[sdiy] [synth-diy] Frequency shifter which rejects negative frequencies

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Sun Jan 26 19:17:46 CET 2014


You can make a frequency shifter that only outputs the sum frequencies, 
or only outputs the difference frequencies, but the design is more 
complex than a simple DSB ring-modulator.  You have to use a quadrature 
sinewave local-oscillator, and you have to produce a quadrature version 
of the audio input.  The latter is either done by shifting the audio by 
90 degrees across all frequencies of interest, or by producing two 
completely new versions of the audio that both have their phases messed 
about with, but which track exactly 90 degrees apart.  Either way you 
need a quadrature sinewave local oscillator and quadrature version of 
the audio input.  You put the in-phase audio and local-osc into one 
ring-mod, and the 90 degree shifted versions of both signals into the 
other ring-mod.  Depending on how you combine the outputs of the two 
ring-mods you can get only the sum frequencies, or only the difference 
frequencies.  These are usually called upper sideband or lower sideband 
respectively.

If this explanation isn't clear, do some googling for "SSB modulator" 
for the gory details.  You can do it with analog or digital techniques, 
but it's easier to get accuracy and repeatability with digital.

-Richie,



On 2014-01-26 17:24, cheater00 . wrote:
> On Sun, Jan 26, 2014 at 6:10 PM, Magnus Danielson
> <magnus at rubidium.dyndns.org> wrote:
>> On 26/01/14 16:55, cheater00 . wrote:
>>> 
>>> Hi guys,
>>> 
>>> In a parallel thread, I have explained what lock-in amplifiers are 
>>> and
>>> how they work.
>>> I have asked myself: what if rejecting negative and positive
>>> frequencies, leaving only frequencies near DC, you could reject only
>>> negative frequencies?
>>> 
>>> I'm not sure, but I believe in the digital world you can do the
>>> four-quadrant multiplication and then take a Fourier transform of
>>> that. But I'm very hazy on the details. Can anyone confirm or deny?
>> 
>> 
>> It works, if you have the phase quadrant of both signals.
> 
> I don't know what this means. Could you explain a bit more please?
> 
>> This is why you have an elaborate filter to achieve this for the audio
>> signal.
> 
> Again.. not sure where to begin with that. Please bear* with me :-)
> 
>>> Is there an analog process that allows you to do this?
>> 
>> Yes.
> 
> What is it? I'm not talking about high-pass filtering before the
> mixing, if that's what you mean. That would give no advantage to just
> a simple high-pass filter.
> 
> Cheers,
> D.
> 
> * here's a very bad ascii art of a couple teddy bears for you:
> 
>            __..._.-.               .-._...__
>           /.-.   '-.)             (.-'   .-.\
>           \',       \             /       ,'/
>            |       o'--D       c--'a       |
>            \      /    |       |    \      /
>             ;._  _\ '-/         \-' /_  _.;
>           .'    __ `\`.-"-. .-"-.`/` __    '.
>         .'    .'  '.|'     '     '|.'  '.    '.
>        /      \     '._,       ,_.'     /      \
>       ;        '-._     \     /     _.-'        ;
>      (|           /'-.__/     \__.-'\           |)
>       \  __     ,'     '-.   .-'     ',     __  /
>        `/  `\.-'|         '.'         |'-./`  \`
>         |    |  '-.                 .-'  |    |
>         |    '-.   )               (   .-'    |
>         \       )-'                 '-(       /
>          '-----'                       '-----'
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