General SDR information

General discussions
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General SDR information

Postby Toontje » Thu Sep 03, 2015 11:39 am


New SDRplay user here.

I am trying to resolve an issue where i get WBFM images on my HDSDR waterfall in the 10m amateur band. I tried to fiddle around with the gain control, but i have (almost) no clue what i am doing.

I read a couple of books, but most of them deal with the mathematics of DSP instead of SDR as such. I am looking for SDR for dummies explaining LNAs, Mixers, IF Amps, ADCs, Tuner gain, LO Frequency plans, DC Offset Compentation, etc. All that good stuff that makes SDR so interesting.

Does anyone have suggestions for a good read up on these (practical) aspects of SDR?


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Re: General SDR information

Postby DaveB » Thu Sep 03, 2015 11:12 pm

It would be really helpful if the SDRPlay would post some links for us to be able to peruse. I've got a copy of the RSBG Handbook with a section on SDR radio - but 8 years is a very long time in the development of SDRPlay. Meanwhile you might try to google "beginner guide to SDR"

I get the same sort of problems with SDRPlay with SDR# - I've I have done some testing and found in my particular case that for HF use in the lower bands setting the LNA gain threshold to 24 dB - which basically turns it off - only one really strong FM station remains in the low VHF region.

I can then adjust the overall gain to compensate without getting the stations coming back. I haven't tried it out when the 10m band is open to see what the effect is on the sensitivity.

I can't give an answer as to exactly how we are getting broadcast FM down around 28-36MHz but one possibility is that the 3rd harmonic of an internal 24MHz oscillator is mixing with the strongest transmitters eg 102.8 - 72 = 30.8. Another possibility is that the SDR is acting in an under-sampling mode as the in-built filters are not deep enough to totally block the broadcast FM signals.

A 30 MHz low pass filter with an attenuation of over 60dB by the time you get to the bottom of the FM band can be got from Mini-circuits with either BNC or SMA connectors. The problem with the SMA connector is that you don't want to be screwing/unscrewing it all the time if you are also interested in HF so an aerial switch between HF / VHF would be needed.

There is also a problem with MW reception as I've found that signals from the 75 m band (Radio China International) can appear on the lower part of the MW band. Careful selction of the cente frequency though pushes it below 540kHz - but a 2MHz LPF would be needed.

None of the above says that the SDRPlay is 'no good' - it has transformed my FM-Dxing - and is excellent value for the money. Just do some reading about other SDR radios and comments about fitting filters to avoid FM band interference or interference to MW.

I wonder if the next generation of the SDRPlay will come with an aerial socket for HF and one for VHF?


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Re: General SDR information

Postby Tech_Support » Fri Sep 04, 2015 5:21 am

Dear Ton & David,
The following thread discusses spurious FM images that result from the architectural use of a block converter to cover HF and frequencies from 250 - 420 MHz


If either of you are Facebook users, this has also been discussed on the public Facebook group along with proposed solutions at:

We will be publishing a white paper that covers this in more detail shortly, but there are two options for dealing with this. Either (as has been suggested) add an in-line FM filter or alternatively you can tweak the frequency plan of the block converter using the Advanced settings in the ExtIO window. This won't remove the spurious responses, but it may allow you to move them well away from the regions of spectrum that you are interested in.

For a future version of the RSP, we are considering both improved FM filtering to reduce these spurious responses and possibly a second RF connector for HF operation. We are certainly looking for inputs from the community as to what features people consider to be most important and we will do our best to implement these if we can. At this stage, we have no specific schedule for the next version, but please keep an eye on the website for future announcements.


SDRplay Tech_Support
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Re: General SDR information

Postby pokpook555 » Sat Sep 05, 2015 8:05 am

i think so in your post

ร่วมเล่นสนุกๆไปกับพวกเรา ได้ที่นี่ คาสิโนออนไลน์
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Location: Ridgeville, SC

Re: General SDR information

Postby NN4F » Sun Sep 06, 2015 6:03 pm


in the extio panel, click on advanced and change the LO from Auto to one of the other values, this should get rid of images in the 10m band, I had the same issue with 6m, changed to the 120mhz while on 6m and the images disappeard....

Paul - NN4F
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Paul Jones - NN4F, Charleston SC

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