panadapter "noise" without antenna connected

[jsdroyster]

Greetings from KT4JR and K4MWE to you smart folks...

Our panadapter regularly (but not always) shows wide green bands of "noise" before the antenna is connected.  When present, the bands are visible in the same panadapter screen location regardless of wbich band the radio is turned to, but the locations vary day to day (sometimes at the low end of the screen, other times in the middle).  The white line on the screen rises to peaks above these green areas.

How would we figure out what the source of this is?  It is present when no other appliances are being used in our house, so it is not our own AC, dishwasher, etc.

THANKS for suggestions!
73,
KT4JR
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[wayne burdick]

On Oct 14, 2014, at 6:35 AM, [hidden email] wrote:

> Our panadapter regularly (but not always) shows wide green bands of "noise" before the antenna is connected.  When present, the bands are visible in the same panadapter screen location regardless of wbich band the radio is turned to, but the locations vary day to day (sometimes at the low end of the screen, other times in the middle).  The white line on the screen rises to peaks above these green areas.

Are you using a PX3 or P3? Assuming it's a PX3, try this:

- make sure RX I/Q is set to ON (KX3 menu)
- make sure the PX3->KX3 cables are fully seated
- connect an antenna to the KX3
- switch to 20 meters
- turn the preamp ON and set the PREAMP menu entry on the KX3 to 20 dB
- set the PX3's SPAN to 50 kHz or lower
- adjust the REF level so the "grass" (noise floor) is close to the bottom of the spectral plot

With the above setup, there will typically be no visible fixed spectral lines on the PX3.

As described in the PX3 FAQ on our web site, very wide SPAN settings and low front-end gain (e.g., turning off the preamp) will reduce the signal-to-noise ratio of an RX I/Q-based pandapter, including making wideband spurs more apparent. The same limitations apply when using the KX3 (or other quadrature downconversion radio) with a PC sound card. Typically this doesn't impact usability, e.g. locating signals of interest. In fact most sound cards are limited to displaying about 48 kHz of spectrum, while the PX3 can go wider thanks to its high-performance/high-speed A-to-D converter.

The P3 uses a completely different method of acquiring the signal for spectral display: It digitally down converts the K3's 8.2-MHz I.F. This provides a S/N and spur advantage over an RX I/Q approach, but requires much more complex circuitry -- hence the difference in cost between the two units.

Wayne
N6KR



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[Guy Olinger K2AV]

Before the antenna is connected, the input is unterminated. When that is
true the internal noise is anywhere from 20 to 40 dB higher.  If you want
to view the true level of the internal noise, connect the input of the P3
to a 50 ohm termination.

Seeing internal noise on an unterminated high impedance input is not an
equipment malfunction, it's usual.

73, Guy K2AV

On Tue, Oct 14, 2014 at 9:35 AM, <[hidden email]> wrote:

> Greetings from KT4JR and K4MWE to you smart folks...
>
> Our panadapter regularly (but not always) shows wide green bands of
> "noise" before the antenna is connected.  When present, the bands are
> visible in the same panadapter screen location regardless of wbich band the
> radio is turned to, but the locations vary day to day (sometimes at the low
> end of the screen, other times in the middle).  The white line on the
> screen rises to peaks above these green areas.
>
> How would we figure out what the source of this is?  It is present when no
> other appliances are being used in our house, so it is not our own AC,
> dishwasher, etc.
>
> THANKS for suggestions!
> 73,
> KT4JR
> ______________________________________________________________
> 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
> Message delivered to [hidden email]
>

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[jsdroyster]

Thanks to Guy and top Wayne for their comments.

I was not clear in my question:  The unit is a P3, not a PX3.

The vertical columns of noise are present with or without the antenna connected.

As they change from one time to another, I do not think they are internal noise.
I see in the issue of QST that just arrived there is a RF interference article which I am now reading...

Thanks for these and any more helpful comments!
Julie KT4JR


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