Monitoring transmit spectrum with a P3

[k6dgw]

The P3 is tunable over quite a range to accommodate various 1st IF's.
Would it be feasible to disconnect it from the K3, tune it to 40 meters,
transmit AFSK A RTTY at low power into a dummy load, and observe the
transmitter's spectrum?  Might need a short "antenna" in the BNC connector?

In the NAQP RTTY, I observed a station on the other side of the
continent with a 20-25 KHz "noise bump" above [but not below] his
signal.  It was 8-10 dB above the ambient noise baseline.  I watched it
for quite awhile and every time he transmitted, it appeared, and when he
finished it disappeared.  No other station, even very strong ones,
exhibited this effect.

It's hard to conjure up how any form of phase noise would be asymmetric
like this.  It could be some form if IMD in his TX RF chain, I guess.
Another possibility is he was running AFSK USB and the noise was being
generated in the sound card.  This could easily be me as well and I'd
like to check.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 50th Running of the Cal QSO Party 3-4 Oct 2015
- www.cqp.org
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[Alan Bloom]

On 07/19/2015 02:16 PM, Fred Jensen wrote:
> The P3 is tunable over quite a range to accommodate various 1st IF's.
> Would it be feasible to disconnect it from the K3, tune it to 40 meters,
> transmit AFSK A RTTY at low power into a dummy load, and observe the
> transmitter's spectrum?  Might need a short "antenna" in the BNC connector?

Yes.  The P3 will tune up to about 22 MHz (and above that with reduced
sensitivity).  If you go into the "Xcvr Sel" menu item and select the
last item in the list, called "0 Hz". the P3 is tuned directly to the
frequency on the screen.

Tune using the CENTER control.  (Center En has to be enabled in the
menu.)  The tuning rate is proportional to span, so select wide span for
coarse tuning and then narrow span to dial in to the exact desired
center frequency.

One warning:  The P3 IF input is unprotected so it is easily damaged by
static discharge or high power.  I recommend adding a pair of
back-to-back switching diodes in parallel across the IF input.  I did
that using a BNC T connector and BNC plug so I wouldn't have to modify
the P3.

Alan N1AL

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