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AM Broadcast Stations on 22 MHz

Posted: Tue Jul 16, 2019 9:26 am
by VK3MHZ
I have been away from SDRPlay for a little while now, and recently installed the latest version of SDRUno. As far as ease of use is concerned, not a lot has changed there, it still is quite a beast to drive. That said, I do really like the program and the RSP.

Listening around tonight, I noticed that if I have the sampling rate higher than 4 MHz, I was receiving local AM broadcast stations around 22657 KHz. I find this rather frustrating, as you can sometimes spend a long time waiting for a station ID only to learn what you are listening to is an image of a AM Broadcast station. Is there anyway, other than using a low sample rate to avoid this?

Also noticed trying to edit a post, after you edit and then click submit it keeps telling you to enter two poll options! Only way around this is to delete the original post and start again.

Re: AM Broadcast Stations on 22 MHz

Posted: Tue Jul 16, 2019 9:42 am
by Mike2459
I have 22MHZ loaded with images at night from the AM BCB. I'm using the hi_z port. I was going to say use mw/fm notch filter. I just tried that after switching to the A port on the RSP2. That doesn't help a bit. Wondering why. That gonna be just great when good propagation returns.

Re: AM Broadcast Stations on 22 MHz

Posted: Tue Jul 16, 2019 11:04 am
by glovisol
Hi VK3MHZ,

Yes, use a high pass filter, as shown here:

HIGH PASS FILTER FOR OPTIMUM HF RECEPTION
https://www.sdrplay.com/community/viewt ... f=5&t=3973

If you go to the last posts of thread above, you will see how the filter eliminates all spurious & images.

Please understand there are limitations even to an extremely wideband, high class, Spectrum Processor such as the SDRplay RSP2. These limitations take effect when you experience high amplitude signals even remoted from your receive frequency, like the MW broadcasters. The High Pass filter referenced above will completely eliminate offending interference from MW.

If you wish to better understand what happens, look here:
SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
https://www.sdrplay.com/community/viewt ... f=5&t=3926
and the SDRplay application note therein referenced

I forgot to mention a very, very important point. If you use an antenna amplifier or an amplified antenna ahead of the RSP receiver, it is very likely that the spurs you experience could be generated inside these amplifiers: then it is evident no filter as proposed above will have any positive effect: you will simply have to modify your antenna setup.

There is a problem with the first post of every thread in the Forum: this has been mentioned countless times, but evidently cannot be removed. The remedy is to just write a sentence sending to the next post and write your text in the next post which you can then correct at will.

Re: AM Broadcast Stations on 22 MHz

Posted: Wed Jul 17, 2019 12:30 am
by Mike2459
If you set your SDR to 24000000 (24 mhz) you will be tuned to the SDRplay's local oscillator (within a few hertz). Center this frequency in the screen, enter ZIF mode & select a sample rate of 4.0 mhz. You will see any image products generated from 0 to 2 mhz starting at 24mhz up to 26mhz with a mirror image starting at 24mhz and going down in frequency. You must be connected to an antenna that receives the 0 to 2mhz range fairly well.

As glovisol said, a high pass filter will block these signals up to the filters pass frequency.
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Re: AM Broadcast Stations on 22 MHz

Posted: Wed Jul 17, 2019 8:50 am
by Tech_Support
I would be surprised if you see this on the RSP1A or RSPduo. Overload/IMD performance below 30 MHz for these devices was improved considerably when compared to the RSP2. People often talk about the performance of "the SDRplay", but all devices do behave differently and the design has been improved considerably since the introduction of the original RSP1. the RSP2 delivers the best HF sensitivity for people with very low noise antennas and in weak MW AM signal areas, but it is more prone to second order IMD issues from the MW AM band, particularly on port A. The second order IMD occurs in the input antenna switching network which is why the built in notch filter may not always resolve the problem. Port B should be better and the Hi-Z port better still.

You should not see this issue with newer models, where the design was improved to address this issue and as observed by others, an external HPF will resolve this issue on the RSP2

Sincerely

Tech_Support

Re: AM Broadcast Stations on 22 MHz

Posted: Thu Jul 18, 2019 1:35 am
by VK3MHZ
Tech_Support wrote:You should not see this issue with newer models, where the design was improved to address this issue and as observed by others, an external HPF will resolve this issue on the RSP2
I have an RSP2, which I brought about a year or two ago, can't recall precisely right now. Re MW on 22MHz, it was purely operator error, I had the receiver in Zero IF mode, once I click Low IF all was good.

