. To receive 150 MHz the VCO frequency would have to be (150 MHz + 10.7 MHz = 160.7 MHz injection). So (160.7 MHz injection - 150 MHz = 10.7 MHz) and the receiver is tuned to 150 MHz, but the image (171.4 MHz - 160.7 injection = 10.7 MHz) also tunes in 171.4 MHz. Because the Preselector Filter passes both 150 MHz and 171.4 MHz, both frequencies would be received together simultaneously. A good voice communications receiver can never allow the unwanted image frequency into the mixer. The IF Filter frequency is doubled to find out how close an image frequency will be (either high or low). So in this example the 10.7 MHz IF has a 21.4 MHz image and the preselector is 150 MHz to 174 MHz for a total of 24 MHz bandwidth. This simple bit of math shows us it is impossible to exclude the mixer image frequency using this example. In fact, cutting the preselector down to 150 MHz to 170 MHz would not really work. The math might show the preselector rejects the 171.4 MHz image, but no real world preselector is perfect enough to exactly cutoff everything above 170 MHz, so 171.4 MHz is too close to 170 MHz and the image will still get into the receiver mixer anyway.

Using the above formula, by doubling the Syntor X 53.9 MHz IF Filter frequency the image frequency is 107.8 MHz away. The Preselector Filter passes from 150 MHz to 174 MHz for a total bandwidth of 24 MHz. This means if you tune in at 150 MHz the mixer image will be at 257.8 MHz. The closest the Preselector Filter gets to this frequency is 174 MHz for a difference of 257.8 MHz - 174 Mhz = 83.8 MHz. This huge 83.8 MHz frequency difference can easily be filtered out by the Preselector Filter, even an imperfect real world preselector.

Whenever the VCO frequency is higher than the desired receive frequency, the 1st mixer is said to be "high side injected". Like the above example of using a 203.9 MHz injection frequency to receive a 150 MHz signal. Whenever the VCO frequency is lower than the desired receive frequency, the 1st mixer is said to be "low side injected". In this case an injection frequency of 96.1 MHz would be used to subtract with 150 MHz to produce the 53.9 MHz IF signal (150 MHz - 96.1 MHz injection = 53.9 MHz). Either high or low injection is used by different Syntor X radios (the example range 2 radio is high side injected).

It should be apparent the Preselector Filter performance is vital to the receiver's performance and the Syntor X radios have an excellent preselector. The Preselector Filter is actually a series of tuned circuits (5 tuned circuits for the VHF radios) that all work together to cover the entire range of frequencies. They must be adjusted together with the correct test equipment and procedures, or gaps in the frequency coverage can be created. For example, a 150 to 174 MHz Preselector Filter that is not correctly adjusted could loose coverage from 155 to 157 MHz. These frequencies could be tuned in with the VCO, but they would not work very well in the receiver.

The Injection Filter must also be tuned correctly or problems with the receiver coverage can be created. The injection filter is a series of tuned circuits (3 tuned circuits for the VHF radios) that all work together to cover the entire range of frequencies. Like the Preselector Filter they must be adjusted together or gaps can be created.

Because the Preselector Filter has to filter signals in the low microvolt amplitude range, its adjustment is much more critical than the Injection Filter which filters a much larger amplitude signal.

Basic PLL synthesizer operation:

PLL diagram

PLL stands for Phase Locked Loop. It is actually the combination of all the above circuits working together to produce a programmable, stable, VCO frequency for the radio. The dashed lines above show the actual loop part of the PLL. The stability of the VCO signal depends directly on the stability of the Programmable Reference Frequency oscillator (i.e. if the reference oscillator frequency drifts then the VCO drifts also).

The Voltage Controller Oscillator (VCO) is an oscillator whose frequency is controller by by a Steering Line Voltage. If the Steering Line Voltage increases so does the VCO frequency and if the Steering Line Voltage decreases then so does the VCO frequency. The VCO uses the V0 and V1 inputs to extend its frequency range. PIN diodes are used to switch the different extended frequency ranges so these are also sometimes called the PIN settings (more on this later). These two V0 and V1 inputs are programmed in the EEPROM code-plug and the microprocessor sends them to the VCO (see the next section for more details on the V0 and V1 settings).

VCO chart

Above is a simplified graph showing the VCO Steering Line Voltage in relation to the VCO frequency. The diagonal line in the chart is a function of the hardware design of the VCO circuit itself. Notice this design includes the highest and lowest frequencies the VCO is capable of reaching. If you follow the red line from 4.5 volts of VCO Steering Voltage to the diagonal line, then follow the green line to the frequencies on the left side of the graph, you will see this VCO example is operating at about 154 MHz with a 4.5 volt steering voltage. If we want the VCO to operate at 155 MHz then the graph shows us a 5 volt Steering line voltage is needed.

The Programmable Reference Frequency will determine the frequency step the VCO is capable of. If the 5 KHz reference is programmed then the VCO can only be locked every 5 KHz (i.e. 150.005 MHz , 150.010 MHz, 150.015 MHz, etc.) If 6.25 KHz is programmed then the VCO can only be locked every 6.25 KHz (i.e. 150.00625 MHz, 150.01250 MHz, 150.01875 MHz, 150.0250 MHz, etc.). The Programmable Reference Frequency is derived from a 14.4 MHz fixed frequency crystal oscillator that is divided down to 6.25 KHz or 5 KHz as programmed by the Syntor X computer. This crystal oscillator is designed to resist frequency changes due to environmental conditions like temperature changes. As mentioned above, the stability of this oscillator directly determines the stability of the VCO frequency. The reference frequency is also programmed in the EEPROM code-plug and the microprocessor sends it to the Programmable Reference Frequency circuit.

