Behavior of AM broadcast images in longwave spectrum?

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franko
Posts: 48
Joined: Mon Aug 28, 2017 9:33 pm

Behavior of AM broadcast images in longwave spectrum?

Postby franko » Sun Feb 10, 2019 8:03 pm

I'm scratching my head a little over the behavior that I'm seeing with AM broadcast images in the longwave spectrum, and would be interested in any help or ideas.

I have an RSPduo attached to a Wellbrook ALA1530LNP mag loop antenna, controlled by SDRUno 1.24 on a Windows 7 PC. Normally the longwave spectrum is totally overwhelmed with noise and entirely unintelligible, due to numerous strong MW AM signals in our urban Los Angeles area.

I just received a low-pass filter I bought from Dale Parfitt at Par Electronics, which does a really nice job of knocking back the noise. Thanks to it, I was able to decode a number of WSPR signals on the 630m ham band last night.

Today I was tuning around to see what if anything I could receive during daylight hours in the longwave spectrum. Around 200-300 kHz I noticed images from a couple of the stronger MW AM broadcasters located nearby. This didn't surprise me, as the filter attenuates but does not entirely remove medium-wave RF; the stations I'm seeing typically put out 50,000 watts and are just a very few miles away.

My question: I noticed as I tuned that each broadcast image changed frequency as I moved the tuning cursor and the LO changed. For example, when LO was 427.2 kHz, a broadcaster transmitting at 1430 kHz appeared in SDRUno at ~278 kHz. If the LO was increased by only 11 kHz to 438.2 kHz, the frequency of the broadcaster image increased by ~45 kHz to ~323 kHz.

I'll confess that I haven't yet paid much attention to the inner workings of the SDRplay RSPs. I assume this has to do with an intermediate frequency or some such issue. Could anyone explain what causes the frequency behavior I'm seeing?

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Tech_Support
Posts: 452
Joined: Mon Jun 01, 2015 7:00 pm

Re: Behavior of AM broadcast images in longwave spectrum?

Postby Tech_Support » Sun Feb 10, 2019 8:38 pm

Hello Franko
Try selecting Low IF mode. These are probably higher order mixing products as explained in this document:

https://www.sdrplay.com/docs/SDRplay_Op ... _MW_HF.pdf

Also, have you tried using the MW notch filter that is built into the RSPduo?

Sincerely

Tech_Support

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franko
Posts: 48
Joined: Mon Aug 28, 2017 9:33 pm

Re: Behavior of AM broadcast images in longwave spectrum?

Postby franko » Sun Feb 10, 2019 9:48 pm

Thanks very much for the link to the paper. Since the broadcast images do move when the LO changes, that makes it clear that they are higher-order mixing products and not the result of intermodulation.

In initial daytime experiments, I've found that by using both the RSPduo's MW notch filter and the external low-pass filter I bought at the same time, the MW signals are essentially zeroed out. I'll have to wait until tonight to see how this works out relative to the longwave signals I'm trying to receive.

I tried switching the IF mode from Zero to Low, but did not see an obvious immediate effect. I'll have to experiment with this further.

Thanks again,

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glovisol
Posts: 449
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Behavior of AM broadcast images in longwave spectrum?

Postby glovisol » Mon Feb 11, 2019 7:49 am

Hi Franko,

You can find a clear example of the advantage of the LOW IF operation here:

viewtopic.php?f=11&t=3936

with a problem very similar to yours.

Cheers,

glovisol

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