I need to measure deviation on a couple of HT's. The problem is finding a way to do it. I have a receiver that has a 10.7 MHz IF output I could feed to a detector but I cannot find any circuits to use to do it. Any pointers would be appreciated. If I can get it detected I can do gain and calibration. The problem is finding a detector IC or schematic with parts.
10.7 MHz IF & detector
Jan 28, 2015
8 Replies
One simple method is to use a delay discriminator. A Mini Circuits mixer, a splitter, and a couple of coax patch cords is all it needs.
If you want to do it many times, and can control the modulation frequency, you can use a comparator and an xor gate.
Cheers
Phil
** HT = handheld transmitter - right ? ** One way is using a frequency counter and a scope.
The scope will reveal the peak voltage being used to modulate the local oscillator. You can then apply that same voltage as a DC level.
The counter will give you the zero modulation frequency and the new frequency with the DC level applied.
Subtraction gives the peak deviation.
.... Phil
The Bessel Null is the cat's meow.
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The other technique is to take a cheap FM IF chip like CA3089 or MC3362 and replace the 10.7 Mhz tunable quadrature coil with a 10.7 Mhz Quartz Crystal.
I have a schematic for the later, and I'll see if can get it on Dropbox or something.
Steve
Here you go.
At 1 KHz sine input to the HT TX input, the output will be 100 mV per KHz of Deviation with components as shown.
From RSGB UHF/VHF Handbook, G.R. Jessop Editor, 4th Edition, 1991 Printing.
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Steve
On a sunny day (Wed, 28 Jan 2015 16:55:40 -0500) it happened nothermark wrote in :
Maybe trigger a oneshot (fixed output pulse length) and average the spikes. Its the old FM video detector.
Thanks! That is the kind of circuit I was looking for.
FWIW I will probably take it at face value to get started. I can get time on an a communications analyzer as a final check but I cannot get one to play with while I figure out what is going on and tweak my HT. That was how I verified the modulation was low. Eventually I intend to get a good signal generator that will let me calibrate it but I still need the meter.
Thanks for the help. There is a Hamfest Saturday where I will see what I find then order some parts based on what I find there. At least I have a place to start now. ;-)
Hi Mark, there are some U tube clips on adjusting deviation the easiest way is to put another radio about 20 feet away from yours on the channel below or above, turn the deviation up until it breaks through on the adjacent ch annel, then take it back a bit, or even talk to some friend close to you an d turn it up so the audio sounds goodm and get him to monitor adjacent cha nnels for any breakthrough.....this is for FM Deviation....Fred g4vvq in Es sex UK
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