The Programmable Loop Divider is used to program the VCO lock frequency. It is programmed with a division constant that divides the VCO frequency. This division constant is calculated to make the desired VCO frequency match the Programmable Reference Frequency. If a division constant for 155 MHz is programmed and a 5 KHz Programmable Reference Frequency is used, when the VCO operates at 155 MHz the Loop Reference Frequency will be 5 KHz (if the VCO frequency is higher or lower than 155 MHz the Loop Reference Frequency will be higher or lower than 5 KHz Programmable Reference Frequency respectively). It is possible to program VCO frequencies that the VCO can not reach (i.e. a VCO lock frequency of 400 MHz can be programmed on a VHF radio, but the VCO will never have any chance of reaching this frequency, so the PLL will never be able to lock at this frequency). The VCO frequency is programmed in the EEPROM code-plug and the microprocessor sends it to the Programmable Loop Divider circuit.

The Phase Detector compares the Reference Input with the Loop Reference Frequency and generates a Steering Line Voltage as a result of the comparison. If the Loop Reference Frequency is higher than the Reference Input frequency, then the Steering Voltage is decreased causing the VCO frequency to go lower and therefore the Loop Reference Frequency to go lower. If the Loop Reference Frequency is lower than the Reference Input frequency, then the Steering Voltage is increased causing the VCO frequency to go higher and therefore the Loop Reference Frequency to go higher. If the Loop Reference Frequency and Reference Input frequency match then the Steering Line Voltage is held at that voltage (this is the locked condition). When the Phase Detector is not locked on frequency the "VCO Unlock Light" is lit, otherwise, when it is locked the light is not lit.

The Steering Line Voltage from the Phase Detector has a limited range. If the VCO frequency can not be moved far enough to bring it into a lock condition, the Steering Line Voltage will simply be stuck at one of its extreme voltages (high or low) and the VCO unlock light will remain lit. The Phase Detector can not change the V0 and V1 VCO inputs, it can only use the Steering Line Voltage to change the VCO frequency. The factory VCO ranges are tuned to match the standard V0 and V1 programming. If you use the VCO microstrip tuning pads (more info below) to move the VCO frequency coverage far enough, you will need to program the code-plug with custom V0 and V1 settings.

For the PLL to operate, the code-plug sets the Programmable Reference Frequency, Programmable Loop Divider and the VCO V0 and V1 range control. The Phase Detector will use the Steering Line Voltage to try and make the VCO frequency match the frequency programmed by the code-plug. If it succeeds the PLL will be locked and the radio will operate correctly.

VCO chart

Lets use the above graph to help discuss how we get this simplified VCO to operate at 155 MHz. For this example lets say the VCO is currently at 150 MHz (3 volt VCO steering line voltage). First we select a Programmable Reference Frequency. Either 5 KHz or 6.25 KHz will work in this case so I will go ahead and use 5 KHz. Next the Programmable Loop Divider is loaded with the data that will divide a VCO frequency of 155 MHz down to 5 KHz. Since the Programmable Loop Divider is set to divide a 155 MHz signal down to 5 KHz, the beginning 150 MHz VCO frequency will be divided down to a Loop Reference Frequency lower than 5 KHz. The Phase Detector will compare these two frequencies, see the Programmable Loop Divider Loop Reference Frequency is lower then the 5 KHz Programmable Reference Frequency and raise the VCO Steering Voltage. The Phase Detector will also light the VCO Unlock Light. The beginning 150 MHz frequency had a VCO Steering Voltage of 3 volts which is now rising causing the VCO frequency to raise with it. As long as the Phase Detector sees this frequency imbalance it will continue to raise the VCO Steering Voltage and therefore the VCO frequency. As this progresses the Programmable Loop Divider Loop Reference Frequency gets closer and closer to the 5 KHz Programmable Reference Frequency. When the VCO Steering Line Voltage finally gets to 5 volts the VCO frequency will be at 155 MHz. Now the Programmable Loop Divider Loop Reference Frequency matches the 5 KHz Programmable Reference Frequency. At this point the Phase Detector will stop changing the VCO Steering Line Voltage and the extinguish the VCO Unlock Light. Because the Phase Detector continuously looks for any difference between the Programmable Loop Divider Loop Reference Frequency and the Programmable Reference Frequency, it will continue to keep the VCO locked on the programmed 155 MHz frequency.

The above simple example might raise the question, why not just set the VCO Steering Voltage at 5 volts and forget about all the reference frequencies and PLL circuits? In reality it is not possible to make identical VCO circuits that will all work exactly the same when a particular VCO Steering Line Voltage is applied. This would mean one radio would require 5 volts and another 4.8 volts and another 5.3 volts, etc. By far the biggest problem is that temperature variations will change the VCO frequency requiring the VCO Steering Line Voltage to dynamically change to hold a particular frequency. This is why the VCO Steering Line Voltage is derived from a frequency comparison against a stable reference frequency (5 KHz or 6.25 KHz) and all these PLL circuits are actually needed.

