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G8MNY > TECH 27.01.17 12:06l 51 Lines 2226 Bytes #999 (0) @ WW
BID : 32836_GB7CIP
Read: GUEST
Subj: Simple Carrier Squelch
Path: IW8PGT<CX2SA<GB7CIP
Sent: 170127/1050Z @:GB7CIP.#32.GBR.EURO #:32836 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New Dec 06)
(8 Bit ASCII Graphics use code page 437 or 850)
Wanting to add a squelch to a FM deviation meter, I came up with this 2
transistor circuit. I did not want the normal noise squelch system as that is
delayed & I do not want to see the large peak noise on the peak reading meter.
ÚÄÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ High Level ÃÄÂÄ´ Limiter ÃÄÄ´ Discriminator ÃÄÄ10kÄÂÄÄÄ>To AF
Ä´ IF stage ³ ³ ÀÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ amp
ÀÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ ³
³ +12VÄ¿ ÚÄÄÄ´ÃÄÄÄÙ
1V p-p ³ 27k D2 ³/ 10u +
³ ÃÄÄÄÄÂÄÄ´>ÃÄÄ´T2 C3
³ 47p ³/ ³ ³\e
ÀÄÄ´ÃÄÄÄÂÄÄÄ´T1 ===33n ³
C1 _³_ ³\e ³ C2 ³
D1/_\ ³ ³ ³
ÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄ
It works on a 600kHz IF in my application, so I expect the circuit will work on
most 455kHz IF systems without too much of a tweak. With a scope I first
checked there was a pre limiter stage with enough IF level to drive a
transistor stage into class C, if there was not I would have added another IF
amp to raise the level, as it happed I was lucky large signals to work with.
HOW IT WORKS
With no signal T1 will be off & T2 will be turned on by the 27k through D2.
Note T2 is used as a PURE ON OFF RESISTIVE ATTENUATOR with no DC present
through C3 on to its collector. T2 ON can short out small amounts of +/- AF
current in this mode.
When there is enough IF signal present D1 & T1 base clip the signal & T1
conducts at the IF frequency. C2 discharges quickly & T2 will turn off when C2
is below 1V some IF ripple/noise can occur across C2, but D2 ensures this has
to be big to activate T2.
There is no Squelch pot in this circuit, I selected a small value of C1 to not
load the IF stage & give the wanted squelch sensitivity. A pot could be used
there as well if there is enough signal.
See my Tech bul "Comms Noise Squelch IC"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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