P3 amplitude calibration -4dB

[Mike Scott-7]

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

 

 

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[Alan Bloom-2]

Hi Mike,

> 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 dB

Other way around, right?

The default P3 calibration is nominal, basically what I measured on my
particular K3 and P3.  It will vary somewhat from radio to radio.  Also,
I'm not sure what the level accuracy is of the SG2 but I don't believe
it is intended to be a precision instrument.  

> I did an S-Meter calibration but this shouldn't make a difference.

Right.  The K3's IF output is before all the signal processing, so
S-meter calibration has no effect on the P3.

On my list of features to add is amplitude calibration, similar to the
S-meter calibration in the K3.  I plan to make it per-band to account
for variations in the K3 RF chain and to separately measure the gain of
the K3's preamp and the attenuator.  This level of accuracy isn't really
needed for normal operation of a radio, but as an ex-Agilent/HP engineer
I'm kind of a perfectionist when it comes to measurement accuracy.  :=)

Alan N1AL


On Sun, 2010-08-29 at 14:02 -0700, Mike Scott wrote:

> 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
>
>  
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

______________________________________________________________
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Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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