[sdiy] 4051 mux with caps and buffers?

[Eric Brombaugh]

On Jul 3, 2012, at 12:01 AM, Gordon JC Pearce wrote:

> On 02/07/12 21:56, Kyle Stephens wrote:
>> One mux/S&H application I've been wondering about is using this amateur radio demodulator circuit...:
>> 
>> http://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdf
>> 
>> ...as a phase splitter (with of course more than merely 180 degrees of shift). Might make a neat LFO utility.
> 
> Well you've actually got two things going on there, you've got two local oscillators 90 degrees apart and a centre-tapped transformer.  Now what happens is that depending on which switch is on, you get in-phase non-inverted, quadrature non-inverted, in-phase inverted or quadrature inverted outputs on.  You don't actually get the input signal shifted in 90 degree steps, exactly.
> 
> The practical upshot is that you stick in a local oscillator at four times the desired centre frequency, say 28.4MHz, divide by four using a Johnson counter or similar to get your quadrature clocks at (in this example) 7.1MHz, then pass that through the synchronous mixer.

The Tayloe sampling quadrature converter is a pretty nifty bit of engineering. I happened to be at the Flagstaff ARA Hamfest when he first presented it during a QRP session and it's one of those ideas where as soon as you hear it the light bulb goes off and you wonder why you never thought of it. Truly unique and yet simple.

That said, it's hard to imagine how this would be useful in an audio synthesis context. It's really only useful for RF downconversion since it combines the functions of mixer and lowpass filter into one block. I suppose you could use it to create a quadrature sine output by driving it with two oscillators, one of which is tunable over a narrow range, but there are a lot of other ways to do that which are probably simpler and easier to control.

Eric