[hpsdr] VHF/UHF downconverter for HPSDR

[Ante Vukorepa]

On ponedjeljak, 3. listopada 2011. at 04:05, Phil Harman wrote:
> In which case an input signal at 383-50MHz = 333MHz, i.e. the image
> frequency, will also give a signal at the IF frequency. If you want say
> 100dB of image rejection then your 70cm bandpass filter(s) before the
> mixer will need to provide this level at 333MHz.
> 

Oh THAT image.
I've sort of always assumed the input would be pre filtered independently of the board, but never really gave it much thought, especially to the fact near-microwave frequencies would pose a problem (just as they would have with the original idea of making a preselector and using under sampling up to 500ish MHz).

Hm.

I've (half-seriously) toyed once with the idea of using two parallel LOs and mixers for the purpose of making image elimination easier. I.e. if the signal is at 433 MHz, the IF is 50 Mhz and the LO is at 383 MHz, then the image will be at 383-50=333 as you've said, but if i introduce another LO at 333-50=283 and another mixer, that will produce an image of the 333 MHz interference at the IF that i can subtract from the "mirror" image generated by the first down-converter.

That would, of course, leave another mirror from 283-50=233, but that way i've moved the problem lower in frequency and (supposedly) made it easier to filter. 

Never gave that idea more thought, though, as it's far too messy, silly and impractical to be of any use.

-- 
Ante Vukorepa
Sent with Sparrow (http://bit.ly/sigsprw)

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