Receiver & Transmitter Mods
for G1MFG / Comtech units



into the


I started my FM ATV adventure using a standard 23cm receiver. Everything was ok until I tried a Gold receiver. Since, I have spent a significant amount of time investigating problems to make my Gold receivers useable. This page describes my  long DETOUR  from other projects to fix a problem with the Gold receiver that eventually resulted in the  Platinum Modifications  and improvement mods for all of the G1MFG 13cm & 23cm receivers and transmitters. These modifications may also apply to similar Comtech units from other suppliers. This page is to share my modification experience, information and hopefully make a worthwhile contribution to on-line information. On this page

  • find links to schematics and component data sheets,

  • what initiated my investigations,

  • the video waveforms used to investigate problems and
    the effect each modification has upon these waveforms,
  • and the  Platinum Modifications .

If you have a Gold receiver, check out the Remove Gold Mods Mod that shows why
each modification should be removed and how to do it.

Please if you have any information or comments to share.

 Note: The web site no longer exists so previous links to site have been removed.

Also Note: Much has changed since these pages were written. I don't believe Comtech Rx's & Tx's are any longer available and there's now much better equipment designed for R/C aircraft FPV to use, but the pages are being left for those that still have and use Comtech units to reference. And for those still using these Comtech units, please note that the vertical sync problem is actually a transmitter design problem that's next to, if not impossible, to correct and that the receiver mod's are simply an attempt to try and make the problem less sever.


I know how much work and time is involved in maintaining a web site, so I normally excuse a few errors or disparity.
However, though out this page I have commented on information that I found confusing at in order to help others make sense of it. Perhaps I am simply slow, too critical or expect too much. If so, I apologize to Giles @ G1MFG.
My comments intend no offense and I hope none is taken.

P.S. It was never my intention to discredit the Gold mods during investigations.
I am simply reporting what was discovered.


Data Sheet, Schematic &
Other Technical Information Links

It was very difficult to find any technical information, other than what is was on the G1MFG modification page(s), on these or any other of the ATV modules manufactured by Comtech, a manufacturer of tuners in Taiwan. Poor quality JPG images of a Comtech 13cm receiver and transmitter schematic are were on this G1MFG page, but as noted, they are not 100% accurate. Other than the voltage regulators, the 13cm & 23cm receivers look alike, however they have slightly different circuitry and many component values differ. For what it is worth, here are better copies of the Comtech Receiver & Transmitter schematics that I made and used before creating my own schematics which are accurate for the receivers & transmitters received in 2003.

Receiver Schematic
Receiver Schematic
13cm & 23cm
Transmitter Schematic
13cm Tuner
Partial Schematic
23cm Tuner
Partial Schematic
13cm Tx Module
Partial Schematic
23cm Tx Module
Partial Schematic
13cm Tuner Data Sheet
23cm Tuner Data Sheet
13cm Tx Module Data Sheet
23cm Tx Module Data Sheet
Data Sheet
App. Note
Tuner I.C.
KIA6003S Audio
Demodulator I.C.
DC/DC Converter


In the Beginning . . . .

Investigations started with the 23cm Gold receiver and it's
frame lock (vertical hold) problem.

Giles said "We've had reports of poor frame lock (vertical hold) on signals sent by our 23cm transmitters" and added "It seems the low frequency response of the 23cm transmitter is a bit poor"

Yes, there is a vertical hold problem and the transmitter low frequency response is poor. However, this problem does not seem to affect the 23cm basic receiver or the 13cm receivers. The 13cm & 23cm receivers and transmitters share nearly identical circuitry and video waveforms from the tuners is similar which indicates that the problem was created, or a less serious problem was made worse, by one or more Gold modifications.


These are a couple of the

Video Waveforms used to Investigate Problems

Square Wave Waveform
@ Field Rate
A Square Wave waveform (left) was used to view low frequency response problems as the waveform distortion becomes more pronounced the longer the period without video, from just the vertical sync period.

A Multi-Burst waveform (below) was used for more typical video with video during all but the vertical sync period.

Video Monitor Display
of Waveform

Multi-Burst Waveform
@ Line Rate

A Multi-Burst Waveform was used to check the video circuit frequency response. The five freq bursts are at: 500KHz,
1.0 MHz, 2.0 MHz,
3.0 MHz, 3.58 MHz,
and 4.2 MHz.

