I preparation to install Dave White, VE6DRW's AGC modification, I thought that
I would perform some measurements on my K2's AGC to get a baseline to measure
changes once the mod is made. The results surprised me.
First off my K2 is serial # 5093, has the 160 Meter, SSB, noise blanker, and
KAF2 audio filter modules installed and has no modifications. For
measurements, a DL1 dummy load was connected to antenna connector. The mode
was set to CW, the SSB filter OP1 selected, the attenuator switched on, AF
and RF gain set to maximum, and the frequency set to 28.002 MHz. My 8V line
measures 8.27 volts.
With the AGC threshold voltage set at 3.80 volts, the AF output averaged 11.9
mV with the AGC on and 13.4 mV with the AGC off. A difference of 1.5 mV or
1.0 dB.
Regardless of where I set the AGC threshold voltage via R1, the K2 always has
more gain with the AGC off than with the AGC on. The lower the AGC threshold
voltage the less the difference between AGC off and on.
In accordance with the MC1350 datasheet, to find the maximum gain of the
MC1350, I grounded pin 5 of the MC1350 via R4 on the Control Board. I lifted
the end of R4 on the U2 side and attached a ground.
The gain increased slightly. With the AGC on the AF output averaged 12.9 mV
about 1 mV higher (0.7 dB) than with the normal AGC circuit. What was
interesting was that with the AGC off, the AF output averaged 13.9 mV,
about 1 mV higher (0.6 dB) than with the AGC on. With the AGC off the gain of
the K2 was about 0.3 dB higher.
With the output of U2A pin 1 disconnected at R4, I would have expected no
difference with the AGC switched on or off. My guess is that the AGC mixer U1
(SA612) is loading down the IF OUT lead connected to pin 8 of the MC1350
which is reducing the level of the IF signal sent to the product detector.
My AGC circuit is slightly different than most. To get the AGC to work,
Elecraft had me change C8 from 39 pF to 10 pF. I wonder if that may have
anything to do with the difference in receiver gain between AGC on and off.
Next I connected the 8 volt rail to one end of a 20 Kohm trimmer and the other
end to ground. The wiper was then connected to the lifted side of R4. The
trimmer was adjusted to find the AGC voltages where gain of the MC1350 was
maximum and minimum.
The maximum gain occurred at 3.25 volts or less and the minimum gain at 4.45
volts or higher. This is a range of 1.2 volts for full swing of the AGC
action of the MC1350.
Resistor R4 was reinstalled and R1 set to 3.80 volts. The AGC voltage at U2A
pin 1 was measured throughout the range of the RF gain control. At maximum
gain it measured 3.80 volts and at minimum 4.84 volts. A range of 1.04 volts
and a high enough voltage to ensure that the MC1350 would reach minimum gain.
With R1 set to 3.25 volts, the RF gain would now only reach 3.99 volts at
minimum gain. This is not enough to reach the minimum gain of the MC1350 and
the range of the RF gain control is now only 0.74 volts.
Working backwards, and setting the AGC voltage to 4.45 volts with the RF gain
control at minimum gain, the AGC threshold voltage becomes 3.6 volts. However,
when adjusting R1 with the RF gain at minimum, it becomes clear that 4.45
volts is not enough to minimize the gain of the MC1350. The minimum gain
occurs at about 4.85 volts. This corresponds to a AGC threshold voltage of
3.80 volts. I suspect that U2 is playing a role in the different minimum gain
voltages seen here as compared to the directly applied voltage on the MC1350
via R4.
That then made me question if R1 could be set to MC1350 maximum gain. Looks
like 3.6 volts or so gets there. However at this setting the RF gain control
does not have enough range to minimize the gain of the MC1350.
So it would appear that to get the full range of gain of the MC1350, the range
of the RF gain control must be controlled. Dave's mod allows the minimum and
maximum voltages of the RF gain control to be set as needed. Rather than set
these limits to certain voltages, they should be set for maximum and minimum
gain.
What the effects on the calibration and linearity of the S-meter are yet to be
determined.
Installing Dave's mod will be my next K2 adventure.
Darrell VA7TO K2 #5093
--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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