Hi Paul and all,
I did the IRC mod to my 1000MP last
weekend. I set
the IF gain as follows:
Before changing anything, I used a
sound-card spectrum
analysis program to measure total AF
noise power in
CW mode with 500 Hz filters at both
IF's, on 15m,
using "dead-band" noise (i.e. antenna
connected).
Other settings were:
AGC OFF, DSP demod OFF, FLAT preamp,
pitch=600 Hz
I did the measurement with an antenna
connected to get
enough front-end noise so that the total
AF output noise
would be dominated by the front-end
noise rather than the
wideband IF noise.
Then, without changing the volume
control setting, I
installed the 70 Mhz preamp and
decreased the IF gain
setting until I got the same AF output
noise power
that I had originally. On my unit, this
meant reducing
the gain setting from 15 to 11. I
noticed that the IF gain
control is very non-linear, i.e. the
gain seems to fall
off rapidly for settings below "10".
Your mileage may vary!
Basically, doing the gain adjustment the
way that I did it
should mean that I have reduced the IF
gain by an amount
approximately equal to the gain of the
preamp, i.e. total
RF/IF gain should be the same before and
after the mod.
Correspondingly, the IF-generated noise
which shows
up at high audio frequencies should be
reduced by an amount
approximately equal to the gain of the
preamp. I verified
this with the spectrum analysis program,
which showed that
the power spectral density at high
frequencies (i.e. above
1 kHz) is 8-10 dB lower after the mod.
I didn't time to investigate the AGC
characteristic
before and after the gain change, and I
haven't listened
enough to be able to tell if the AGC
dynamic response has
been degraded by the mod. Your
observation is interesting,
however. I wouldn't have thought that
the relatively broad-
band preamp would add enough delay to
change the AGC dynamic
response significantly - but who
knows...
I should note that I was the originator
of the capacitor mod,
so I already had that installed. Thus,
my "before" measurement
includes the modified AF frequency
response characteristics.
Hope that my experience will be of some
use to others who
choose to perform the mod. So far, I am
very pleased with the
results.
73,
Steve, K9AN
sfranke@uiwpls.ece.uiuc.edu
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