[hpsdr] VHF/UHF downconverter for HPSDR

[Ante Vukorepa]

On 3. 10. 2011., at 14:10, "Phil Harman" <phil at pharman.org> wrote:

> Hi Ante,
> 
> Yes, you could use an image rejection mixer. This is basically what we do in Mercury - but since it's done digitally the image rejection is effectively perfect.
> 
> The practical issue with an analogue image rejection mixer is this.  In order to get 40dB of image rejection then the I and Q channels need to be matched in amplitude to within 1dB  AND in phase within 1 degree. For every 20dB increase in image rejection these values increase by a factor of 10. So for 60dB of image rejection the figures become 0.1dB and 0.1degree etc. These figures needs to be maintained over the entire frequency range.

Yeah, i was aware of that being an issue, but didn't realise how much of an issue. Thanks a lot for the figures!

That reminds me... I've just ran across a site that brought another issue to my attention. Si570, apparently, has a pretty variable amplitude over its frequency span. That's not much of a problem with just a mixer, but quite probably would/will cause problems with any/all image rejection techniques.


> I'm not trying to discourage you, just applying the "get it to work on the whiteboard before spending any time or $'s" approach that I normally take.
> 

No worries there, i'm perceiving this as a learning experience first and foremost. And learning by trial and error, as fun as it often is, would quite probably burn through an insignificant amount of time and money in this particular case :)

I'm grateful for your patience and any and all the knowledge and insight provided.


> Wide band image signals e.g TV stations and infrequent image signals e.g. two way radios, could be a problem.

Slightly off topic, but i've got the additional "local" disadvantage of living right next to a GSM/UMTS cell phone tower (BS), that seems to have increased its ERP above all sensible limits recently. I'm getting interference from it (the modulation "pattern" is pretty recognisable) all over the spectrum. Some kind of common mode RFI effect, i presume, as its present both far below and above the carrier frequencies and affects the IF of the receivers. I can even hear the 935 MHz downlink signal just pointing a FunCube Dongle SDR without an antenna connected towards the window. And no, the RFI does not appear to be induced in the receiver itself, as no amount of shielding helps and due to the fact it's not there unless connected to an antenna. (the FCD isn't the only receiver affected either, although the effect on it is easiest to notice - i'm getting nasty desensitising on two Icoms too)


> Have a read of the user manual for the SignalHound Spectrum Analyser, page 21
> 
> http://www.signalhound.com/manual.pdf
> 
> they appear to get very good results even though they are deliberately using LO and IFs that will result in image responses.
> 
> Solving these problems is 99% perspiration and 1% inspiration!

Thanks a lot, i predict a lot of reading and perspiring in my very near future (weekend, in fact) :)


Ante Vukorepa
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