[hpsdr] VHF/UHF downconverter for HPSDR

[Phil Harman]

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.

Whilst it is possible to compensate for amplitude and phase errors in 
software, just like Flex do for their range of radios,  to cover 100's of 
MHz could be a tall order.

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.

We have the advantage when using Mercury as the IF that we have a huge 
number of possible IF and LO frequencies that will tune the wanted 
frequency.

Lets say we tune to the wanted signal with  IF's of 10MHz then  20MHz then 
30MHz. If the FFT of each of these looks sensibly the same then there is a 
good chance we don't have any images.  If one say gives a different FFT but 
the others are the same then perhaps use one of this pair as the LO/IF pair. 
If all three give different results the try some other LO+IF combinations.

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

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!

73's Phil...VK6APH

----- Original Message ----- 
From: "Ante Vukorepa" <o.orcinus at gmail.com>
To: <phil at pharman.org>
Cc: <hpsdr at lists.openhpsdr.org>
Sent: Monday, October 03, 2011 7:04 PM
Subject: Re: [hpsdr] VHF/UHF downconverter for HPSDR


> On Mon, Oct 3, 2011 at 11:38 AM, Ante Vukorepa <o.orcinus at gmail.com> 
> wrote:
>> 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.
>>
>
> Now that i've mentioned it, would an image rejection method as
> implemented in image reject mixers work, or would that produce
> rejection levels too low / losses too high?
>
> (Disclaimer: BTW, please bear in mind that, as i've said, i've never
> actively meddled in RF electronics of any kind, apart from an
> introductory course on RF comms in university some time ago, and am
> just beginning. Meaning i'm pretty clueless re: most of the matter
> concerned here.)
>
> -- 
> Ante Vukorepa
>
>
>
> -----
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