Folks,
After playing with the AGC threshold vs. S-9 = 50 uV on several K2s, I can
state that the value of 3.80 volts is only a nominal value, and in my
experience should only be considered as an upper limit. I have found the
proper AGC threshold to range from 3.65 volts to 3.8 volts.
There is some slight varaition in receiver gain from K2 to K2. This is
likely due to manufacturing varaiations in the components and could also be
influenced by the loss through the particular IF filter, so the really
correct settings for your K2 may not match another's.
First and most importantly, check with AGC on vs. AGC off to be certain that
there is no AGC activation with receiver noise (no antenna conenected - and
check all bands). If the background noise increases with the AGC off, the
the AGC threshold is too high. That means the AGc is being activated on
receiver noise alone, and does not allow hearing the full sensitivity
capabilities of your K2. The highest voltage point where your reciever
shows no change from AGC on to AGC off is the highest voltage YOUR AGC
threshold should be set.
Now, how does this relate to S-9 indications, you may ask -- well, different
AGC threshold voltage settings will make the S-meter more or less lively
that is for certain, and by 'playing' with the settings of AGC threshold,
S-Lo, and S-hi, one can make the S-meter read almost anything within reason.
If you have a calibrated 50 uV signal (such as the ouptut of the XG-1), you
can set the S-meter on 40 meters for S-9 at 50 uV (I have found that this is
best set with the pre-amp ON).
I find it helpful to make a chart of the AGC threshold voltage, S-low
setting, S-hi setting, and the resultant s-mater reading. This chart will
tell you which way to move the AGC setting to achieve a higher or lower
s-meter reading with any given signal input (50 uV is ideal). I have tried
doing it by 'guess and by gosh', and believe me, creating the chart is the
fastest way to get there - frustration will quickly set in without it - this
is an exercise with 3 variables, and my brain gets confused with only 2
variables.
Bottom line - the K2 S-meter can be made to be 'stingy' or 'lively'
whichever is your choice. There will be some variation from band to band (a
lot depends on the bandpass filters settings - do that part of the alignment
first), so make your choices and set it to please you. For my clients and
my own K2s, I set the S-meter to read S-9 with 50 uV input on 7040 Mhz with
the K2 preamp turned on.
Oh yes, for my personal viewpoint -- the S-meter reading is entirely
relative because it only indicates that one particular signal is stronger
than another signal. The real 'proof of the pudding' is the 'R' in the
RS(T) report which indicates how well you can read the other station - and a
properly aligned K2 can dig them out of the mud, and !!!that!!! is what
really counts.
73,
Don W3FPR
----- Original Message -----
Howard,
With regard to S meter readings it does appear at least with my K2 #4168
and
a later K2 #4324 tested a few weeks back that 80, 40, 30, 20 and 17m
bands
come out with similar sensitivity in the level required for S9 when
switched
to the same mode and bandwidth. For 15, 12 and 10m there was a
discrepancy
with the later serial number being slightly less sensitive. The alignment
appeared to be fairly well correct for both K2.
With the Elecraft XG1 signal generator the testing for S9 is done at
7040kHz
with the higher level output at 50 microvolts PD (50uV PD =
pprox -73dBm).
With the AGC threshold set at 3.80V, 50uV was insufficient to make S9
with
both these K2. They both needed about 63uV PD to make S9 with the preamp
switched off and were slightly more sensitive when checked with CW at
700Hz
bandwidth compared to SSB with the SSB filter in circuit.
However during testing it was found that small variations in the AGC
threshold voltage had a substantial effect on the level required to make
S9. With
AGC threshold at 3.80V, as noted 63uV was required for S9. Drop this by
0.1V to
3.70 V, only 40uV was required and again by another 0.1V to 3.60V, only
25uV
was now needed for S9. With all of the changes in threshold voltages the
CAL
S HI and CAL S LO needs to be reset at each step.
The effect noted was posted on the Elecraft Reflector though only raised
two
replies in confirmation. Again has anybody else seen this effect of
rising S
meter sensitivity when lowering the AGC threshold whilst testing the K2
receiver?
The optimum point for the setting of the AGC threshold with my K2 #4168
came
out at 3.70V, which meant the S meter sensitivity came out at 40uV PD for
S9. This can be fudged to 50uV for S9 by adjusting CAL S HI if desired.
Left
mine as is for ease of S meter HI/LO setting as per the manual.
Assuming the alignment is correct on the K2 reported with low S meter
sensitivity, it may be worth checking that AGC threshold has been set to
at least
the 3.80V initial setting. Well worth checking the K2 Alignment
Instructions
on Don, W3FPR's web site to ensure that you get everything correct.
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