Multi-Burst Waveform
@ Field Rate

Video Monitor Display
of Waveform

Note: Transmitter audio sub-carriers were disabled in most cases in order to view un-filtered base band video waveforms.


Received Waveforms
These Field Rate Waveforms show the cause of the vertical hold problem.

For the 13cm and 23cm Basic receiver, the sync level shifts some during the vertical sync period, however this causes few problems for most equipment.

13cm Advanced Rx

23cm Basic Rx
The 23cm Gold receiver waveform shows that the sync is shifted very high, even up into the picture video level area, during the vertical sync period. This makes it difficult, or impossible, for monitors and other equipment to differentiate between sync and video.
23cm Gold Rx

A capacitive coupled signal will center itself about the average signal level at a rate dependent upon the circuits R/C time constant.

These Vert Sync Period Waveforms (close-up view on right) show how the waveform centers itself about the average signal value (yellow line) due to poor performance of the receiver's DC restoration circuit.

Many sync circuits look for sync at a level slightly above the sync tips (blue line above) so they are unable to recover sync from signals with shifted sync.

DC restoration circuits restore video signals by resetting sync tips to a consistent DC reference level. The yellow arrow indicates the sync tip reference level in this waveform. One thing that would really improve these ATV receivers, is a good DC restoration circuit.

See the Platinum Modifications for further waveforms and analysis of this and other problems.




Test equipment setup while working
on the receivers and transmitters.


With such interesting electronics available it is easy to
get addicted to technology. While institutions such as Morningside Recovery deal with extreme addictive behavior, the steps to recovery from technology addiction can be as easy as turning off your computer or cell phone.

Giles at G1MFG has his "Gold" modifications and now I have some of my own, but they need a name.
Someone suggested to use what the credit card companies use to indicate better than Gold, so here are

The Platinum Modifications
for G1MFG 13cm & 23cm Receivers and Transmitters

These receivers & transmitters may not perform any worse than similar units and the receivers have very good sensitivity, but they can use improvement. The Platinum modifications improve the receiver and transmitter performance as much as possible without adding additional circuitry.

There is also a mod to provide a "S" meter signal from the 23cm tuner, where to find the13cm tuner "S" meter signal and how to add a "S" Meter and Expanded Scale Battery Volt Meter to the receivers. Digging deep into the tuner and transmitter module circuitry and by custom programming the PIC chip, there are some neat things possible that I plan to explore and present here in the future.

Note: These modifications are not for everyone and many will require
a very small tipped soldering iron, tweezers and a good pair of magnifying glasses!

Warning! Perform these modifications at your own risk!

Use the following links to quickly locate individual modifications and other information.

1 13cm & 23cm Transmitter - Voltage Regulator Mod
2 13cm 8mW Transmitter - Increase Output Power Mod
info 13cm & 23cm Transmitter Video Peaking
3 13cm Transmitter - Improve Video Mod
4 23cm Transmitter - Improve Video Mod
5 13cm & 23cm Transmitter - CCIR pre-emphasis (coming)


info Receive Sensitivity Comparison Table
Notes Regarding the Use of Un-Regulated Wall-Warts
1 23cm Gold Receiver - Remove Gold Mods Mod
2 13cm Receiver "S" Meter Output Info
3 23cm Receiver "S" Meter Output Mod
4 13cm & 23cm Receiver - Improve Sensitivity Mod
5 13cm Receiver - Improve Video/Audio Mod
6 23cm Receiver - Improve Video/Audio Mod
7 23cm Receiver - AFT Mod
8 13cm & 23cm Receiver - CCIR De-emphasis (future)



For Gold receivers, Remove Gold Mods Mod must be done in conjunction with the

23cm Receiver Improve Video/Audio Mod


Modifications are quite simple and only require changing a few components, so don't let all of the waveforms and information scare you away from trying these mods. I wanted to document, for my own reference, what I found and I may have included more than necessary of the 400+ recorded waveform images. I hope explanations are clear and thorough enough to properly explain things and answer any of your questions.


Before  and  After
Platinum Modification Waveforms
Receiver & Transmitter Waveforms Before Platinum Modifications After Platinum Mods
13cm Advanced 23cm Basic 23cm Gold 13cm or 23cm Rx & Tx

Field Rate Multi-Burst Signal shows typical Vertical Sync Period Waveform.

