R:190720/0943Z 9604@N7HPX.#BOI.ID.USA.NOAM BPQ6.0.18
R:190720/0934Z 33441@N3HYM.MD.USA.NOAM BPQK6.0.18
R:190720/0942Z @:CX2SA.SAL.URY.SOAM #:10444 [Salto] FBB7.00e $:12947_GB7CIP R:190720/0940Z @:GB7CIP.#32.GBR.EURO #:12947 [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.

┌────────────┐ ┌─────────┐ ┌───────────────┐
│ High Level ├─┬─┤ Limiter ├──┤ Discriminator ├──10k─┬───>To AF
─┤ IF stage │ │ └─────────┘ └───────────────┘ │ amp
└────────────┘ │ │
│ +12V─┐ ┌───┤├───┘
1V p-p │ 27k D2 │/ 10u +
│ ├────┬──┤>├──┤T2 C3
│ 47p │/ │ │\e
└──┤├───┬───┤T1 ===33n │
C1 _│_ │\e │ C2 │
D1/_\ │ │ │
─────┴─────┴────┴─────────┴──

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


--- Mystic BBS v1.12 A43 (Linux/32)
* Origin: HAMRADIO telnet lu9dce.dynu.com (21:5/101)