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Broadcast AM station carrier disappears in certain bandwidths

Posted: Thu Apr 18, 2019 4:03 pm
by W1VLF
Hello Folks,

W1VLF here.
This is my first time posting to this group.

I have a SDR RSPpro2 that I am using on an older windows 7 desktop. Using Console version 3

I was testing this receiver for a friend. A peculiar thing happened while I was testing.
While listening to the AM broadcast band here in the states, I notice that one station appeared to be transmitting in DSB no carrier.
Being unaware any testing of that type was going on, I thought this was very curious. All the adjacent signals looked normal on both the waterfall and the frequency display. Except this one station. There was no carrier.

The station is at 1410 KHz It could be received in DSB LSB and USB but not using the AM detector. With all the adjacent stations looking and sounding ok with an AM detector , I couldn't understand what was going on here. I tried attenuation, band pass filtering thinking there was some kind of overload situation to no avail.

I happened to change the bandwidth for some reasons and suddenly the station's carrier returned. Well I thought the 1410 Khz station fixed what ever problem they had going on. But no anytime the bandwidth is set to 3 MHz or higher the carrier for the 1410 KHz station disappears.

The bandwidth change does not appear to affect any other adjacent frequencies. Is will quite reliably affect the 1410 KHz.

Is there some kind of software sampling issue going on here. 1410 KHZ divides into something sampling evenly.
Also I tried SDR uno and the same thing happens to the 1410 accept I cannot change the sample rate in that program low enough to restore the carrier in the station.
Any ideas??


Update !!!!Re: Broadcast AM station carrier disappears in certain bandwidths

Posted: Thu Apr 18, 2019 6:42 pm
by W1VLF

Some of the information I posted here just previously was incorrect. I said that only 1410 KHz was affected by this particular phenomenon.
This untrue. It only seemed that way because had 110 Khz in the center of the screen. When looking at other frequencies I was viewing them off center of the window. How ever when any AM station is forced to center screen, using center screen icon in the upper right corner, it too will loose its carrier. The original victim returns to normal when not forced to the center of the screen.

Again this affect only the signal in the center of the screen

Re: Broadcast AM station carrier disappears in certain bandwidths

Posted: Thu Apr 18, 2019 8:22 pm
by F1BJB
Hi and welcome
This behavior is normal with a zero-if receiver.
You cannot receive correctly AM or FM when perfectly centered.
This can be useful if for example you have an interfering signal on 1411Khz just use 1411 as center frequency and you have a free notch filter. :)
Also if there is a very strong signal somewhere when you center on it you improve the dynamic range for the rest of the band.