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Tayloe Detector : 1:4 Mux Biasing

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jamfletch:
Hi Everyone,

I've been doing some reading on Tayloe Detectors, one thing I'm struggling to understand is the requirement for a DC offset at the input of the 1:4 mux. I've seen this in Tayloe's original paper here : http://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdf along with a couple of example designs such as : https://github.com/thaaraak/Tayloe-Mixer-v2/blob/master/Tayloe%20Mixer%20v2.pdf

In Dan Tayloe's design, he biases up the RF input with a DC voltage prior to the multiplexer. At the outputs of the multiplexer, he uses AC coupling to the inputs of the Op-Amps, therefore I concluded the DC bias isn't required for the Op-Amps, but is for some reason used for the multiplexer. I've read the datasheet of the mux he's using here : https://www.diodes.com/assets/Datasheets/PI3B3253.pdf

I might be missing something obvious, I haven't managed to find an explanation online as to why the biasing is required. My only thoughts currently is that you may want to prevent the negative going part of the AC RF signal hitting the negative clamp voltage at the input?

Any help would be appreciated

Kleinstein:
The CMOS mux chips may need the DC bias, as they can only handle signals inside the supply range (with only a little overrange). So the PI3B3253 and similar chips with single supply need the offset. A 74HC4052 could get away without it (e.g. with a +-5 V supply).  The amplifiers may get away without the AC coupling too, as there usually is not much DC offset in the result (a bit from coupling of the IF back to the input).

jamfletch:

--- Quote from: Kleinstein on May 19, 2024, 01:15:57 pm ---The CMOS mux chips may need the DC bias, as they can only handle signals inside the supply range (with only a little overrange). So the PI3B3253 and similar chips with single supply need the offset. A 74HC4052 could get away without it (e.g. with a +-5 V supply).  The amplifiers may get away without the AC coupling too, as there usually is not much DC offset in the result (a bit from coupling of the IF back to the input).

--- End quote ---

That makes sense, have I missed something in the datashet for the PI3B3253 that states that this is the required case for the inputs? It seems that there isn't any specification for required minimum input voltage to the switch inputs, only for the DC Logic

mawyatt:
We found long ago that for RF signals where 3rd order non-linearity is of significant interest and minimizing such were major design goals (at the chip level), the simple NMOS switch was the best overall performer.

As mentioned keeping the input peak to peak signal within the switch voltage range, whatever switch type is used, is important for achieving acceptable overall linearity.   

You might find this of interest, please note the two IEEE papers and the impossible noise figures achieved, violating conventional mixer theory.

https://www.eevblog.com/forum/rf-microwave/polyphase-or-n-path-mixer/msg3381802/#msg3381802

Best,

jwet:
Digital Bus switches (ie N channel only) are the standard for these circuits- they have low Ron when sourcing, low threshold voltage and are cheap.  CMOS type switches that use and N an P channel devices in parallel offer real advantages in discrete (non integrated) circuits.  We're not talking about 4052's, ADI/Maxim makes wideband, very low Ron switches- look at the Max4684 series- sub .5 ohm with .05 ohm flatness on 2.7v supplies- but there are many others.  These are exceptionally good switches for these applications and offer real benefits over Bus Switches.  I evaluated these ultra low Ron symmetrical switches for this mixer app and they showed a lot of advantages over bus switches.  I never published results because bus switches were so well accepted and were easier to integrate on commodity processes.

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