The technical details of the PLL synthesizer covered in the Syntor X manuals are much more complex than the simple example given here, but the basics are still the same. One difference is the PLL Adapt operation. PLL circuits are not very good at making fast frequency changes that require large changes in the VCO Steering Line Voltage (i.e. switching between 150 MHz and 160 MHz). The PLL normally only makes very small Steering Line Voltage changes to keep the VCO locked on frequency. The Syntor X uses a special PLL operation called Adapt. When the microcomputer switches the PLL to Adapt, it can make fast changes in frequency (i.e. quickly make large VCO Steering Line Voltage changes). After the frequency change is complete the microcomputer will switch out of Adapt and back to normal PLL operation. Without the Adapt operation it would take significantly longer for the PLL to make large frequency changes. Since most Syntor X radios must shift the VCO 53.9 MHz between the same transmit and receive frequency, fast frequency changes are essential. In fact when the Syntor X receiver scans different frequencies it has an impressive frequency change speed. Another difference is the VCO frequency is modulated with an audio signal to produce the FM transmitter signal. Normally the PLL would prevent the VCO frequency from being frequency modulated so both the VCO and Programmable Reference Frequency oscillator are frequency modulated together. This causes the Phase Detector to not be able to detect the VCO frequency modulation so it can not cancel it out. This type of VCO modulation is called Reactance Modulation and is superior to Phase Modulation for low frequency audio modulation like DPL signals.

Syntor X VCO operation:

VCO frequency chart

The above table is just a representation of a factory tuned VCO. VCO Steering Line Voltages were simply made up for the explanations in this section (they are similar to factory voltages). In real life each individual radio will have slight variations anyway. The jagged tear in the chart is there because of the chart's left hand side frequency scale discontinuity created by the large frequency jump between transmit and receive frequencies.

The V0 and V1 bits only effect the VCO circuit itself. They change the frequency range the VCO will operate at. Do not confuse this with the frequency programmed into the phase locked loop (PLL). Any low band, VHF or UHF frequency can be programmed into the PLL (i.e. the Programmable Loop Divider and Programmable Reference Frequency). It is up to the VCO to be able to reach the frequency programmed into the PLL, if it can. The V0 and V1 bits change the VCO tank circuit and that changes the frequencies the VCO can operate at. In the above chart you can see that when V0 = 0 and V1 = 0, the VCO will operate approximately in the 200 to 218 MHz range. When V0 = 1 and V1 = 0, the VCO will operate approximately in the 158 to 177 MHz range. From this you can see the software that programs the radio (it sets the V0 and V1 bits) must match the way the VCO is actually tuned, in order to operate correctly. For example: if the software you programmed the radio with treated a transmit frequency of 150 MHz as a Tx high range (V0 = 1 and V1 = 0), a VCO tuned like the one in the above chart would never be able to reach 150 MHz because it would be operating in the 158 to 177 MHz range. The PLL would drive the steering voltage as low as it can go and the VCO would be stuck around 158 MHz (the VCO unlock light would stay lit) which is low as the VCO could go when V0 = 1 and V1 = 0. In order for this example to actually achieve a lock, either the VCO itself would have to modified so the Tx high range includes 150 MHz (the VCO frequency ranges would all be lowered), or the software that programmed the radio would have to be changed so it treats 150 MHz as a Tx low range (i.e. V0 = 1 and V1 = 1), or you can use a binary editor to edit the code plug and change the V1 bit from a 0 to a 1 yourself.

The above example V0 programming is for a high side injected radio. The V0 programming values would be reversed for a low side injected radio (i.e. V0 = 1 for receive and V0 = 0 for transmit, for low side injection). Of course if you used the VCO frequencies in the above table for low side injection, the radio would be tuned to 200 to 230 MHz operation (you would need a different preselector, injection filter and transmitter circuits than the VHF example radio for it to actually work with low side injection). Recognizing the difference between high side and low side injection will come in handy when you get to the UHF and 800 MHz radios.

To interpret the above graph use this example, lets pick a VCO Steering Line Voltage of 5 volts, V0 set to a 1 and V1 set to a 1. Follow the light gray vertical line from the 5 volt VCO Steering Line Voltage straight up to the first Tx Low Range diagonal line (this is where the V0 sand V1 settings match). Where the 5 volt line meets the Tx Low Range diagonal line go straight across to the left side of the graph and it will read 155 MHz. This means that if you supply a 5 volt steering voltage with V0 and V1 both set to a value of 1, the VCO will operate at 155 MHz. Lets change V0 to a value of 0 and leave everything else as it was. Now you have to follow the 5 volt line all the way to the Rx High Range diagonal line, go straight to the left side of the graph and the VCO frequency is about 222 MHz. You can also pick a VCO frequency like 165 MHz for example. From the left side of the graph at the 165 MHz line follow it to the right until you intersect the Tx High Range diagonal line, then go straight down from this intersection and you will see the VCO Steering Line Voltage will be about 4 volts. The other information tells us that V0 needs to be a 1 and V1 needs to be a 0 for the VCO frequency to operate at 165 MHz in the Tx High Range.

