[R-390] 2 diode agc ssb mod c/l meter linearity

[Larry H]

Hi all,  Jacques Fortin convinced me (easily) to measure the forward
voltage of some pertinent diodes, so here's the detailed results of the
testing that I did yesterday.

The voltage source is 27 V.  The volt meter is an rca senior voltohmyst
(7.3 M ohms/volt on 1.5 V scale).  The current is approximate.  Across the
top of the chart is listed the dropping resistor used to set the current
flow.
Silicon Junction diodes:
Resistance:  8.2 M    2.2 M    270 K    100 K    85 K
1N540         .28 Vf    .32 Vf    .39 Vf    .42 Vf    .48 Vf
1N543         .30 Vf    .36 Vf    .44 Vf    .48 Vf    .53 Vf
1N1694       .32 Vf    .38 Vf    .46 Vf    .50 Vf    .56 Vf
1N4004       .32 Vf    .36 Vf    .44 Vf    .48 Vf    .53 Vf
1N4148*      .31 Vf    .38 Vf    .47 Vf    .50 Vf    .57 Vf
~ Current:    3.3 uA    12 uA    100 uA  270 uA  318 uA
The bottom line of the chart shows the approximate current flow through the
diode with the resistor used as specified in the first line of the chart.
This range of current was chosen because of the low level of current flow
(about 25 uA) when the rx signal and noise levels are low (minimum AGC
voltage generated).  At higher RF signal and noise levels, the diode is
reverse  biased and not providing the clamping effect.

As you can see, most Silicon Junction diodes have a very similar forward
voltage (Vf) for the same current at low values.  The reason a Silicon
Junction diode was chosen, is for their current and voltage
characteristics.  This circuit requires a 200 volt  400 ma diode. *
IMPORTANT* - the 1n4148 is included in the chart for reference only, but
is  *NOT* suitable for this circuit.  It is a 75 V 300 ma diode.

I'm attaching a revised schematic.  Please discard the previous one.  Sorry
for the confusion.

Regards, Larry

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