[hpsdr] VHF/UHF downconverter for HPSDR

Mon Oct 3 06:46:30 PDT 2011

Ante,

On Oct 3, 2011, at 7:10 AM, Phil Harman wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> [snip]
> 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.

Another issue is the harmonic and spurious content of your LO.  Since the output of the SI570 comes from a buffer fed directly from a digital divider, it's more like a square wave than a sine wave, at least at lower frequencies where the output buffer isn't limiting it, so it will have strong odd harmonic content.  Each of these harmonics, though weaker than the fundamental, will still mix with everything in the mixer input signal, and any mixing product that falls below your LPF cut-off will be admitted.

If you can easily move the LO injection and IF frequency in sync, as you can with HPSDR and an SI570, it is easy to distinguish an image from a wanted signal -- the wanted signal won't move, but an image will.  Images caused by harmonics of the LO will move much farther.  This observation was used to advantage in spectrum analyzers over 50 years ago to extend their range to take advantages of harmonics of the LO: a known change (amount and direction) in the LO frequency could be observed to move a signal on the display by either that amount or a multiple of it, and in the same or the opposite direction.  With experience, you could punch the "shift" button and immediately distinguish unwanted from wanted signals and estimate their frequency.  (I have one of these old units that I occasionally warm up for fun, and it's quite an amazing piece of work -- I suspect it weighs almost as much as I do.)

Spectrum analyzers also sometimes have tuned preselectors on the input that track the LO change and will knock down out-of-band signals by perhaps 30-40 dB.  (These are generally YIG-filter based.  You can often see YIG preselectors advertised on ebay, seldom ranging below 1GHz, but going up to tens of GHz.  They are often used in wideband microwave test equipment.)  A friend of mine used to design TV tuners for a living in pre-digital TV days.  He designed varicap-tuned tracking preselectors into the front-ends of the tuners, which was necessary to reduce interference from images.  This is quite a technical challenge in high-volume production, given manufacturing variations in components, but these days a DAC driving the tuning diodes would make it (comparatively) a piece of cake.  Designing one of these with a tuning range extending over a very broad frequency span is challenging; you would probably end up with several, and pick the appropriate one for the frequency range of interest.

It's not hard to pre-compute some of the "troublesome" spots, like the FM broadcast band, commercial radio bands, etc. and pro-actively work around them.  It's also not hard to jump-tune and find combinations that give relatively image-free reception around a single frequency of interest, though as Phil points out, some interference sources are pretty broad so it's good to have a lot of choices in LO-IF combinations.  Many commercial radios (e.g., cellphones) use these tricks - the technique is well-known now, but was novel in the not-so-distant past.  You'll find it described in patents from not that many years ago.

Steve, K9SRB

> 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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> 
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