The horizontal black dashed lines shows the factory set frequency at which the high and low ranges are changed (at about 163 MHz and 216 MHz). These changes keep the VCO Steering Line Voltage within reasonable limits (not too high or to low a VCO Steering Line Voltage). In a perfect world the radio would see when the VCO Steering Line Voltage was getting too high or too low (past where the horizontal black dashed lines are) and change the V1 control line as a direct response. What really happens is the code plug programming software just assumes the V1 control line needs to be switched at a preprogrammed frequency (it has no idea what the actual VCO Steering Line Voltage is). If you change the VCO frequency range enough (as explained below in VCO frequency adjustment theory) the factory assumption is no longer valid and the VCO may not be able to lock where it should be changing ranges (i.e. the V1 control line needs to be switched at different frequencies than the original factory setting). If not corrected, this problem will only create small gaps in the VCO coverage, that are only noticeable if you try and use those particular frequencies. To illustrate this, draw your own horizontal black dashed line at 155 MHz. Now the code plug will blindly switch the V1 control line at 155 MHz and you can see the VCO frequencies for Tx High Range V1 = 0 do not go low enough to allow the VCO to lock. In this example the VCO will not lock until you program a frequency around 156 MHz or higher. So this example will create a dead zone from 155 MHz to about 156 MHz where the VCO will not lock (all because the 155 MHz V1 change setting in this example does not match the VCO frequency range). To use the existing above illustration, this example changed the frequency where the V1 control line was switched, but in real life the problem occurs when the entire frequency range is changed (i.e. the diagonal lines in the graph are moved up or down changing the frequency range of the VCO).

The vertical blue dashed lines shows the factory set Low Range VCO Steering Line Voltage vs. the VCO frequency. The vertical red dashed lines shows the factory set High Range VCO Steering Line Voltage vs. the VCO frequency.

Notice the factory does not use the full VCO Steering Line Voltage range, only around 3 to 7.5 volts is used. This is done on purpose to compensate for frequency range shifts caused by temperature extremes. As the temperature varies so will the VCO frequency and the PLL will change the VCO Steering Line Voltage to compensate and try to keep the VCO on the programmed frequency. For example, if the VCO Steering Line Voltage is already at the extreme bottom end of its range and temperature causes the VCO frequency to rise, the PLL can not get the VCO Steering Line Voltage any lower to bring the VCO frequency back down and the PLL lock is lost. If you use the entire voltage range or one extreme end of the voltage range, your radio may be vulnerable to large temperature shifts that can prevent the PLL from locking or make it unlock on these fringe frequencies. Since these radios are designed to live in places like a vehicle trunk, large temperature shifts are normal for these radios. Also not using the full VCO Steering Line Voltage range allows some extra slop when picking code plug frequencies for the V1 control line setting (i.e. each radio's VCO frequency range does not have to be identical from one radio to another as they roll of the assembly line). It also allows extra some room for long term changes caused by component aging.

From this graph you can see what would happen if you set the PLL Programmable Divider for a frequency of 170 MHz and set V0 and V1 both to a value of 1. Because of the V0 and V1 setting the VCO would only be capable of frequencies from about 145 MHz to almost 165 MHz. The Phase Detector would drive the VCO Steering Line Voltage as high as it possibly could and still never be able to get the VCO to the 170 MHz required to achieve a lock. A more practical use of the graph is to help identify the frequencies where V0 and V1 need to be changed to keep the VCO Steering Line Voltage within a reasonable voltage range.

Notice the extreme tops and bottoms of the diagonal line Tx and Rx, High and Low Ranges. The VCO frequencies no longer provide a nice predicable linear frequency response for a given VCO Steering Line Voltage (i.e. when the VCO Steering Line Voltage is below 1.5 volts or above 8.3 volts in this example). These are not exact representations of what each VCO will do at theses voltage extremes, it is just a reminder that VCOs can give weird unpredictable responses when operated at their Steering Line Voltage extremes. They also become susceptible to temperature variations at these extreme ranges (i.e. they may suddenly loose their lock, refuse to lock or become very slow at achieving a lock). This is another good reason for not using the VCO Steering Line Voltage extreme voltage ranges.

From the above graph you can see the VCO operates in 4 frequency ranges as set by V0 and V1. If the code plug sets the PLL Programmable Divider to a frequency and also sets the correct V0 and V1 values, the PLL synthesizer will lock on that frequency and the radio can now use the programmed VCO frequency for its operation.

Syntor X bandwidth and retuning:

These radios have a built in bandwidth (i.e. a radio that covers from 150 to 174 MHz has a 24 MHz bandwidth). The frequencies within the bandwidth can be shifted up (i.e. 154-178 MHz) or down (i.e. 144-168 MHz), the amount depends on the radio. Sometimes you can stretch the bandwidth a little wider (i.e. 144-174 MHz for 30 MHz bandwidth), but not always.

Just because a received signal falls outside the bandwidth does not mean it disappears (i.e. 149.995 MHz signal is just as good as a 150 MHz signal to the radio). What will happen is, the further away the signal gets from the covered frequencies, the worse the receiver gets at hearing it. Most of these radios will cover down to 148 MHz with no real audible loss in sensitivity. In fact most receivers are usable down to 146 MHz, but not that many work as well or at all at 144 MHz. There are three major things that affect the bandwidth of the radio: 1) Voltage Controlled Oscillator (VCO) frequency range, 2) receiver Preselector Filter, and 3) the receiver Injection Filter. The first step is to determine if the VCO is locking on all the frequencies you have programmed into the radio by checking the VCO Unlock Indicator light. If the VCO is locking on frequency and receiver performance is still unsatisfactory, first service the radio and check its actual receiver sensitivity, both within the radio's original bandwidth and the new frequencies that are outside of the factory bandwidth. If the radio is working correctly within its factory bandwidth, but not working well enough outside its bandwidth where you want to use it, you will have to retune the receiver. This retuning requires a test bench and is not easy to do correctly. The 68P81044E40 800 MHz manual and the 68P81111E94 VHF Test Memory manual show a factory approved field retuning procedure.

