[Elecraft] [K2 4922] IF Amplifier Alignment

[David Woolley]

Don Wilhelm wrote:
> 
> Ron Hahn (EI2JP) wrote:
 >> My problem is a constant S3 reading on the S-meter.
 >>

I'm glad I'm not the only one to notice this!

> 
> Attempting to use the S-meter at this stage is not reliable.

The manual does, however, imply one should attempt this, by suggesting 
one gets a good S meter reading before starting.
> 
> You can (and should) re-adjust the S-meter menu settings CAL S Lo and 
> CAL S Hi after you complete all other assembly and test items.

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.



-- 
David Woolley
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