Re: Filters for AirSpy, SDRplay etc


Allan Isaacs
 

The answer’s probably elementary, but can someone explain why I see the broadcast on 198KHz (much attenuated) sitting at 58KHz when I’m tuned to the Time Signal on 60KHz if the view shows 60KHz on the left of the screen but not on the right?

Is the VFO being altered to 140KHz in one view and not the other (ie 198-140=58) ?

I dragged the frequency scale below the waterfall to change the view.

SDR Play plus SDR Console V3 using an unfiltered long wire

Allan G3PIY

 


From: sdr-radio-com@... [mailto:sdr-radio-com@...]
Sent: 03 November 2016 00:38
To: sdr-radio-com@...
Subject: Re: [sdr-radio-com] Re: Filters for AirSpy, SDRplay etc

 

 

 

On 11/02/2016 07:14 PM, paul newcombe G6YZC@... [sdr-radio-com] wrote:

 

Don,

 

You will get no argument from me. On 6m I have 3 6ft cavity filters and the band is still unusable alas.

Perhaps a high intercept preamp followed by a really sharp bpf might solve your problem. Unless you
can vary the coupling on the big cavity filters to make them as sharp as needed. You'd probably still
need the preamp, but it would have to be a pretty stoochy one if you have as much junk there as you
indicate.

--Doug, WA2SAY, retired RF Engineer


On 2m I have a 6 pole bandpass filter and on 222 I have a 7 (?) pole bandpass filter.

My thoughts are eliminate as much as possible before the RX has to deal with it.

 

I think this is the right page..

 

http://qrp-labs.com/bpfkit

 

Paul 

N2EME

 


From: sdr-radio-com@... on behalf of djl djl@... [sdr-radio-com]
Sent: Wednesday, November 2, 2016 7:34 PM
To: sdr-radio-com@...
Subject: [sdr-radio-com] Re: Filters for AirSpy, SDRplay etc

 

 

I'm really glad to see this thread.. front end filters are (nearly
almost) absolutely necessary for the correct use of SDR radios. Any
nonlinearity in front of AND INCLUDING the a/d converter will wind up
with birdies all over the spectrum. This includes overdriving any
preamps or amps (exceeding dynamic range ANYWHERE), any bad solder
joints, dissimilar metals with corrosion, protective diodes, and bird
poop. You will get all multiples of all signals with all of the other
signals and all their harmonics. Especially bad in hf, as has been
noted here. I have seen local broadcast harmonics all the way up to 30
MHz and beyond, even listened to them. Remember, once the a/d
conversion is done, and the fourier transform is taken, you get all the
negative frequencies too!
I've thought in the past that not enough attention is paid to this
problem with sdr incarnations. Somehow, perfection is expected. There's
a reason for the bandpass and roofing filters in "old fashioned" radios.

Sorry for the rant. Carry on...

--
Dr. Don Latham
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304

 

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