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World of Ham Radio 1997
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WOHR97_AmSoft_(1997-02-01).iso
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kam_5.doc
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1997-02-01
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32 lines
KAM.5
The AM/FM push-button switch on a KAM panel (S2) changes not only
compression rate of the input amplifier system (U1-14, Q5 and Q6), but
signal input sensitivity. Reduction of the first is desired for a CW
reception to avoid "swamping" of the circuitry (over-compressing); however,
the second effect could create a problem in interfacing with a transceiver.
I use ICOM-751 transceiver and feed the KAM from high impedance output (10
kohm) before AF gain control and power amplifier (pin 4 of 24-pin plug).
Since the impedance of KAM audio input is only 600 ohms, the signal is much
attenuated. To develop the required 20 mVpp on the input pin for non-CW
reception (the switch in FM position), the transceiver has to output 0.35
Vpp (20*10600/600). But, if the switch is in AM position, the de-sensitized
input (about 100 mVpp) requires at lest 1.8 Vpp. This value, which is close
to distortion limit of the transceiver, I could obtain with strong signals
and with disconnected AGC (in OFF) only.
I have slightly modified the KAM circuitry by disconnecting the AM/FM switch
from ground and connecting this point to the R1-R13 junction. The switch
changes rate of compression only. The sensitivity of AF input remains
constant at 20 mVpp.
Now, I could:
- copy very weak CW signals which hardly move the S-meter,
- use large dynamic range of the transceiver automatic gain control and
- enjoy easiness and speed of tuning to a CW station.
CONTINUED IN KAM.6