When using the radio to receive frequencies outside the factory tuning range, the fact that the Preselector and Injection filters do not perfectly cutoff these frequencies often allows you to use the radio without retuning. If these frequencies are very close the original factory tuning range the difference will not even be measurable. How far you stray outside the original factory tuning range and your own personal level of acceptable receiver performance will determine if you need/want to retune the filters.

If you only intend to use the original factory frequency ranges, the Syntor X radio will not require any retuning at all to achieve optimum performance. Other radios that do not have a broadband 24 MHz bandwidth Preselector Filter like the Syntor X must be retuned when frequencies are changed significantly and usually only cover a small bandwidth (i.e. usually less than 4 MHz for a VHF radio, possibly as little a 1 MHz). This small bandwidth is the reason for the Micor dual receiver radios (i.e. the Micor can not receive at 150 MHz and 170 MHz without two different receiver front ends in the same radio).

Getting a radio model with an Rx preamp will help extend the useful range of the receiver a little bit more without retuning (within limits). However, I do know the 450-470 MHz UHF radio receivers tend to be fairly deaf below 446 MHz and will probably require the full front end retuning for HAM use (i.e. receiver Preselector Filter and receiver Injection Filter). The VCO will usually lock all the way down to 440 MHz without any modification. A friend of mine used just a signal generator, lots of experience and luck to retune his UHF front end and it appears to work OK within the HAM bands. There may however be dips or blind spots within the preselector coverage bandwidth (that can easily be induced by this procedure) that he has not discovered yet. You should use a full test bench and the services or equivalent experience of a radio technician to retune the Preselector Filter and Injection Filter if you want to do it right.

Below is an illustration of a Preselector Filter response curve.

Preselector filter curve

This simplified graph is not an exact representation of the Syntor X Preselector Filter response curve, but it illustrates the principal. The frequencies from 150 MHz to 174 MHz are highlighted to show the factory tuning. An "ideal" filter would look like a box shape (just like the yellow area) with perfectly straight up and down sides instead of the curved sides shown. Since "ideal" filters do not exist this is usually what we get. Notice that there is always some signal loss induced by the Preselector Filter as it never passes 100 % of the signal. Filters are also vulnerable to temperature variations so the filters also need some extra room in the way they are tuned for optimal performance (i.e. notice how the curves do not immediately start down outside the yellow highlighted area). This graph also shows why the receiver is good for signals outside the factory tuning range and what happens the further you get from the factory frequency range. Each radio has its own individual filter response curve so the amount of signal passed outside the factory frequency range varies from radio to radio. Also keep in mind that your ears can not discern small changes in the percent of signal passed so there is a difference in test bench measurements vs. any difference you actually can hear. Remember that 5 separate tuned filters make up the VHF Preselector Filter. Each tuned filter covers a part of the total frequency coverage to create a curve similar to the graph above. If the filters are not all adjusted together correctly there will be dips (i.e. signal loss) in what should be the flat part of the filter response curve.

Preselector filter tuning

Here is a simplified graph to show how each of the 5 individual VHF preselector tuned circuits work together to cover the entire 24 MHz bandwidth. As in the previous simplified graph example, this is not an exact representation of an individual filter response curve. Each of the 5 filters has its own color and the shape of each individual filter response curve is shown on the right. For example; if the red filter 2 was missing or not adjusted correctly there would be a gap between the black filter 1 and blue filter 3. This is why adjusting these is so difficult. Without some way to see the entire 24 MHz bandwidth and how each filter plugs its own gap there is no way to be sure they are all adjusted correctly. Being able to "see" this is why specialized test equipment is required to correctly perform this adjustment.

If we convert a radio to cover from 144 MHz to 168 MHz the result should look like something like the above graph with 144 MHz where 150 MHz is currently and 168 MHz where 174 MHz is currently (i.e. just shift the entire frequency scale over until 144 MHz lines where 150 MHz used to be). With this type of retuning the bandwidth is still 24 MHz. If we try to extend (i.e. stretch) the coverage from 144 MHz to 174 MHz we have to be careful not to create dips in the flat part of frequency range or to cause the entire flat area to pass less signal than before. In this case the bandwidth is being expanded to 30 MHz. This may or may not work well depending on your particular radio.

Of course there are limits to just how far you can retune any radio without getting into component replacement and extensive modifications. These limits will vary from radio to radio. The modifications on this web site do not cover these extensive modifications.

VCO frequency adjustment theory:

VCO tuning chart

The microstrip tuning pads are a series of small square pads on the VCO circuit board. They have a wire soldered between the pads which the factory has cut at some point to tune the VCO frequency range. The pads mostly form a capacitor and there is only one physical connection to the VCO circuit (i.e. the other part of the capacitor is inside the VCO circuit board underneath the pads). When the wire is cut, the pads on one side of the cut are connected to the VCO circuit and they determine the VCO tuning range, while the pads on the other side of the cut are isolated and no longer affect the VCO tuning. The more pads connected to the VCO circuit, the lower the VCO frequency range (i.e. fewer pads are isolated). The fewer pads connected to the VCO circuit, the higher the VCO frequency range (i.e. more pads are isolated). Please note the entire VCO frequency range (lowest to highest frequencies) is affected by these pads.

