Message 24514 in TECH.WW
From: G8MNY
Date: Sun, 21 Jun 26 08:32:00 Z
Newsgroups: TECH.WW
Subject: FM Deviation Calibration
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Dec 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
PHASE MODULATION
This is a form of FM where the carrier phase is changed not the frequency. It
used to be quite common on Xtal bound PMR rigs as the Xtal Q did not affect
modulator gain, as varicaping of different crystals in an FM design does. The
modulator is slightly different as the varicap is not used directly on the
crystal oscillator, but in the following buffer stage.
It is measured differently to FM, in Rad/s, and 1 Rad/s = FM Modulation Index
(Dev/ModF) of 1 at ANY Mod Frequency. However it gives much the same results as
FM except the modulation is treble lifted at +6dB /Octave.
______ _______ _________ _________ _________ __________
~~~~~~~~
So peak modulation has a different meaning in PM, unless there is a clipper set
up before a 6dB / Octave treble cut filter in the modulator (to turn it back to
FM), to limit the frequency related deviation!
FM MODULATION STANDARDS
Using Carson's rule the width of an FM signal is approx..
2x deviation + 2x highest modulation frequency.
This is not all the sidebands the FM process generates, but most of them, to
see the rest of them look into the Bessel function FM sideband harmonic series.
Carson's rule is simply explained by considering what happens at an instant
when a low frequency has given almost full deviation. The instant FM frequency
is at 1 side of the deviation window, and there is still some treble syllibance
diagrams below for 12.5kHz & 25kHz systems, where the lowest & highest
modulation sidebands are added to either side of the FM deviation like 2 side
by side AM signals. So the Rx also has to let in and correctly demodulate all
these wanted sidebands if there is to be no distortion.
12.5kHz CHANNEL FM SYSTEM
TX Bandwidth
Lowest _____________ Highest Rx .--------------. _-3dB
Next ||<3kHz><----5kHz---><3kHz>|| Next |<----11kHz--->|
Channel ||----------11kHz----------|| Channel |<-----12kHz---->|
|----------12.5kHz----------|
N.B. there is next to no Rx protection "GUARD BAND" between channels on the
12.5KHz system! For this reason commercially, adjacent channels are NEVER used
in the same area!
modulation index of 0.833 (Dev/ModF), which gives little capture effect over an
AM system.
The Tx also needs to have the AF response VERY WELL FILTERED, if the FM
sidebands are to be kept out of the adjacent channel.
0 150 200 300 400 600 800 1k 1k5 2k 3k 4k Hz
The ideal Rx IF filter can't be made (see Rx bandwidth diagram), so in practice
narrower filters give better adjacent channel performance, but with quite high
audio distortion (10%), as some of the needed spectrum is lost.
With tight filters, the channel carrier frequency accuracy is important to keep
the Tx signal centre of the Rx IF. This is not so easy on higher bands without
very good Xtal stability (ovens etc), so the 12.5kHz system is NOT used above
VHF!
25kHz CHANNEL FM SYSTEM
TX Bandwidth |GUARD |
Lowest ____________ Highest BAND| Rx .------------. _-3dB
Next | |3.5kHz<--10kHz--->3.5kHz| | Next |<----17kHz--->|
Channel| |---------17kHz----------| |Channel |<------25kHz----->|
|-------------25kHz--------------|
N.B. Here there is the luxury of an 8kHz GUARD BAND between channels on this
system, which is why it can work much better with strong adjacent channel
signals, than the 12.5kHz system, and with very little distortion!
index of 1.4 (1.6 if 3kHz) and has 2x (6dB) more noise rejection & capture
effect than the 12.5kHz system.
The Tx AF filtering, Rx IF filter and frequency accuracy, are far less
stringent than for the 12.5kHz system, and the comms sound quality can be quite
a bit better. Due to the wider guard band the adjacent channels have less Tx
QRM in them and Rx filters can more easily remove the adjacent channel signals.