Since you are tech support, you may be able to answer a question that has eluded me ever since I got it:

I have read posts stating there should be no difference in reception between using LIF and ZIF - this (at least in my case) is simply not true. Listening around 9 MHz to a station, in LIF mode, reception was fine, switching to LIF the signal completely disappeared. Correct me if I am wrong, but I believe what I was hearing in ZIF mode was actually an AM broadcast image.

// I just tried the above on 11 MHz just now, again in ZIF a rather strong signal, which I believe may have been ABC National here (AM BC), once I switched to LIF the signal was no longer present //

Yet on all the framed bands (presets) everything is ZIF mode.
For normal tuning through bands, should I be using LIF or ZIF, or when is one to be used over the other?

Re: AM Broadcast Stations on 22 MHz

Posted: Thu Jul 18, 2019 1:50 am
by VK3MHZ
Mike2459 wrote:I have 22MHZ loaded with images at night from the AM BCB. I'm using the hi_z port. I was going to say use mw/fm notch filter. I just tried that after switching to the A port on the RSP2. That doesn't help a bit. Wondering why. That gonna be just great when good propagation returns.
Mike, this was during the day - at night it only becomes worse.
The nearest AM TX to me is only 7Km line of sight. As it is a Vision Australia site, it is very easy to identify during daytime by it's content.
I to am using the HiZ port, as I am just using a long wire, so the long wire to the active terminal and also have a earth stake connected to the earth terminal.

Re: AM Broadcast Stations on 22 MHz

Posted: Thu Jul 18, 2019 1:51 am
by VK3MHZ
glovisol wrote:Hi VK3MHZ,

Yes, use a high pass filter, as shown here:

I forgot to mention a very, very important point. If you use an antenna amplifier or an amplified antenna ahead of the RSP receiver, it is very likely that the spurs you experience could be generated inside these amplifiers: then it is evident no filter as proposed above will have any positive effect: you will simply have to modify your antenna setup.
Thanks I will look into the filter.
As for an amplifier, no I am not using any amps or the likes. Just a long-wire around 105m long, hope to extend that soon.

Re: AM Broadcast Stations on 22 MHz

Posted: Thu Jul 18, 2019 1:55 am
by VK3MHZ
VK3MHZ wrote:
Tech_Support wrote:You should not see this issue with newer models, where the design was improved to address this issue and as observed by others, an external HPF will resolve this issue on the RSP2
I have an RSP2, which I brought about a year or two ago, can't recall precisely right now. Re MW on 22MHz, it was purely operator error, I had the receiver in Zero IF mode, once I click Low IF all was good.

Since you are tech support, you may be able to answer a question that has eluded me ever since I got it:

I have read posts stating there should be no difference in reception between using LIF and ZIF - this (at least in my case) is simply not true. Listening around 9 MHz to a station, in ZIF mode, reception was fine, switching to LIF the signal completely disappeared. Correct me if I am wrong, but I believe what I was hearing in ZIF mode was actually an AM broadcast image?

I just tried the above on 11 MHz just now, again in ZIF a rather strong signal, which I believe may have been ABC National here (AM BC), once I switched to LIF the signal was no longer present

Yet on all the framed bands (presets) everything is ZIF mode.
For normal tuning through bands, should I be using LIF or ZIF, or when is one to be used over the other?

Just for the sake of clarity:
My antenna is a 105m long-wire, connected directly to the positive (active) terminal on the HiZ block, and I have a lead going to an earth stake connected to the earth (ground) terminal on the block. Originally I did have a jumper between the ground and negative terminals but other members here suggested not to do so. No LNA's, no baluns or filters currently.


My main areas of listening interest are aviation (HF LDOC), MW and LW (LW I haven't had a lot of success with).

Re: AM Broadcast Stations on 22 MHz

Posted: Thu Jul 18, 2019 2:01 am
by VK3MHZ
glovisol wrote:Hi VK3MHZ,

Yes, use a high pass filter, as shown here:
Silly question maybe, is there anywhere I can purchase a hp filter, or at worst someone who constructs them?