[Elecraft] P3 amplitude calibration -4dB

[Mike Scott]

I connected my SG2 on 40M to K3 antenna 1 and ran an amplitude measurement
test on the P3 and find a -4dB difference in expectations.

 

I tested P3 level readings with K3 antenna tuner in bypass and P3 IF OUT
switch set to off, sub receiver off (this is to make sure I am not sharing
the antenna).

Configuration of K3 per S-meter calibration set up: Preamp on, ATT off, AGC
slow, CW mode, bandwidth normalized using 500 Hz filter. K3 has had the IF
modification for the P3 by swapping out a surface mount resistor.

 

For 50 uV on SG2 I read -111 dBm on the P3, expected is -107 dBm

For 1 uV  on SG2 I read -77 dBm on the P3, expected is -73 dBm

 

So I see a -4 dB gain offset at both SG2 settings. The gain offset was
larger when I had the P3 IF OUT switch in the wrong position J

 

Would anyone like to speculate where the gain offset is coming from? This is
not earthshattering, just curious.

 

1.       SG2 needs a new battery or just isn't accurate enough for this
test. (I have a 100 MHz oscilloscope but I don't know about its ability to
measure 50 uV)

2.       K3 amplitude gain is different than the P3 expects, I did an
S-Meter calibration but this shouldn't make a difference.

3.       P3 calibration is off

4.       I didn't do the test right

 

Preamp in or out, ATT in or out didn't change the readings. I have 500 Hz
filter gain set to 4dB but changing this does not affect P3 readings as the
P3 tap must precede the filter.

 

AE6WA

Mike Scott

Tarzana, CA