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G8MNY > TECH 11.04.06 01:50l 142 Lines 6491 Bytes #999 (0) @ WW
BID : 25455_GB7CIP
Read: DL1LCA GUEST
Subj: FM Deviation Calibration
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Sent: 060410/2332Z @:GB7CIP.#32.GBR.EU #:25455 [Caterham] $:25455_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Dec 05)
(8 Bit ASCII Graphics use code page 437 or 850)
MODULATION STANDARDS
Using Carsons' Rule the width of an FM signal is..
2x deviation + 2x highest modulation 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! Commercialy adjacent chanels are never used the the same area!
For the 12.5KHz system a MAX of ñ2.5KHz Peak deviation is used. 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. the comms audio 200Hz-3KHz @ -6dB,
ideally at >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 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 little distortion!
For 25KHz system a MAX of ñ5KHz peak deviation is used. The Tx AF filtering &
the Rx filter are far less stringent than the 12.5KHz system & 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 FM system. With
comms bandwidth the amount of emphasis cannot be very great, but +6dB @ 2KHz is
commonly used.
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 narrow CW
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
the carrier whistles nulling while the other sideband ones get stronger.
Also a an accurate AF 1KHz sine wave generator is needed to feed the mic
circuit via a suitable attenuator (series 100Kê ?)
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 that the AF clipper now hard clips anything above 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 +/- 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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