Re: setting proper deviation
- Do you have a computer with a sound card? You could install one of the
sound card audio analyzer programs or any program with a scope-type
display and use it as an aid to setting deviation. Some of those
programs have audio generator functions as well.
voltage output of a VHF/UHF scanner. But you still needed something to
"calibrate" it. It is difficult to find a constant tone so the voltmeter
would have a stable display. What I did was use the DTMF output from my
HT as a standard level, but you can't even do that if you don't know
what that deviation is. And DTMF isn't accurate either because it is a
Dual Tone signal.
You have to be very careful what tone you provide while setting the
deviation. You have to set the deviation for the HIGHEST frequency tone
in the signal. Because of audio pre-emphasis in the transmitter, if your
target is 3.2KHz deviation, that should be the deviation at the space
(high) frequency. Deviation at the mark (low) frequency will be around
1.7KHz, 6dB per octave lower. If you use mixed tones, the average as
shown by voltmeter will be somewhere in the middle.
The best choice is to modify a scanner to get audio from the
discriminator in the radio, or use the 9600 baud audio output from a
"9600 ready" ham rig. Run the audio to your computer sound card and
watch for the peak deviation on public safety transmissions in the VHF
band. Those transmitters should be very well adjusted for 5KHz peak
deviation. Once you know what the peak voltage is for 5KHz, you can set
your TX for 3.2KHz peak deviation using either space tone or mixed
tones. Or adjust the mark tone for 1.7KHz deviation and assume the space
tone is right.
According to Gary N8UR, "Lets not forget Layer 1", it is actually
preferable if the space tone is slightly louder (has more deviation)
than the mark tone. This was a factor for old TNC designs. This means
you would want even less deviation on the mark tone when the space tone
is adjusted correctly. Some of this is an attempt to make up for radios
with poor de-emphasis circuits, some makes up for the TNC tone detector
> Posted by: "Stephen Brown Jr" stephen.brown75@... sbrown6079
> Date: Sun Dec 3, 2006 9:46 am ((PST))
> What's the best way to set deviation without a scope? Would using the tune
> feature of the calibrate command be reliable? Looks like a pretty nifty
> feature from what I have read about it, but I'm not sure if that will give
> me the desired results.