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G8MNY > TECH 28.06.05 00:01l 137 Lines 6165 Bytes #999 (0) @ WW
BID : 51833_GB7CIP
Read: GUEST OE7FMI
Subj: FM Deviation Calibration
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<DK0WUE<I0TVL<CX2SA<GB7YFS<GB7CIP
Sent: 050627/1932Z @:GB7CIP.#32.GBR.EU #:51833 [Caterham] $:51833_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Apr 05)
MODULATION STANDARDS
Using Carsons' Rule the width of an FM signal is
2x deviation + 2x highest Mod frequency.
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. This gives the diagrams below for 12.5 &
25KHz systems. where the lowest & highest modulations sidebands 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!
For 12.5KHz system we use a MAX of ñ2.5KHz Peak deviation. The Tx also needs to
have the AF response very will filtered if the FM sidebands are to be kept out
of the adjacent channel. eg comms audio 200Hz-3KHz @ -6dB, ideally >3.8KHz
needs to be in the order of -70dB !
The ideal Rx IF filter can't be made, so in practice narrower filters gives a
better adjacent channel performance, but with higher audio distortion as some
of the wanted spectrum is lost.
25KHz SYSTEM
TX Bandwidth
Lowest _____________ Higest Rx .------------. _-3dB
Lower /'³ deviation 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 |<-------20KHz------>|
|-------------25KHz--------------|
N.B. There is the luxuary of a 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 & little distortion!
For 25KHz system we use a MAX of ñ5KHz peak deviation. The Tx AF filtering &
the Rx filter are far less stringent than the 12.5KHz system & adjacent
channels are more with easily removed.
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 FM. With comms
bandwidth the amount of emphasis cannot be very great, but +6dB @ 2KHz is
common.
FM Rx Tx Pre- Rx De- Overall Audio
Noise Floor emphasis emphasis Responce
/|\ _.-' _.-' ```ÄÄ..__ ..............
Level__..,,Ä-' __..,,Ä-' `Ä._ _ _ _ _ _ _ _ _ Noise
1 3 5 1K 2K 3K 1 3 5 1K 2L 3K 1 3 5 1K 2K 3K 1 3 5 1K 2K 3K Floor
Freq ->
DEVIATION MEASUREMENT
Here are 2 simple methods for FM deviation calibration.
1/ Bessel carrier null method.
Mr Bessel modulation index graphs show the 1st order carrier null occurs
when the MI=2.4 then at 3.142 intervals after that. That means with a 1KHz
sine wave modulation tone will produce the 1st carrier null at precisely
2.4KHz deviation & the 2nd @ 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
You will need a SSB Rx with RF gain control, ideally with a very CW narrow
filter, or a spectrum Analyser with narrow filter. eg a soundcard from SSB
AF O/P & an AF spectrum Analyser programme or just good ears listing to just
one of the whistle nulling while others sideband ones get stronger. Also a
an accurate 1KHz sine wave AF generator is needed.
Using a 1KHz sine tone adjust modulation level (deviation) to produce no
carrier on a SSB/CW Rx.
Now note the modulator drive level on the Tx (scope it) at the modulator &
ensure the AF clipper, hard clips anything above this level by adjusting the
deviation pot with the mic gain set at max (shout into the mic).
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 +/- 5 division display. Then 1KHz deviation = 2 divisions
peak to peak.
Now anything U can Rx, will instantly show you the on channel frequency
error & deviation on the scope.
For Tx deviation setting, just ensure the clipper hard clips anything above
this level by adjusting the deviation pot with the mic gain at max (eg 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 ie easy to put a Peak reading meter
circuit (not average VU) onto any Rx & calibrate it.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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