The un-modified 13cm receiver video is very good. The 23cm Basic receiver is worse due to poor low frequency response of the transmitter. The Gold mod that increases the value of C8 in the receiver makes the receiver's output worse, to the point that the receiver becomes unusable due to the vertical hold problem.

Field Rate Square Wave Signal to enhance DC Restoration Problems.

The un-modified 23cm units have trouble handling a signal of this type. Even though the received waveform from Platinum modified receivers is less than ideal, other equipment seems to have no problem dealing with it.

Line Rate Multi-Burst Signal to show the Frequency Response of Transmitter/Receiver Pairs.
Un-modified units show poor high frequency responses. Platinum modified units show a fairly flat response.
Horizontal Sync and Color Burst Signal.
The Platinum modified units provide a much cleaner output signal with no discernable overshoot or ringing.

13cm and 23cm Platinum Receiver Waveforms
show that there is little difference between the two.

13cm Receiver 23cm Receiver


Transmitter Modifications


13cm & 23cm Transmitter Voltage Regulator Mod

Reference: Transmitter Schematic

The manual says "power requirements are 12 to 18 VDC at 150 ma and best results are with a supply of 15V or more" because the 78XX series of regulators require up to nearly 3 volts greater input voltage than output voltage, to provide regulation.
The stock 7812 regulator is not a problem when operating from an unregulated 12V wall-wart that is capable of supplying greater than 15V to a 150 ma load. However, when operating from a 12V battery or regulated 13.8VDC power supply it is best to replace the 7812 with a low dropout regulator such as a LM2940 which requires 12.2V, or less, input to provide a regulated 12V output. If you always use a regulated 12V power supply, one could also simply remove and bypass the regulator.


13cm 8mW Transmitter - Increase Power Mod

Click here if all the G1MFG transmitter output power ratings have you confused?
*** *** November 1 / 2003

This modification increased my 8mW transmitter outputs to 35-40mW.

Refer to G1MFG transmitter technical notes for additional information and help to locate the resistor to be removed for increased output power. The yellow arrow in the right hand photo below points to where this resistor was removed from a modified transmitter.

The technical notes also mention adjusting the two coils in the transmitter module. In each of my four transmitters, which ranged from 8-10mW, the two 2-turn coils were spread open and pressed down, nearly flat, against the circuit board. Output power increased, to typically 20 mW, as the coils were lifted up off the board and squeezed tightly back together (for maximum inductance) as in the following left hand photo.

Wanting MORE POWER meant I may need coils with more inductance. The original 2-turn coils were removed and new 3-turn coils were made from #26 magnet wire wound on a #60 drill bit to obtain the correct diameter.

Original coils after they were reformed tightly together for increased output power


New 3-turn coils after adjustment
for maximum
output power

Size of new coils & removed resistor
compared to a penny.

Before adjustment, with the new coils still wound tightly together, output power was 12mW (typical). After adjustment, two transmitters provided 40 mW, one 35mW and one was 32 mW.

Without data sheets, I don't know if this mod is stressing the output transistor, but it doesn't worry me as 40mW is not very much power, plus I would not be surprised to find the same output transistor in the 25mW unit (40mWmax). The transmitter will get slightly warm after performing this mod.

Good Luck!

You will need Patience and Good Magnifying Glasses!

The components are very


Transmitter Video Peaking Information
Component designations (i.e. R1, C1, L1, etc) refer to components on
13cm Transmitter Module Schematic and 23cm Transmitter Module Schematic

In the transmitter modules, video goes through a parallel C1-R1 (100p // 2.7K) video peaking network before going to VR1, for deviation adjustment, and through C2 to the modulator.

Waveform Before C1//R1

Waveform after C1//R1

Waveforms show the effect of
C1//R1 Video Peaking Network
on video signal going to the
transmitter modulator.

Received Waveform
Without Peaking

Received Waveform
With Peaking

Received Multi-Burst Waveforms
without and with transmitter peaking.

The C1//R1 video peaking circuit effect is removed in the receiver by a series C-R circuit. This is likely some form of noise reduction pre-emphasis, however between manufacturers there seems to be no standard as to the amount of peaking applied. This presents a problem for ATV groups where equipment from any number of different manufacturers could end up being used.

Does everyone have to figure out how to modify their particular equipment for a flat response before adding standard CIRR pre-emphasis? Or is the group forced to use whatever type of equipment the group initially starts out with and forever locked in to using? Or does everyone plead ignorance and ignore the fact that pre-emphasis varies between equipment and simply live with the consequences?