Here is a photograph of the range 2 VHF VCO.

The above graph shows the 4 VCO frequency ranges for the factory tuning and what happens when you change the VCO microstrip tuning pads. The blue information is for adding more microstrip tuning pads (i.e. restoring cut wires) to the chain which lowers the overall VCO frequency. The red information is for removing microstrip tuning pads (i.e. cutting the wire) from the chain which raises the overall VCO frequency. The amount of frequency shift shown above was made up for the purpose of illustrating the principals involved.

It is apparent from the lower set of frequencies that there is over a 5 MHz difference between the red high frequency and blue low frequency VCO microstrip tuning. In real life larger changes can be achieved.

This graph can also be used to depict what happens when temperature changes affect the VCO frequency. Just like messing with the microstrip tuning pads, the entire frequency range will shift up or down in frequency, but not nearly such a large frequency shift as what is shown above. Take note that when the frequency range is shifted up or down, the VCO Steering Line Voltage required to keep the VCO at a fixed frequency changes. For example, at 150 MHz the factory tuning takes about 3 volts, at the red tuning range it takes about 2 volts and the blue tuning range takes a little bit over 4 volts. From this example you can see why the Steering Line Voltage needs a little extra usable range to handle VCO temperature variations. Keep in mind this example exaggerates the frequency shift caused by temperature and therefore also has an exaggerated amount of VCO Steering Line Voltage change, however, the principals involved are still valid.

Lets take the example of the 150 to 174 MHz radio being retuned to 144 to 168 MHz. If you try and use the Factory Microstrip Tuning you can see the VCO Steering Line Voltage is unacceptably low (about 1 volt) at a programmed frequency of 144 MHz and may not even be able lock at this frequency. By changing the microstrip tuning and adding more pads (More Factory Microstrip Tuning Pads), the VCO frequency range is lowered and the Steering Line Voltage is back up to 2 volts at 144 MHz. In this example the original 150 MHz low frequency used a VCO Steering Line Voltage of about 3 volts, so even more microstrip pads than shown in the above example should be added back to the VCO until a 144 MHz PLL lock can be maintained at about 3 volts. Also keep in mind that the code plug VCO V1 settings may have to be changed to keep the VCO Steering Line Voltage at the high end within limits. The factory tuned radio used to change the V1 settings at 161.8 MHz and 215.5 MHz which kept the VCO Steering Line Voltage around 7.5 volts. For the conversion, you will have to determine what new programmed VCO frequencies cause the same 7.5 volt VCO Steering Line Voltage and use these frequencies to change the V1 setting in the code-plug.

Please remember the above sample voltages do not apply to your radio. In real life to perform this tuning you have to measure VCO Steering Line Voltages at the factory frequencies before you convert the radio coverage. Then program the new frequency coverages, retune the VCO microstrip pads until the low frequencies approximately match the old low VCO Steering Line Voltages. Next determine at what new frequencies cause the VCO Steering Line Voltages reach their old high voltages and use this to set your V1 code-plug programming. In some cases there may not be enough tuning range in the microstrip tuning pads and some VCO surface mount capacitors may need changing.

If you do the full conversion from the above example, the PLL synthesizer will be just as reliable within its new frequency coverage as it used to be within the original factory frequency coverage. You can now program frequencies anywhere within the complete new frequency coverage with total confidence that everything will work, even at temperature extremes within the radio's operating specifications.

All of the above is a fair amount of work and in reality for HAM radio use only, range 2 VHF conversions can get away with just adding all the microstrip pads back to retune the VCO frequency range because they do not use any frequencies near the old V1 shift frequencies. In this case retuning the VCO frequency range only means lowering its frequency range. If frequencies other than HAM frequencies are programmed they will probably work, but might fail to lock under certain conditions so all that is sacrificed is reliability for some non-HAM frequencies.

In reality there is quite a bit of VCO frequency coverage available outside the normal factory VCO Steering Line Voltages. The closer these VCO Steering Line Voltages get to their extreme edges the less reliable the PLL lock becomes. Allot of VHF range 2 radios are used for HAM radio frequencies without any conversion at all and do not have any reliability problems in the environments and frequencies they are used at.

Syntor X programming software that is designed or customized to program specifically within the HAM frequency range usually have new V1 shift frequencies (they may also be called PIN Codes or PIN Shifts). These new V1 settings are based on assumptions about how a retuned VCO might behave. If your VCO can not lock on certain frequencies where you would expect a high VCO steering line voltage, these settings may be the cause. The good news is retuned radios rarely encounter in this problem in actual use.

Actual VCO frequency adjustments:

Here is a photograph of the range 2 VHF VCO.

The VCO microstrip trimming capacitor is used to adjust the frequency coverage of the VCO. This is a row of small square pads on the VCO circuit board with a wire soldered across them. The wire will be cut between two or more of the pads (this was done by the factory). The more pads wired together the more capacitance there is and the lower the VCO frequency range. First, determine if you really want to or need to retune the VCO before you mess with these pads. Identifying the pads and which end to cut from or add to will require the radio's service manual. The wire between the microstrip tuning pads is the only thing ever cut on the VCO board. The VHF radios are the only Syntor X radios to use separate Tx and Rx VCO circuits. All the other radios use a single VCO circuit. Tuning the VCO will be different for each individual radio. However, if you want to lower the frequency coverage for HAM use only, you can just restore the factory cut which will probably cause some loss at the high end frequency range. Of the 5 T83 VHF radios I have seen, only 1 had a VCO that would not lock at 144 MHz. This radio would not lock below 146 MHz on Tx. All I did was restore the factory cut connection on the Tx VCO microstrip trimming capacitor and it worked fine. I never did check to see how much of the top end I lost by adding all the pads back instead of just some of them. Of course, even though the other T83 radios were locking at 144 MHz, the VCO Steering Line Voltage was probably far to low for reliable operation at this frequency.

