Second Panadaptor on Different Band Causes Spurs

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  • Updated 4 years ago
I like to run JT-65 on multiple bands at the same time...

With V 1.2.1 I started to get spurs showing up every 1875 Hz on the Second (15M) Panadapter and on other bands when I had the 20M Panadapter open

I initially thought that this was a local problem on 15M so I put toroids on everything possible to no avail.  So I tried multiple different computers thinking it might be a sound card issue inside my control computer...

But I soon saw that it was an issue with SmartSDR 1.2.1

Here is a 15M Screen Showing NO Spurs



Here is a 2nd Panadapter with 15M and 20M with Spurs on 15M every 1875 Hz

The Spurs also show up on 20M (2nd Panadapter)




The spurs show up in the second panadapter
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KY6LA - Howard, Elmer

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Posted 4 years ago

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Tim - W4TME, Customer Experience Manager

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You have ADC overload.  In the first image, the WIDE indicator is off, indicating that you have the band specific preselectors enabled.  When you opened the 2nd panadapter on the same antenna (ANT-1) which is the same SCU, the preselector was turned off.  ADC or SCU overload can happens when the sum total RF energy that is presented to the SCU is greater than the overload point (just above +0 dBm).

Since you have a 6700 with two SCUs, if you set the 2nd panadapter to a different antenna (ANT2 or RX-B IN) which will force the radio to use the 2nd SCU and the preselectors for each band are enabled (no WIDE indicator on either panadapter), I suspect that the display will clean up.
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KY6LA - Howard, Elmer

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Yes - that does clean it up...

The issue unfortunately is that I only have one Antenna (SteppIR MonstIR) which is up high (85') in the air.   I usually use it to monitor 2 -5 bands at the same time... when I hear something I want to work, I click on TX on the band in question, the antenna quickly tunes to that band and I work them.

I do have a multi-band dipole at 30' but by comparison to the MonstIR even tuned to a completely different band that the one I am listening to, the dipole barely ever hears anything...

Any suggestions on how I can use only one antenna input?
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Tim - W4TME, Customer Experience Manager

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Howard - Then what you probably want to do is see if you can identify what part of the band the strong signal component is originating from.  I suspect it is the BCB portion of the sampled spectrum.  If this is the case, then a good RX only BCB filter placed in the RX-A Loop should attenuate enough of those signal to prevent the SCU overload.  I have used the Morgan (previously ICE) BCB filters successfully in the past.  http://www.morganmfg.us/radio-products/bcb-interference-filters/
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KY6LA - Howard, Elmer

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My tower which is 600'ASL is 2,900' as the crow flies andL from 23 FM Stations and 18TV station antennas +US Navy Secret Stuff broadcasting at least 10MegaW  at about 900' -1,000' AS- so there is a lot of RF around here..  I will buy one of the BCB filters to see if that improves things...


(Edited)
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Tim - W4TME, Customer Experience Manager

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That is a lot of RF flying around.
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KY6LA - Howard, Elmer

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So I bought the BCB filter and installed it.. Didn't seem to make much difference.  Still get the  1875Hz spurs when I open panadaptors on  both 15 and 20 at the same time..Likely most of my incident RF occurs from much higher frequencies....

I wish there was a simple way to direct each of the SCU to capture a different band on the same antenna?
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Tim - W4TME, Customer Experience Manager

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Howard, we can't redirect RF from one antenna to two SCUs.

I do have a question, what happens if you change the RF preamp to -10 when you are experiencing the spurs?

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KY6LA - Howard, Elmer

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The noise floor rises from -130dbm to -120dmb but the spurs stay relatively the same about 15 db above the noise floor on the second display which is on 15M when the 20M display is open
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Tim - W4TME, Customer Experience Manager

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OK.  Good info.  Do the spurs seem to be modulated in any way?  Like they are a mixing product?
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KY6LA - Howard, Elmer

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Spurs seem to vary in intensity from 5dB to 20dB over the noise floor.. sort of like someone keying up a transmitter.. never goes totally away but not specific period for the spur intensity changes.
They seem to be evenly spaced about every 1875Hz