EMPHASIS
With FM it is usual to apply some Tx treble pre-emphasis and Rx treble de-
emphasis, this is to mask the increased treble Rx noise (hiss) with the FM
system, and reduce overall system AF harmonic distortion. With comms bandwidth
the amount of emphasis cannot be very great, but up to +6dB @ 2kHz can be used.
FM Rx Tx Pre- Rx De- Overall Audio
Noise Floor emphasis Response
2 3 5 1k 2k 3k 2 3 5 1k 2k 3k 2 3 5 1k 2k 3k 2 3 5 1k 2k 3k Floor
Freq ->
DEVIATION MEASUREMENT
Here are 3 simple methods for FM deviation calibration. Phase modulators use
6dB / Octave LF lift on the modulation to mimic FM and these need some care when
scoping @ the Tx to realise what you are seeing!
1 Bessel carrier null method.
Mr Bessel modulation index graphs show the 1st order carrier null occurs when
the Modulation Index (Dev/ModF) = 2.4, then again at 3.142 intervals after
that. This means a 1kHz sine wave modulation tone will produce a 1st carrier
null at precisely 2.405kHz deviation & a 2nd at 5.54kHz deviation. USEFULL!
No Mod Some Mod 1st Null More Mod 2nd Null
MI= 0 <2.4 2.4 >2.4 2.4+Pi
To monitor the modulation spectrum, you will need a SSB Rx with RF gain
control, ideally with a very narrow CW filter, or a Spectrum Analyser with a
narrow filter. E.g. a sound card from SSB AF output and an AF Spectrum
Analyser programme, or just good ears listing to just the carrier whistles
nulling while the other sideband tones get stronger.
Also an accurate and pure AF 1kHz sine wave generator is needed to feed the
mic circuit via a suitable attenuator (series 100k ?)
Method.
Using a 1kHz sine wave tone, adjust modulation level (mic gain/deviation) to
produce no carrier on a SSB/CW Rx.
Now note the modulator drive level (e.g. scope it) @ the modulator,and ensure
that the AF FM clipper now hard clips anything at this level by adjusting the
deviation pot with the mic gain set at max (e.g. shout into the mic etc.)
2 Discriminator DC & Scope method. (can be used on air with a Rx/scanner)
Access to monitoring FM Rx's discriminator is needed to display the DC level
on a oscilloscope. Make sure the scope is connected to the discriminator
point before any de-emphasis components, and that the deviation sidebands
being measured will all fit through the IF filter, otherwise the display will
lie.
SCOPE TRACE Fc+5kHz _____ _
.' `.
Send a carrier, and change the Rx/Tx frequency +/-5kHz & adjust the scope
gains and position to give a +/- 5 division display.
Now anything you can Rx, will instantly show you the on channel "frequency
error" and "deviation" on the scope trace. N.B. any AC loading of
discriminator my give errors for this method!
Method
For Tx deviation setting, just ensure the clipper hard clips anything @ this
level by adjusting the deviation pot with the mic gain at max. (E.g. shout
3 Accurate by design DDS Osc source.
With a DDS osc programmed to construct FM, the modulation source is correct
by design. I have a "FeelTeck Dual DDS Osc FY6600-60M" one, that generates
many orders of FM deviation accuracy, from a fraction of Hz dev to MHz
deviation. Above 60MHz signal harmonics are needed. Use as the calibrated
source with method 2 above.
SETTING UP A DEVIATION METER.
At this point with a calibrated reference, it is relatively easy to make a peak
reading meter display, and calibrated in peak deviation for your Rx. Once you
have a calibrated source, it is easy to put a peak reading meter circuit (not
an average VU circuit!) onto any Rx and calibrate it. For accurate work a wide
Rx is needed.
HARMONICS
Using these the deviated signal will increase by their harmonic factor. E.g. 2M
check.
N.B. With some deviation meters, it is easy to accidentally measure a harmonic
and not the fundamental's deviation!
Also see my buls on "FM Stereo Radio Principles", "DYMAR Mod Meter Type 1785",
"RTT Mod Meter (100)"."RTT Comms Test Set","RTT Comms Test Set use" & "FeelTeck
Dual DDS Osc FY6600-60M".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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