Remember to replace the VCO cover (it does not have to be screwed in place until you are done) before checking the effect of your frequency adjustments. When the cover is off it can effect the VCO frequency slightly.

The VCO circuit board is an alumina thick film substrate that looks like a ceramic compound. It is an excellent heat sink. When you solder on this you will need a hot soldering iron and only use the absolute minimum time required to make the connection. Allow everything to completely cool off before placing the solder iron on the VCO board again. If your soldering iron only sinks a little bit into the solder then appears to freeze or stick you will need a hotter soldering iron. There are surface mount components all over this board. If you ever solder near any of them (you should not need to) be aware that overheating the metal ends of the surface mount components can cause them to leach onto the component body and destroy the component. There are special solders designed to minimize leaching. Some of the components like certain resistors are built into the board itself and can not be replaced or field serviced. Be very careful to never damage or gouge the VCO board or you could accidentally damage it beyond repair.

One person had an experience where a VHF range 2 VCO (that was working in the factory frequency range) would not lock at 144 MHz after it was modified. Surprisingly, the problem was traced to bad solder joints on the RF Board (it is the largest board in the radio). The radio worked just fine and the VCO locked below 144 MHz after some suspicious looking RF Board solder joints were resoldered.

Another person added extra materials to the VCO microstrip trimming capacitor to increase its capacitance. A small piece of removed PC board copper foil should work if it carefully soldered flat on the VCO board and does not come near or touch any other components or traces. The end of the VCO microstrip trimming capacitor chain would probably be the best place to connect it and run it alongside the microstrip pads. Another trick is to make big solder blobs between the VCO microstrip trimming capacitor squares to increase the capacitance. Keep in mind these techniques are the last resort. As an alternative you can replace the SMD capacitors on the VCO with different value ones (SMD soldering is definitely a more advanced operation). The reason this VHF range 2 would not go low enough without extra help could be from component aging, something like the previous problem above or maybe it was just a radio where all the individual component values tended to be at one extreme of the component value tolerance range, predisposing it to higher frequencies than most other range 2 VHF radios?

VCO lock test:

After your Syntor X is programmed here is a simple test you can perform to make sure the radio's VCO is covering the entire programmed frequency range. To setup the test:

  1. attach the antenna connector to a suitable dummy load
  2. open the top cover with the push button under the handle
  3. turn the radio on
  4. turn the scan off *
  5. locate the "loss of lock indicator" on the circuit board

If you have trouble finding the "loss of lock indicator" turn the scan on and off a few times while you look for the light. You should see the light when the scan is on as long as it does not stop on an active frequency. If you do not have a "PRI" scan switch then try changing modes while looking for a brief flash.

To check the frequencies:

  1. select mode 1
  2. if the indicator does not stay lit up you are OK (a quick blink is normal)
  3. press the PTT
  4. if the indicator does not stay lit up you are OK (a quick blink is normal)
  5. release the PTT
  6. select the next mode and repeat from step 2 until all modes have been checked

The "loss of lock indicator" lights up when the synthesizer's voltage controlled oscillator (VCO) is not on its programmed frequency. This will normally occur for a fraction of a second (a blink of the light) when changing modes or when the PTT is pressed.

When you have the VCO locking over its entire frequency range the following can be observed. If you activate the scan and no active frequencies are found (i.e. the receiver squelch is not opened), the indicator light will look like it is on, even though it is really flashing on and off very fast (it is an optical illusion). However, if you use priority scan modes another behavior can be observed. If the radio scan finds any active mode other than the highest priority mode, the indicator light will pulse at a slow rate. This pulse is from the frequency change required to go check the priority mode and then return to scan mode (if the priority mode was not found to be active). If the scan locks on the highest priority mode then the indicator light will go out.

If the radio did not lock on any frequencies check the code plug programming for errors and then try the original code plug frequencies or program a code plug with frequencies in the original factory frequency range. If the VCO still will not lock, repair the radio.

If the VCO will not lock on some frequencies check for a pattern (i.e. it will not lock on frequencies lower than or higher than some frequency). Unless the radio has been modified or is damaged, it should always lock on frequencies in the original factory frequency coverage. If you are trying to exceed the factory frequency coverage, simultaneously above and below, it may or may not be possible. If you are simply trying to shift the frequency coverage up or down then you can adjust the VCO microstrip trimming capacitor and V0 and V1 code-plug settings.

Beyond the basic assumptions:

The actual radios are different, but showing detailed examples of each one would take large amount of time and space. So instead I will give a quick summary of the basic differences.