I would appreciate hearing from anyone who has answers for any of the above questions or how different groups have addressed this problem. I would also appreciate hearing from anyone who has good links to information regarding this topic. Any information received will be shared with everyone here.


13cm Transmitter - Improve Video Mod

Component designations (i.e. R1, C1, L1, etc) refer to components on 13cm Transmitter Module Schematic.


23cm Transmitter - Improve Video Mod

Component designations (i.e. R1, C1, L1, etc) refer to components on 23cm Transmitter Module Schematic.

23cm Transmitter Module

Comtech FM1394TSIM
1340-1450 MHz
40mw (typical)

This modification improves the low frequency response of the transmitter by changing two capacitors in the transmitter module.

NOTE: This mod will make the Gold receiver vertical hold problem worse, so the Gold receiver mods must be removed. Then keep going and do it right by performing the Platinum receiver mods.

Place cursor over text to show
component identification arrows.

For access into the PLL compartment, this this tab must be temporarily bent up and out of the way.

Replace 36 pfd capacitor at this location with a .01 ufd capacitor. The 36 pfd cap is so tiny that it is easiest to replace it. Photo shows a larger .01ufd cap. used for mod.

Stack & solder a 0.5 ufd capacitor on top of the 1.0 ufd capacitor at this location or simply replace the 1.0 ufd capacitor with a 1.5 ufd one. It is usually easiest to simply stack & solder components if they are of similar physical size.

Replacing this 1.0 ufd capacitor with a 2.2 ufd 6.3V capacitor will make a further slight improvement to the transmitter low frequency response, but the improvement is too small to make this mod worth doing.

Video Peaking Components C1 & R1.
These are the components refered to in the previous Video Peaking Circuit information.

VR1 Video Deviation Control

This Modifications Effect

Received Field Rate Waveforms
Before & After Transmitter Modification
(Waveform video taken directly from the
receiver's tuner output pin.)

Un-Modified Transmitter

Modified Transmitter

Close-Up of Waveform Vertical Sync Period

After modification the sync level remains more
consistent. This is half of the solution to the Gold
receiver's vertical lock problem.
(Removing Gold Mod-3b is 2nd half of the solution)

The low frequency response problem is caused by a 36 pfd capacitor being too small a value, in comparison to the varactor capacitance, to effectively modulate the varactor diode at lower frequencies.

Increasing the value of the 36 pfd capacitor also increases the PLL response time. This makes it impossible for the PLL to correct large frequency step changes so the PLL filter 1.0u capacitor must be increased in value to reduce the response time. Before increasing the 1.0 ufd capacitor's value, it was interesting to watch received video fluctuate for a second or more while the transmitter's PLL corrected small transmitter frequency changes.

It is nice to have frequency agile transmitters, but PLL circuits in FM video transmitters can be a big problem. Think about it. First, we want a PLL with a fast response time to quickly respond to transmit frequency changes. Even if we don't mind waiting, the response time must be at least fast enough to correct for the largest frequency step change that will be made. Then, when we modulate the VCO with video, we want a PLL with a slow response time that will not correct the VCO frequency changes due to FM modulation. We can not have it both ways so compromises must be made. So if you want a good FM video transmitter, forget about being frequency agile and use a crystal oscillator type transmitter.


Receiver Modifications


13cm Receiver "S" Meter Output Info

13cm Tuner

"S" Meter Signal
A signal strength voltage is available
on pin 9 of the receiver's tuner.
(Yellow arrow indicates Pin 9)

This voltage varies from ≈ 0.9V (no signal) to 4.3V (AGC saturated).

See Receiver Sensitivity Table for
"S" meter voltage versus signal strength.

See "S" Meter Mod for details of how to add a "S" meter.


23cm Receiver "S" Meter Output Mod

23cm Tuner

The 13cm & 23cm tuners look (and are) very similar.

A signal strength voltage is available on pin 19 of the TA8804F receiver I.C. in the tuner. Unlike the 13cm tuner, a jumper must be installed to make the "S" meter signal available on pin 9 of the tuner.

See Receiver Sensitivity Table for
"S" meter voltage versus signal strength.

This modification is the G1MFG "S" Meter Mod, with some additional information.
Reference: G1MFG - "S" Meter modification

In the tuner, locate the two pads (indicated by yellow arrows) on either side of a compartment 'wall'. There is just enough room under the wall to solder a small wire link between the pads. The signal strength voltage will now be on pin 9 of the tuner (indicated by red arrow).

