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Long Wave and AM Broadcast interference

Posted: Wed Dec 12, 2018 3:35 am
by VK3MHZ
Gidday all;

I am pretty keen on long-wave DX, although right now I am struggling with AM Broadcast stations, for want of a better description - 'suprious images', since I don't know the correct technical term.

Example: 3CS (Colac) 83 Km (52 Miles) ESE 102ยบ from my location, S9+30/-42.5dBm on 1134 KHz, is also quite strong on 336 KHz.

This is not the only AM station that I am getting images of on LW, just an example. Is there any way I can reduce, or even completely eliminate AM broadcast station images on the long-wave band (below 500 KHz)? Any help would be greatly appreciated.


System Specs:

Operating System
Windows 7 Home Premium 64-bit SP1
CPU
Intel Core i3 380M @ 2.53GHz
Arrandale 32nm Technology
RAM
4.00GB Single-Channel DDR3 @ 532MHz
Graphics
Intel HD Graphics (Acer Incorporated [ALI])
Audio
High Definition Audio Device

SDRPlay RSP2.
Antenna: HiZ input, 150 meter long wire.

Re: Long Wave and AM Broadcast interference

Posted: Wed Dec 12, 2018 6:41 am
by Tech_Support
Hello VK3MHZ
Have you tried low IF mode?

Please see the following applications note: https://www.sdrplay.com/docs/SDRplay_Op ... _MW_HF.pdf

Whilst this is predominantly aimed at the RSP1A, some of this is relevant to the RSP2 as well.

Sincerely

Tech_Support

Re: Long Wave and AM Broadcast interference

Posted: Thu Dec 13, 2018 5:58 pm
by glovisol
Hi VK3 MHZ,

The incoming level of a MW broadcast station between -50 and -40 dBm is perfectly normal and most common, so we must live with it, but RSP class receivers are perfectly capable to cope with these levels while working in LW below the MW band, by using the LOW IF mode, as advised by Tech_Support. I am uploading three PC screens (taken with several MW signals with levels -40 to -50 dBm operating on the MW frequency band immediately above and not shown) as follows:

a) LW reception at ZERO IF with MW NOTCH FILTER IN - no spurious.
b) LW reception at ZERO IF with MW NOTCH FILTER OUT - very many spurious signals and most of them are higher order, not intermodulation.
c) LW reception at LOW IF with MW NOTCH FILTER OUT - no spurious at all!

So for even stronger signals you still have the notch filter in reserve. This is an RSPduo in operation as I do not have the RSP2.

Further know how on IIP3 and IIP2 here:

https://www.sdrplay.com/community/viewt ... f=5&t=3926

Cheers,

glovisol

Re: Long Wave and AM Broadcast interference

Posted: Thu Dec 13, 2018 6:50 pm
by glovisol
Sorry....I forgot to ask: how are you coupling the long wire antenna to the HI Z input? Are you using a transformer type balun? This is what you should be using anyway.

Re: Long Wave and AM Broadcast interference

Posted: Fri Dec 14, 2018 12:43 pm
by VK3MHZ
As I am using the HiZ input I am not able to use the MW notch.

The antenna is a 105m long-wire, to the positive terminal, N & G are bridged to ground (Earth stake).

Re: Long Wave and AM Broadcast interference

Posted: Fri Dec 14, 2018 1:45 pm
by glovisol
1) I have just demonstrated yesterday with the PC screens (as also explained in the SDRplay white paper) that you can get rid of the spurious signals by using the LOW IF setting: go to the top left of the SDRuno main, click SETT. and in IF Mode select LOW-IF and put Decimation to 8 in the Decimation window: the interference will disappear. As I wrote, this works even if you DO NOT HAVE / DO NOT USE THE MW notch.

2) Direct coupling a 105 m long wire antenna (I myself am using a 135 m Beverage) to the P terminal, with N&G terminals to ground is extremely dangerous to the receiver (and to yourself, perhaps?): even a small indirect lightning transient could kill your RSP2 at any moment. Not only that, but you are probably bringing in a lot of noise which significantly reduces your system sensitivity. Please look at:

https://www.sdrplay.com/community/viewt ... f=5&t=3352

Where a lot of info is available, especially in the mid-posts of this long thread.

3) I am going to post a lowpass filter design (CAUER C05 15 40) which kills all MW stations and therefore radically eliminates the problem in the LF band once and for all with any kind of receiver, see:

https://www.sdrplay.com/community/viewt ... f=5&t=3943

Re: Long Wave and AM Broadcast interference

Posted: Mon Feb 11, 2019 10:32 am
by fotoralf
glovisol wrote:go to the top left of the SDRuno main, click SETT. and in IF Mode select LOW-IF and put Decimation to 8 in the Decimation window
Given that SDRuno is still not available for Mac OS, how can this be replicated in GQRX or CubicSDR?

Ralf

Re: Long Wave and AM Broadcast interference

Posted: Mon Feb 11, 2019 2:51 pm
by ON5HB
In CubicSDR it's no problem, select the IF-mode before you start the device.

Decimate is not needed to select, makes not that much of a difference.

I myself use Zero-IF where possible combined with RfNotch if needed.
Cubic-sdr.png
Cubic-sdr.png (87.65 KiB) Viewed 3586 times

Re: Long Wave and AM Broadcast interference

Posted: Wed Feb 13, 2019 1:27 pm
by kiheiman
It is pretty easy to build out a band stop filter using the Mini-Kits bare filter circuit board. I built out one for 500 Khz to 1600 Khz for use with my Elad Duo and SDRPlay RSP1A. Please see attached sweep of the filter. A number of AM stations at my location have signal levels of -30 dbm. For specific frequencies, you could use the same board to build out a notch filter. Another option, using the same board, is to build out a low pass filter with a cutoff of 500 Khz.

Regards
Clint
W7KEC