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G8MNY > TECH 24.12.13 13:05l 179 Lines 8557 Bytes #999 (0) @ WW
BID : 49304_GB7CIP
Read: GUEST
Subj: FM Deviation Calibration
Path: IZ3LSV<IK8VKW<F1OYP<GB7CIP
Sent: 131224/0948Z @:GB7CIP.#32.GBR.EU #:49304 [Caterham Surrey GBR] $:49304_GB
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Feb 10)
(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 in crystal FM designs. The modulator is slightly different as
a the varicap is not used directly on the crystal oscillator, but in the buffer
stages following. However it gives much the same results as FM except the
modulation is seem to be 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 limit the frequency related deviation!
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 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
modulation 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 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! Commercially adjacent channels are never used the same area!
For the 12.5kHz system a MAX of ñ2.5kHz Peak deviation is used, giving a
modulation index of 0.833, 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% 2.5kHz
-6dBÄ´ / >Þ ÃÄ 50% 1.25kHz
³ / Very tight audio filtering> Ý ³
³ / > Þ ³
³ / for no adjacent ch QRM > Ý ³
-70dBÄ´ / > Þ ÃÄ0.03% 0.75Hz
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 high audio
distortion, as some of the wanted spectrum is lost.
With tight filters, the channel carrier frequency accuracy is important to keep
the Tx signal center of the Rx IF. This is not so easy on higher bands without
very good Xtal stability etc, so the 12.5kHz system is not for used above VHF.
25kHz SYSTEM
TX Bandwidth
Lowest ____________ Highest 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) the noise rejection capture effect of
the 12.5kHz system.
The Tx AF filtering & the Rx filter are far less stringent than 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 2.4kHz 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 ones 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.
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. Then 1kHz of deviation = 2
divisions peak to peak.
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.)
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.
Also see my buls on "FM Stereo Radio Principles"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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