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G8MNY  > TECH     11.07.18 09:03l 66 Lines 3357 Bytes #999 (0) @ WW
BID : 28638_GB7CIP
Subj: Comms Noise Squelch IC
Sent: 180711/0730Z @:GB7CIP.#32.GBR.EURO #:28638 [Caterham Surrey GBR]

By G8MNY                           (Correction Nov 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is an old circuit that was published in Sept 1982 Electronic's World
By LJ Forrest.

I have used a HiFi version on the HiFi sound channel on 23cms ATV repeater
GB3HV in an attempt to remove radar zips, as well as providing a squelch.

It uses an KB4432, KB4436 or TDA1001(A) IC, that has 5 NPN emitter follower
buffers, an FET switch, a Schmitt Trigger, a monostable & a regulator. So it is
possible to make up the circuit with discreet components if you can't source
this old IC.

+ve 컴100컴쩡컴컫컴컴컫컴컴컴컴컴컴컴컴컫컴컴컴컴컴컫컴컴컫컴컴컴컴컴컴
             +===   150k     47n      ===220n     1K8                
            47u쳐컴컴)컴컴컴컴늘커     쳐컴컴5k6컴캑               /e
     旼컴컴)컴컴)컴컴)컴컴커          쳐컴컴컴컴컴)컴컴)컴컴6K8컴캑T1
           旼캑     쳐캑쳐                                    \
                  1n  4k7   3k3         旼컴컴)컴컴)컴컴쩡커    
            16쳐68k캑15   14   13   12   11   10   9    330   
            旼탠컴컴탠컴컴탠컴컴탠컴컴탠컴컴탠컴컴탠컴컴탠 6k8 2n2 
            -ve          旼컴좔컴컴좔커 憫컴컴컨  reg    ===   
                   읕>컴좔 shcmit agc >  mono         좔컨   
         33k                읕컴컴컴컴컴켸 읕컴쩡컴               6k8
Rx           旼>컴     旼>컴쩡o/'o쩡캑>커     旼캑>커           
Input       읕탠컴컴탠컴컴탠컴컴탠컴컴탠컴컴탠컴컴탠컴컴탠           Output
>캑쳐)3k3컨컴컴1    2    3    4    5    6    7     8              +
 1u +                6n8===   1k8         읕6k8컫컴컴컴컴>쳐컴컴컴)컴쨈쳐>
     쳐컴컴컴컴)컴늘캑     읕12k캑    1k8           \                   1uF
    47k           1n 읕12k컴컴컴                T2쳐쩡컴쩡컴쩡6k8캑 6k8
    Pre        ===                     ===6n8       e/ un 6k8+ 10u  +  
 Sens       n22                                    mute   === 10u=== 

The input signal is fed via a RF filter to biased up the emitter follower amp
on Pin 1. Pin 2 output feeds two paths 1/ the audio path, where it goes to a
low pass Butterworth filter Pin 3 & 4 amp, & 2/ the noise path, where it feeds
an adjustable high pass Butterworth filter on Pin 15 & 16 amp. The noise amp
feeds the Schmitt trigger with a level following AGC. With no noise the
monostable is not triggered. The un-triggered monostable operates the FET hold
level switch that lets the audio pass to the storage CR & to the last buffer
amp Pin 5 & 6.

With impulsive noise present, the high frequency content will operate the
Schmitt & triggers the short time mono, which turns off the FET path for a few
mS. During this time, the mean level stored on CR is used to hold the AF level
during the break into the last buffer.

With constant noise the Schmitt trigger output also drives PNP T1 that after a
fraction of a second charges up the squelch time constants to operate NPN T2
that mutes the output.

See my Tech bul "Simple Carrier Squelch"

Why don't U sent an interesting Bul?

73 De John, G8MNY @ GB7CIP

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