[sdiy] 4051 mux with caps and buffers?

[Gordon JC Pearce]

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.

"Great", you say, "but what for?"

Well, your outputs are the incoming RF at 7.1MHz mixed down to audio 
frequencies - but the clever bit is that you now have the in-phase and 
quadrature versions, which form the real and imaginary components of a 
rotating vector.  The faster the vector spins, the greater the 
difference in frequency from the centre.  The centre?  Yes, the centre - 
if the vector rotates one way it represents a frequency above the centre 
point, and if it rotates the other way it represents a frequency below 
the centre point.  So with a bit of surprisingly un-scary maths you can 
demodulate radio signals across the full bandwidth of your sample rate - 
so grabbing them into a PC at 48kHz would give you +/-24kHz about 7.1MHz 
with the ability to tune anywhere instantly, or view the spectrum of a 
chunk of band, or all sorts of other goodies.

I wrote some software to do that, which is available here:
http://gordonjcp.github.com/lysdr/

Other software for Windows is out there.

-- 
Gordonjcp MM0YEQ