May 1, 2022
“Building an FM Deviation Monitor”
Deviation monitor project ( Draft)
First off… Obviously I don’t claim that this project is meant to replace a professional FM Frequency Deviation Monitor.
One aspect of communications radios that has always plagued operators is having a proper sounding signal whether it is the modulation of an AM transmitter or the deviation of an FM transmitter.
Amateur radio transmitters should be modulated or “Deviated” to a full 5 khz level to sound rich and full of audio characteristics of the spoken voice.
When listening to the repeaters you have heard stations that you have to turn up the volume to hear what they are saying. These stations are often heard asking for a radio check and the disturbing thing I find, is the stations going back to them and telling them that they sound fine into the repeater!
That’s ridiculous, some folks will be honest and tell them that they sound a little low in audio, at least they are on the right track.
Your transmit audio is THE SINGLE MOST IMPORTANT characteristic of your signal.
Here is an example…if a station is only deviating their transmitter to a level of say 2.5 Khz deviation and is using an HT to talk to the repeater AND has noise on his signal at about 1.5 to 2Khz in level he will not be heard.
BUT….if the station is fully deviating a 4.5-5 khz signal his voice will rise above the noise and be totally intelligible, even though his signal is still noisey! AMAZING!
A good example of variable audio levels is on the Sunday night net at times. Listening to the different stations you will notice that everyone’s audio level goes up and down a lot.
This project was first built and used in 1978.
I was looking for a way to monitor my 2-mtr transmitter’s deviation without the use of a commercial service monitor. Those were only had by the two-way technicians working on FM two-way radios.
At the time using a tube receiver crystaled up on the local repeaters input frequency, I was able to see my audio signal with a scope from my portable or mobile radio.
One of the problems to overcome was getting a consistent audio level out of the receiver. That’s not a problem with today’s technology.
I’m a big fan of building projects with items from my parts supply (Junk box) that is already on hand, but still inexpensive to build with getting your compo0nenta off line.
What I have done here with the SA-818 walkie talkie module is add a circuit to take the receiver audio output and look at it with an oscilloscope, which is an ac signal and then add a meter driving circuit to rectify the ac audio signal into a dc signal able to drive a 50ua meter.
Using a service monitor’s generator output and feeding an on-frequency signal with a calibrated 5Khz deviated 1000hz tone, I will get a known audio level out of the SA-818 driving the meter that can be set for a full scale deflection.
What makes this possible is the SA-818 digital structure having 8 preset consistent levels out of the module for volume and 8 selectable settings for the squelch.
When the full-scale deflection is observed, a mark on the meter face will indicate that any received audio signals can then be viewed and compared to our 5Khz full scale deflection.
Who cares what audio level is if its not at optimum level, that’s what we are striving for.
Using this device, here is a way for a station to be able to tell if their transmitter needs to be adjusted for full deviation ( programming analog vs digital ), Possibly change microphone or change the way you talk into your microphone to get your voice peaks to get as close to 5Khz deviation as possible.
How to talk into your microphone was covered in a previous Tek Net and is a whole different discussion.
If you care about the way your signal is received by others and want to see it for yourself…..
This project will do that.
After trying several different SA-818 modules I have found them to be extremely close to each other as far as the signal being received, so the success rate from folks building this project will be very consistent.
What parts do I need?
(parts list)
Using the NiceRF.com SA-818 module (once programmed ) it is going to be built into an enclosure with a front facing meter and a rear connector to be connected to an oscilloscope.
Again once we have adjusted our meter for full scale deflection, by connecting a scope to this device you will see that audio level is ok or not.
This project is being presented as a two part project.
First part is the Deviation monitor project. How to build, wire, program and calibrate it for use.
The second part of the project ( most like 2 weeks later) will be the addition of a hand held microphone and audio amplifier with speaker to create a single channel radio capable of talking to a repeater and being able to display the output deviation of that repeater’s users .
That is when you will see how different the stations audio varies among operators, that all sound close to being the same but aren’t.
You’ll be able to tell a station, your audio is clear but is about half or a little low compared to others.
If your wondering, what about over deviating a signal, DON’T.
Because of the way the modulation circuits in most computer controlled transmitters are designed it is pretty hard to over deviate because of the limiting in the exciter stages, if someone has altered their transmitter to the point of more than 5 khz deviation it will most likely be distorted and very obvious.
Building this project is simple and straight forward.
Parts list consists of: SA-818 (programmed to your frequency)
50ua Meter
20K potentiometer (Calibration control)
9Vdc battery packs
7805 5 Volt regulator
.1uf ceramic capacitor
.22uf ceramic capacitor
1N4001 Diode
(4) 1N270 germanium diodes
1000 ohm 1/4w resistor
Led (power indicator)
On/Off switch
(2) Female BNC panel mount connectors
Enclosure (your choice)
Misc. standoffs or screws and nuts to mount the boards
The power supply board with the 7805 regulator is where all the power distribution and parts come together.
A small piece of prototyping circuit board material is used for the power supply and the meter diode board.
(Schematic here)
There is no critical wiring in building this unit. Everything is low impedance, making shielded wiring unnecessary except for the RF coax to connect the board to the antenna connector.
DE WB6AMT Earl
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