The low band radio uses a 75.7 MHz 1st IF frequency instead of the 53.9 MHz used by all the other Syntor X radios. It is the only Syntor X to use a mixer circuit in the transmitter RF circuit. The Tx mixer allows all 4 VCO ranges to simply overlap from the low to high coverage and eliminates any need to jump the VCO frequency by the 75.7 MHz IF frequency between Tx and Rx. All the other Syntor X radios except the 800 MHz radios need a 53.9 MHz VCO frequency shift when switching between the same transmit and receive frequency. The preselector filtering is different for this radio. It uses several low pass and high pass filters with no external adjustments. Changing the tuning of this radio will require the manual and component replacement. The 3 adjustable tuned circuits found where the VHF radio preselector and injection filters are located is for the low band extender receiver front end tuning. For normal HAM use this radio usually does not require any preselector or injection retuning. This radio uses high side injection.

The VHF radios come in a factory range 1 (136 to 154.5 MHz) and range 2 (150 to 174 MHz) coverage. Both of these use high side injection. There are 5 preselector tuned circuits and 3 injection tuned circuits. The range 2 injection frequencies are perfect for a low side injection 220 MHz radio conversion, but the rest of the 220 MHz conversion would involve extensive Preselector Filter modifications or replacement, 1st Mixer modification as well as lots of transmitter modifications.

The UHF radios come in 5 different ranges. Range 1 (406 to 420 MHz), range 2 (450 to 470 MHz), range 3 (470 to 488 MHz), range 4 (482 to 500 MHz) and range 5 (494 to 512 MHz). There are 6 preselector tuned circuits and 3 injection tuned circuits. If you want to convert a radio, start with the radio closest to the frequencies you want to use. For example, trying to convert a range 3 to cover 440 to 450 MHz would be a major project. The range 1 radio uses high side injection, ranges 2 through 5 use low side injection. This injection difference makes converting a range 1 to 440 to 450 MHz a major project also. The best radio for this commonly performed conversion is the UHF range 2. Ranges 2, 3, 4 and 5 are all low side injection.

The 800 MHz radio uses Tx frequencies of 806 to 825 MHz and Rx frequencies of 851 to 870 MHz. Because there is already a built in 45 MHz difference between the transmit and receive frequencies, the same VCO range is used for both transmit and receive (i.e. the VCO V0 control line is not used at all). The 800 MHz radio uses a VCO doubler to get the VCO up to the 800 MHz region. For example, to get an 820 MHz VCO frequency, 410 MHz is programmed into the VCO. The doubler also doubles the minimum frequency step (i.e. the 5 KHz reference is now equivalent to a 10 KHz step and the 6.25 KHz is equivalent to a 12.5 KHz step at the doubler output). The doubler also doubles the VCO bandwidth range so the high and low VCO range switching used on other Syntor X radios is not needed or used (i.e. the VCO V1 is not needed, but may be used for Talkaround). Not all 800 MHz radios are capable of simplex operation. If they do not have the Talkaround option then they are not capable of simplex operation. The Talkaround option uses the VCO V1 to kick the transmitter frequency up 45 MHz to operate in the 851 to 870 MHz region (of course the transmit VCO frequency only moves 22.5 MHz before it gets doubled). Any reasonable effort to convert an 800 MHz radio to the 900 MHz band will require the Talkaround option to be present in the radio. This radio is low side injected. This means the Rx VCO frequencies are from 398.55 to 408.05 MHz and the Tx VCO frequencies are from 403 to 412.5 MHz. The entire standard VCO range is from 398.55 to 412.5, a total of 13.95 MHz bandwidth (27.9 MHz bandwidth after it is doubled) which is why a single VCO range works. Talkaround adds 425.5 to 435 MHz by setting VCO V1 to a value of 1 on radios equipped for it. This radio has an additional 2 tuned circuit preselector, located in the housing that contains the receiver preamplifier (both are tuned through holes in the top RF board). There are also 6 preselector tuned circuits and 3 injection tuned circuits.

All Syntor X radios only have a single synthesizer that is shared between the receiver and transmitter, so using these radios for repeater conversions requires two separate radios. Using a "VXJ" model radio will provide a ready made Carrier Operated relay (COR) signal from the J1-35 EMS Mute line. The EMS Mute line is conditioned by the receiver PL / DPL circuit so the PL or DPL will work correctly with this as a COR.

Wrap up:

As shown above many Syntor X radios are simply used as is with no conversion other than programming a new code-plug and they work very reliably. There are simple partial conversions that help mostly for a limited frequency range and there are full blown conversions that maintain the original factory performance over the entire new frequency range.

The VCO retuning is covered in some detail. Unfortunately, the receiver Preselector Filter and Injection Filter tuning is much more difficult and requires expensive specialized test equipment. The two Motorola manuals with field tuning procedures were pointed out above. I have been collecting test equipment to perform these Motorol tuning procedures. The 68P81111E94 VHF Test Memory manual failed to specify the model of HP preamplifier is used in its retuning procedure so I have to wing it with a substitute preamplifier. I will add whatever I learn from the procedure to the web page. Of course I would love to have an expensive spectrum analyzer with a tracking generator instead :-).

When desired or needed, the preselector filter tuning is something most people will probably pay for to have it done correctly, by a fully equipped radio shop, and some people will wing it like my friend did. However, if I find a better way I will add it to this web page.

Please do not get the idea that a standard factory tuned Syntor X radio used outside its factory frequency range is not reliable. All I have attempted to point out is that without retuning they can be pushed outside their range to the point they become unreliable at some frequencies. This point is different for each individual radio. For many people converting the frequency range is more of a matter of maintaining peak performance than anything else. Others get radios that need to be converted just to work at the frequencies they want to use. The good news is the Syntor X is such a nice radio it is well worth the time and effort to convert.








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