David,
Jack Smith has characterized the K2 AGC curves (along with some other
rigs) and you may find some useful information on his website:
http://www.cliftonlaboratories.com/receiver_agc_curves.htm
The IF AMP AGC input is really current driven, so trying to obtain a
direct relationship between the voltage and the AGC action is
problematic at best - yes, I know the data sheet expresses the AGC
voltage in the curves, but that is based on the operating conditions
stated in the data sheet.
When you set up the AGC Threshold voltage in the K2 to 3.8 volts, you
are not reading the voltage into the MC1350, but rather reading an
indirect control point voltage - the actual AGC drive to the MC1350 is
two active devices 'down the road' from where the AGC Threshold is set.
What I can say for certain is that I have had to re-visit the AGC
Threshold and S-meter settings after completing assembly of any K2 that
I have built or repaired (I have built many and repaired many more) -
for instance, the filter passband alignment will influence the S-meter
response, so the S-meter should best be set after the final filter
alignment.
True, the manual does not explicitly state that one should re-adjust the
S-meter settings after all the other alignment is done, but it does hint
at it.
One can 'play' with the AGC Threshold voltage setting to achieve better
receive sensitivity with no input signal, but move it very far below 3.8
volts and the S-meter settings and response gets 'squirrelly', so if you
choose to do that, plan on a lot of interactive 'playing' with the AGC
Threshold, CAL S Lo and CAL S Hi - the manual settings are a compromise,
but they do work and work quite well - vary from them as you wish, but
do so at your own peril, the 'standard' settings can be easily restored.
73,
Don W3FPR
David Woolley wrote:
I don't believe the manual says this, although I've only got to the 40m
receive only stage.
I'm actually having great fun trying to work out how the AGC system
really works and really should be calibrated. One complication is that
the MC1350 is only characterised for a 12 volt supply, but the K2 uses
8. If I understand the internals of the chip properly, the AGC range
will be constant but the threshold will reduce by one volt for every two
volts reduction in supply voltage, so the 5 to 7 volts range in the data
sheet will correspond with 3 to 5 volts in the K2 (at the V AGC point,
not at U2 pin 5). Is that right?
Another question is how one gets 88dB+ of S meter range (6dB * (9 - 1) +
40dB) from an AGC system with a maximum range of 68dB, especially when
the initial S meter calibration uses less than 1.3 volts of the
available 2 volt control range?
Another one is that the design seems to assume a gain balance between
the AF and AGC IF chains, but the differential gain is subject to
production spread in the NE612 chips (minimum to typical gain of 3dB and
unspecified typical to maximum gain). It also depends, to a lesser
extent, on the output impedance of the 612 in the AGC chain. The AF
chain will depend on the loss in the second filter; I don't know enough
about crystal filters to estimate this. There are probably some other
factors.
(I think there is a deliberate 6dB difference in that the nominal gain
of the U2a op amp is about 22 (33k explicit over 1.5k output
resistance), and the LM 380 has a nominal gain of 50).
I wonder if the feedback resistor on U2a really could do with being a
variable. Although you can get a similar effect with R1 (because the
main part of the AGC response is exponential), you lose range on the RF
gain control.
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