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G8MNY > TECHNI 19.10.19 18:52l 201 Lines 9456 Bytes #999 (0) @ WW
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Subj: FM Deviation Calibration
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jan 16)
(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 different crystals in FM designs do. 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, & 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 as the FM process generates, but most of them, to
see the rest look into the Bessel functions.
His 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, & there is still some treble syllabance mod to
carry with its ñ3kHz sidebands like an AM signal. This gives the diagrams below
for 12.5kHz & 25kHz systems, where the lowest & highest modulations sidebands
are added to either side of the FM deviation. So the Rx also has to let in all
these wanted sidebands if there is to be no distortion.
12.5kHz CHANNEL FM SYSTEM
TX Bandwidth
Lowest _____________ Highest Rx .--------------. _-3dB
Lower /'³ deviation ³`\ Upper Bandwidth ³ ³
Sideband/' ³ +/-2.5kHz ³ `\Sideband (ideal) ³ ³
_________,/_____³___________³_____\._______ ____,'_-70dB `._____
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 reasion commercially adjacent channels are NEVER used
in the same area!
For the 12.5kHz system a MAX of ñ2.5kHz Peak deviation is used, giving a
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.
0dBÄ´ .-ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ-. ÃÄ100% 2.5 kHz
-6dBÄ´ / >Ý ÃÄ 50% 1.25kHz
³ / Very tight audio filtering>Þ ³
³ / > Ý ³
³ / for no adjacent ch QRM > Þ ³
-70dBÄ´ / > Ý ÃÄ0.03% 0.75 Hz
ÃÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÙ
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
Lower /'³deviation ³`\ Upper Bandwidth ³ ³
Sideband /' ³ +/-5kHz ³ `\Sideband ³ ³
___________,/_____³__________³_____\._______ ____,'_-70dB `.____
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, & with very little distortion!
For 25kHz system a MAX of ñ5kHz peak deviation is used, gives a modulation
index of 1.4 (1.6 if 3kHz) & has 2x (6dB) more noise rejection & capture effect
than the 12.5kHz system.
The Tx AF filtering & the Rx filter are far less stringent than for the 12.5kHz
system, & 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 & Rx filters can more
easily remove the adjacent channel signals.
EMPHASIS
With FM it is usual to apply some Tx treble pre-emphasis & Rx treble de-
emphasis, this is to mask the increased treble Rx noise with the FM system.
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
/|\ _.-' _.-' ```ÄÄ..__ ..............
Level__..,,-Ä' __..,,-Ä' `Ä._ _ _ _ _ _ _ _ _ Noise
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 2 simple methods for FM deviation calibration. Phase modulators use
6dB/Octave LF lift on the modultion to mimic FM & 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 2.40kHz deviation & a 2nd at 5.54kHz deviation.
dBs ³ |
³ ³ || || |, ,|
³ ³³³ |³³ ³³| .|³³³³³³³|. ,|³³³³³³| |³³³³³³|,
ÄÄÄÁÄÄÄf ÄÁÁÁÁÁÄ ÄÁÁÁÁÄÁÁÁÁÄ ÄÁÁÁÁÁÁÁÁÁÁÁÁÁÄ ÄÁÁÁÁÁÁÁÁÁÁÄÁÁÁÁÁÁÁÁÁÁÄ
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 & 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 & pure AF 1kHz sine wave generator is needed to feed the mic
circuit via a suitable attenuator (series 100kê ?)
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, & 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 (shout into the mic etc.)
2 Discriminator DC & Scope method. (can be used on air with a Rx)
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, & that the deviation sidebands being
measured will all fit through the IF filter, otherwise the display will lie.
SCOPE TRACE Fc+5kHz _____ _
.' `.
Fc ÄÄÄÄÄÄÄ MOD | | |
Fc-5kHz _____ ñ5kHz `._,'
Send a carrier, & change the Rx/Tx frequency +/-5kHz & adjust the scope gains
& position to give a +/- 5 division display.
Now anything you can Rx, will instantly show you the on channel "frequency
error" & "deviation" on the scope trace.
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
into the mic.) Then set deviation = ñ2.4 or ñ5.0 divisions peak to peak.
SETTING UP A DEVIATION METER.
At this point with a calibrated reference, it is relatively easy to make a peak
reading meter display, & 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 & calibrate it. For accurate work a wide Rx
is needed.
HARMONICS
These will deviate more by their hamonic factor. E.g. 2M rig at ñ2.5 kHz dev
will have a 70cms harmonic deviation of ñ7.5 kHz.
N.B. With some deviation meters, it is easy to measure a harmonic & not the
fundimental!
Also see my buls on "FM Stereo Radio Principles", & "RTT Mod Meter (100)"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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