Shortwave broadcast aliasing into 40m band

  • 2
  • Problem
  • Updated 5 years ago
  • Solved
  • (Edited)

Setup: Flex 6500, SDR ver1.3.0


I have noticed aliasing in the 40m band when there is a high-strength shortwave broadcast near. The issue is dependent upon the spectrum bandwidth I have selected. Curiously the problem occurs when the bandwidth is small. The first picture below is when I am zoomed-out on the band - no aliasing and the received audio is clean. Notice the strong out of band shortwave broadcasts. The second picture shows the aliasing problem when I have decreased the bandwidth by clicking on the +zoom button a couple of times. It becomes hard to discern the desired signals from the aliased shortwave signal and the aliased signal is very noticeable, err annoying, in the baseband audio. 


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SteveM

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Posted 5 years ago

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Tim - W4TME, Customer Experience Manager

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SteveM - I'd like to investigate this situation in more detail with you via the HelpDesk, so I am going to convert this Community post to a HelpDesk ticket and will be in touch.
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SteveM

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Sounds good Tim. I've no experience with the HelpDesk. Do you remote in to my PC somehow? If so, give me some time to figure out the best time to do this. I think we'll need the strong adjacent signals present - not sure what time propagation brings them to me, probably in the evening.

Also, I've searched the forum for similar issues. I came up with this:

https://community.flexradio.com/flexradio/topics/interference_on_20_meters_non_wide

Seems he had the exact same issue as me (top picture shows WIDE, bottom picture doesn't). He called it interference, but I know for a fact it is aliasing. The birdies' amplitudes followed the broadcast signals and I can hear broadcast audio along with the in-band audio. Was Bob's issue ever resolved?

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Tim - W4TME, Customer Experience Manager

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You should have received an e-mail from the HelpDesk this morning.  Check your spam folder if you didn't get it.
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Tim - W4TME, Customer Experience Manager

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This topic has been solved in Zendesk
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Jon - KF2E

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Tim,

Forgive my ignorance but how can I read the solution? I went to zendesk and searched "Shortwave broadcast aliasing" but got zero results.

Jon...kf2e
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DrTeeth

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You beat me to it ;-).
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Tim - W4TME, Customer Experience Manager

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Official Response
The images issue took a bit of troubleshooting to characterize and ultimately resolve.

In direct sampling radios with no analog mixers, IM mixing (images) is not technically possible, unless there is some unexpected component failure that would act as a rectifier. In SteveM's case, a faulty lightning arrestor was "re-purposed"  as a mixer, demodulating foreign shortwave broadcast stations. These were real signals and the radio received them like it is designed to.  Disconnecting the faulty lightning arrestor removed the source of the extraneous signals and resolved issue.
(Edited)
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Ken - NM9P

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Thanks for that report, Tim.  It is something to add to my notebook of unexpected interference sources!  I would bet that having no panadapter would have made it even harder to diagnose.  Score another one for SDR's!
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Tim - W4TME, Customer Experience Manager

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"I would bet that having no panadapter would have made it even harder to diagnose." 

Absolutely!
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Bob Wright, N7ZO

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Tim,

Thank you for reporting the resolution here.  It helps the rest of us learn.
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Ned K1NJ

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Yes, indeed! Very cool.  Good to know.

Ned,  K1NJ
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SteveM

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For anyone who is interested, the lightning arrestor that I was using was a Jetstream JTLA1. It seemed like it had pretty decent specs so I bought it just a couple of weeks ago to "protect" my Flex-6500. I suggest you stay away from that one. Tim recommended these to me instead: http://www.arraysolutions.com/Products/surge_arrestorAS-3xx.htm.

I apologized to Tim for having not eliminated as many external components from the system before raising an issue to support. But in my defense, this was a weird issue that disarmed me of any useful troubleshooting skills. The panadapter-zoom can turn-on or turn-off a mixing circuit in the external lightning arrestor?!?

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Andrew Russell

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G 'day all, I recently had a similar problem with broadcast IM after upgrade of the firmware.
It would be fine for a while after transmitting, 1:1.4 SWR. Eventually I found a fractured solder joint in the solder on the centre pin of a connector. That was the rectifying junction. Not so passive inter modulation.
(Edited)
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Tim - W4TME, Customer Experience Manager

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This topic has been solved in Zendesk
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Jon - KF2E

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Again?

Jon...kf2e

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