See "S" Meter Mod for details of how to add a "S" meter.

The G1MFG "S" Meter modification fails to mention that pin 9 of the tuner is connected through a 220 ohm resistor to pin 10 of the SP5055 PLL I.C. This has no effect on the signal strength voltage, but if one ever wanted to make use of the PLL I.O. signal, then it would need to be relocated. Fortunately, it is easy to relocate the PLL signal to the same tuner pin that the 13cm tuner has this signal available on.

1. Remove the 4 MHz crystal.
2. Relocate the 220 ohm resistor indicated in photo A to the position indicated in photo B.
3. Re-install the 4 MHz crystal. The PLL Port 3 I.O. signal is now available on pin 12 of the tuner.


13cm & 23cm Receiver - Improve Sensitivity Mod

There is a problem with the Gold receiver sensitivity modification, but when I have more time I will see if there is another way to improve this and the 13cm receiver sensitivity some. The receivers, which already have better sensitivity than any other receivers that I know of, have no sensitivity problems, but it is always fun to try and find free improvements. As for weak signals, the extra 3 dB gain provided by the Gold mod will not really make that much difference compared to a good 16 dB low noise pre-amp installed at the antenna.


13cm Receiver - Improve Video/Audio Mod

Component designations (i.e. R1, C1, L1, etc) refer to components on 13cm Receiver Schematic.

See the following 23cm receiver mod for similar step-by-step instructions and waveforms. This mod is almost exactly the same as for the 23cm receiver except for several component values. The component differences are:

1. Capacitor C3, which compensates for transmitter pre-emphasis, differs from the 23cm receiver.

2. Capacitor C4 does not need to be increased as for the 23cm receiver because of the different biasing arrangement used for the 13cm receiver.

3. Capacitors C5, C6 & C7 are different from the 23cm receiver because the 13cm  receiver audio sub-carriers are much higher in level and require more attenuation.

4. There is no need to confirm R8, C8 or C10 values as these should not have been modified by previous mods.


13cm Receiver Component Locations
(Place cursor over text to show component identification arrows)

1. Replace C1 (22 ufd) with a
    220 ufd, 16 volt capacitor.

2. Replace C2 (2.2 ufd) with a
    22 ufd capacitor (use capacitor
    from step 1.)

3. Replace C3 with a 390 pfd cap.

4. Replace C5 with a 150 pfd cap.

5. Replace C6 with a 390 pfd cap.

6. Replace C7 with a 390 pfd cap.


23cm Receiver - Improve Video/Audio Mod

NOTE - The "23cm Transmitter - Improve Video Mod" should also be done!

Component designations (i.e. R1, C1, L1, etc) refer to components on 23cm Receiver Schematic.

This mod is not as complicated as it appears due to all of the waveforms and descriptive text.
This mod only involves replacing 5, adding 1 and confirming 3 component values.

Referring to the 23cm Receiver Schematic, L1, L2, C5, C6 & C7 form a low-pass filter which removes the audio sub-carriers from the video. After a lot of trial & error trying various component value combinations to improve the filter's response, I gave up and switched to the 13cm receiver which had a better video response to begin with. I had better luck, but it was slow going so I resorted to my computer and a circuit modeling program to determine better component values for a flat low-pass response with a sharp cutoff at the highest cut-off frequency that would provide sufficient sub-carrier attenuation. Component values to improve the receiver's low-freq response were also determined at the same time with the program.

After modifying the 13cm receiver and confirming that the computer calculated values worked, I simply used the same values to modify 23cm receivers with equal success. One thing I learned from this experience was not to waste so much time before using the computer to model a circuit first. It does take time to enter the circuit information, but overall, it is much faster. (Plus it is so much easier and takes so much less time to modify the circuit by simply keying in new data and clicking a mouse button a few times than it does to do un-solder and solder tiny SMT components.


1. Replace C2 (2.2 ufd) with a 22 ufd, 16 volt capacitor to improve the video low-frequency response.

C2 = 2.2u   C4 = .22u

C2 = 22u   C4 = .22u

C2 = 22u   C4 = 22u
Upper Trace -
C4 voltage (NE592 pin 1)

Lower Trace -
NE592 Output (pin 5)

Waveform is 3 Fields

Improvement can be seen in the lower waveforms when C2 is increased in value. More noticeable is the increased signal appearing across C4 which also needs to be larger in value.

