Minimum gain but still have images

Post information or questions regarding SDRplay products here
wildham
Posts: 2
Joined: Sat Apr 09, 2016 5:49 pm

Minimum gain but still have images

Postby wildham » Sat Apr 09, 2016 6:00 pm

I have been using the sdrplay with SDRConsole V2.3. Native support is built in for SDRPlay with this software. On 75 meters I continue to have AM station images with the gain set to minimum. This is using the 250 KHz sample rate. I don't see this problem when I use HDSDR with the SDRPlay. Does anyone have suggestions on eliminating the images. Perhaps, I need to try a different IF Frequency. IN any event having lots of fun learning the ins and outs of the software and hardware. Tom K5RY
Last edited by wildham on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

WhoaNellie
Posts: 18
Joined: Thu Oct 22, 2015 12:37 pm

Re: Minimum gain but still have images

Postby WhoaNellie » Tue Apr 12, 2016 2:47 pm

By "gain set to minimum" what do you mean? Do you have a number for that?
Last edited by WhoaNellie on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

wildham
Posts: 2
Joined: Sat Apr 09, 2016 5:49 pm

Re: Minimum gain but still have images

Postby wildham » Wed Apr 13, 2016 3:42 am

Thanks for reading my question. Hope the following description answers the question. Using the values available in SDR-Console, IF gain is set to -80 and I can select either LNA on or OFF, which I am given to understand controls a 24 db gain step. So with SDR-Console, I can get a minimum gain of -104db. With HDSDR, I usually, run the overall gain setting at -90 to -93 db for best performance and have no images with the SDRPlay. The images only happen when using SDR-Console, so I don't suspect the hardware is at fault, rather the way the software is controlling the HW gain distribution.

One thing I did confirm is that the AM Station image moves within the IF bandwidth as the center frequency of the IF bandwidth is shifted. The interesting thing is that the AM Station image moves by 30 KHz when the IF center Frequency moves 10 KHz. Been trying to digest the MIRICS chip documents and the SDRPlay schematics to track down what the mixer products would be for the frequencies involved. Might have to break out the RF Sig gen and do some testing. I might play with some antenna input filtering also. I am using a 200 foot dipole center fed with open wire line. Gathers, lots of RF energy,

Tom, K5RY
Last edited by wildham on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

DaveB
Posts: 130
Joined: Sun Feb 01, 2015 6:04 pm

Re: Minimum gain but still have images

Postby DaveB » Wed Apr 13, 2016 10:26 pm

By "AM station" you mean a medium wave broadcast station I presume. Your description suggests an aliasing problem which is why it is moving at x3 the frequency shift. I had the reverse problem - Radio China International on the 75 m band appearing at the bottom end of the AM band and moving at a multiple of the frequency shift. I would say it is an inherent problem with Software Defined Radios - the ARRL handbook ought to have a chapter or more devoted to SDR radios and may contain a useful explanation. The answer in your case is a steep high-pass filter to block out AM stations - in my case it would be a low pass filter to block out SW stations.

Why it would affect SDR-Console and not HDSDR I don't know - but there is a preview of v3 of SDR-Console and by the weekend there should be a new release. If you try it bear in mind it is a pre-beta "taster". It would be interesting to see how that performs. v3 pre-release I've tried certainly has a better implementation of the latest Mirics API gain distribution. With the aerial you have I'd certainly run with the LNA off.

David
Last edited by DaveB on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

User avatar
Harry
Posts: 35
Joined: Mon Nov 02, 2015 9:47 pm

Re: Minimum gain but still have images

Postby Harry » Sat Apr 16, 2016 12:43 pm

DaveB wrote:By "AM station" you mean a medium wave broadcast station I presume. Your description suggests an aliasing problem which is why it is moving at x3 the frequency shift. I had the reverse problem - Radio China International on the 75 m band appearing at the bottom end of the AM band and moving at a multiple of the frequency shift. I would say it is an inherent problem with Software Defined Radios - the ARRL handbook ought to have a chapter or more devoted to SDR radios and may contain a useful explanation. The answer in your case is a steep high-pass filter to block out AM stations - in my case it would be a low pass filter to block out SW stations.

Why it would affect SDR-Console and not HDSDR I don't know - but there is a preview of v3 of SDR-Console and by the weekend there should be a new release. If you try it bear in mind it is a pre-beta "taster". It would be interesting to see how that performs. v3 pre-release I've tried certainly has a better implementation of the latest Mirics API gain distribution. With the aerial you have I'd certainly run with the LNA off.

David


I too notice significant differences between HDSDR and SDR-Radio Console (both v2.3 and v3 preview) regarding overload and images as well as the DC offset spike. HDSDR employs the SDRplay's native ExIO while Simon implemented his own for SDR-Radio Console. As of now I find HDSDR handles images and the DC offset far better than SDR-Radio while Simon's software offers more overall software flexibility.

BTW and FWIW, I find that using Low IF with HDSDR greatly improves managing those images.

Harry
Last edited by Harry on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

DaveB
Posts: 130
Joined: Sun Feb 01, 2015 6:04 pm

Re: Minimum gain but still have images

Postby DaveB » Sun Apr 17, 2016 4:25 pm

BTW and FWIW, I find that using Low IF with HDSDR greatly improves managing those images.


Hi Harry,

The problem with using Low IF is that you then have to contend with IF images spaced at twice the sample rate i.e. 4.096 MHz below a strong station - and these are only around 45 dB down.

Airspy will also have Low IF images - but how good the suppression is - is the question that has yet to be answered for me. I'd love someone who owns it - and does FM broadcast DX - to post some objective information eg screen shots if there are significant images.

What I have found is that HDSDR is better at nulling out the zero IF images than the in-built correction in the API. I can manage to get the image of a 70 dB station into the noise floor - with the API it is 5-10dB above. You have to turn off the auto correction in the API for best results.

David
Last edited by DaveB on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

User avatar
Harry
Posts: 35
Joined: Mon Nov 02, 2015 9:47 pm

Re: Minimum gain but still have images

Postby Harry » Sun Apr 17, 2016 9:51 pm

David,

Thanks for the tip. Will give it a try.

Harry
Last edited by Harry on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

jackb
Posts: 2
Joined: Thu Apr 28, 2016 3:34 am

Re: Minimum gain but still have images

Postby jackb » Thu Apr 28, 2016 5:46 am

This should solve your image problem: http://www.dxing.com/tnotes/tnote06.pdf
I noticed the same thing with my RSP and another radio I have.
Last edited by jackb on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason


Return to “SDRplay related”

Who is online

Users browsing this forum: Google [Bot] and 4 guests