Reduction in the signal across C4 can be seen after C4 is increased in value (see step 2.)
Note: Any signal appearing on C4 is signal that will not be amplified.

Note: It was while taking these photos that I noticed the distortion along the video waveform's lower edge which was later found to be due to the one Gold Mod that I thought was worth while. See Remove Gold Mod to improve received picture quality (2) for details.


2. Replace C4 (.22 ufd) with a 22 ufd, 16 volt capacitor to improve the video low-frequency response.
I found it easiest to solder the 22 ufd cap across the existing .22 ufd capacitor.

C4 = 0.22u

C4 = 22u

Upper Trace -
NE592 pin 1 (C4 voltage)

Lower Trace- Video from
NE592 pin 5 (Output)

Field Rate Waveforms

Increasing C4's value significantly reduces the voltage across C4 and
the vertical sync period distortion which contributes to the vertical hold problem.


3. Replace C3 (470 pfd) with a 180 pfd capacitor to improve the video high-frequency response.

C3 = None

C3 = 470 pfd

C3 = 180 pfd

Multi-burst Waveforms from NE592 Output Pin.

C3 and R3 form a high-frequency filter to compensate for peaking applied in the transmitter by C1//R1. The first waveform shows the received video with no compensation. The second waveform shows over compensation by the original 470 pfd capacitor. The third waveform shows compensation provided by a 180 pfd capacitor.


4. Confirm R8 = 150 ohms. R8 must be 150 ohms for a proper video low-pass filter response.
5. Add C5, a 150 pfd capacitor. Steps 5, 6 & 7 provide for a proper video low-pass filter response
6. Confirm C6 = 390 pfd
7. Replace C7 with a 390 pfd capacitor.
8. Confirm C8 = 0.05 ufd. If C8 is larger in value, the time constant of the DC restoration circuit will be too long for the circuit to operate properly. If C8 is made smaller, the DC restore circuit actually performs better, however the video line rate low frequency response will then suffer.
9. Replace C10 with a 20 pfd capacitor. 20 pfd should work fine and provides sufficient signal to the audio demodulator circuits. I have not used the sound circuits much yet so I will reserve further comment for later.
10. Replace C1 with a 220 ufd, 16 volt capacitor if you plan on using an unregulated wall-wart or any other poorly filtered power supply to power the receiver.


23cm Receiver Component Locations
(Place cursor over text to show component identification arrows)

1. Replace C2 (2.2 ufd) with a
    22 ufd, 16 volt capacitor

2. Replace C4 (.22 ufd) with a
    22 ufd, 16 volt capacitor

3. Replace C3 (470 pfd) with a
    180 pfd capacitor

4. Ensure R8 = 150 ohms

5. Add C5, a 150 pfd capacitor

6. Confirm C6 = 390 pfd

7. Replace C7 with a 390 pfd cap.

8. Confirm C8 = 0.05 ufd.

9. Replace C10 with a
    20 pfd capacitor.

10. Replace C1 with a
     220 ufd, 16 volt capacitor.


23cm Receiver - Add AFT Output Mod

Refer to 23cm Receiver Tuner Schematic

This mod will extend either the Analog AFT (Automatic Fine Tuning) or Digital AFT signal from the TA8804F I.C. to pin 5 of the 23cm tuner. This signal is not important to have available at this time, but I plan to make use of the AFT signals to provide Automatic Fine Tuning. When sucessful, I will present details in a future article.

Place cursor over text
to show component identification arrows.

For the Analog AFT signal, add a 100 ohm resistor
at this location.

For the Digital AFT signal, add a 100 ohm resistor
at this location.

For now, here is some basic information on the AFT signals.
  • When "Low  in Frequency" the Analog AFT voltage is > Vcc/2 or 2.5 volts.
  • When "High in Frequency" the Analog AFT voltage is < Vcc/2 or 2.5 volts.
  • When "On Frequency" the Analog AFT voltage is = Vcc/2 or 2.5 volts.
  • When "Low  in Frequency" the Digital AFT Out2 = 'Low' and Digital AFT Out1 = 'High'.
  • When "High in Frequency" the Digital AFT Out2 = 'High' and Digital AFT Out1 = 'Low'.
  • When "On Frequency", one or both of the Digital AFT signals may be pulsing and at some point,
    both will be High. The period of the pulse train (on the unit I experimented on) was 16 ms.