Single transistor mixer

On Wednesday, February 24, 2016 at 2:04:08 PM UTC+11, mixed nuts wrote:
> On 2/23/2016 7:52 PM, bill.sloman@ieee.org wrote:
> > On Monday, February 22, 2016 at 4:51:49 PM UTC+11, mixed nuts wrote:
> >> On 2/21/2016 9:47 PM, ChesterW wrote:
> >>> On 2/21/16 6:52 PM, whit3rd wrote:
> >>>> On Sunday, February 21, 2016 at 2:22:18 PM UTC-8, ChesterW
> >>>> wrote:
> >>>>> I need to make a mixer (a multiplier, not an adder) ... for
> >>>>> a lock-in-amp.
> >>>>
> >>>> The classic one-transisor mixer is a dual-gate MOSFET. If your
> >>>> 'lock-in amp' has a squarewave reference, analog-switch
> >>>> polarity reversal can be very effective, too.   The 'switch'
> >>>> elements can be diodes, or FETs, or any other kind of
> >>>> transistor; you could even consider photocouplers, at your
> >>>> relatively low frequency range.
> >>>>
> >>> I've never heard of a dual-gate mixer, so thanks for the tip!
> >>>
> >>> In an ideal world I'll use a sine wave for my carrier and
> >>> demodulate with a sine. That way my noise bandwidth will be lower
> >>> I think than using a square wave.
> >>>
> >>> My DUT doesn't give a linear response to a square wave, but does
> >>> to a sine wave. The carrier used to demod the signal can be
> >>> thought of as a sort of matched filter, so since my signal is a
> >>> sine the best demod carrier is also a sine.
> >>
> >> Mixers are always driven with a square wave local oscillator to
> >> guarantee linear response to the signal and eliminate LO amplitude
> >> variations as a factor.
> >
> > They aren't. If the amplitude of the local sine wave oscillator is
> > stable - which can be arranged - there aren't any local oscillator
> > amplitude variations, and people who are worried about detecting odd
> > harmonic content in the signal they are looking at do go to the
> > trouble of building clean and stable sine wave local oscillators.
> 
> I know that, but try to find 

     something that is sold as

> a mixer in which conversion gain, IIP3 and NF
> depend significantly on LO power.  They put limiters in 'em and tell you 
> feed 'em with enough voltage to make things nice and square.  If you 
> want a linear multiplier, you make or buy a multiplier.
> 
> I think it was an ADI app note (or dialog article) where that 
> distinction was made by Barry Gilbert and Bob Clarke.

It's a marketing distinction, not a technological distinction. Every four quadrant multiplier is a potential mixer, but they tend not to be sold as such.

-- 
Bill Sloman, Sydney

On 2/21/16 8:39 PM, Bill Sloman wrote:
> On Monday, 22 February 2016 11:04:34 UTC+11, Jim Thompson  wrote:
>> On Sun, 21 Feb 2016 16:22:20 -0600, ChesterW <iamsnoozin@yahoo.com>
>> wrote:
>>
>>> I need to make a mixer (a multiplier, not an adder) for sinusoid signals
>>> somewhere between about 10 kHz and 1 Mhz. This is for a lock-in-amp. My
>>> main constraint is low cost. I'd use a microcontroller with an A to D
>>> and a hardware multiplier, but I'd like to keep the cost down by using a
>>> cheaper micro. I could use a simple power-of-2 window in the micro and
>>> then only need additions to demod, but that would give me a wider
>>> bandwidth for noise to get in. I though a single transistor mixer design
>>> might give better results. Anyone know of a good reference?
>>>
>>> ChesterW
>>> +++
>>> Dr Chester Wildey
>>> Founder MRRA Inc.
>>> Electronic and Optoelectronic Instruments
>>> MRI Motion, fNIRS Brain Scanners, Counterfeit and Covert Marker Detection
>>> Fort Worth, Texas, USA
>>> www.mrrainc.com
>>> wildey at mrrainc dot com
>>
>> For RF, a simple diff-pair makes an excellent mixer... think half of a
>> Gilbert multiplier (you don't need the half that eliminates DC from
>> the source... tank loads do that).
>
> The Faulkner and Harding phase sensitive detector works fine at audio frequencies as well (though it was claimed that it worked up to 30MHz).
>
> E.A.Faulkner and D.W. Harding J.Sci. Instrum. volume 43 page 97-99 (1966)
>
> http://iopscience.iop.org/article/10.1088/0950-7671/43/2/305
>
> Faulkner improved it a bit
>
> E.A.Faulkner and J.B.Grimbleby Electronic Engineering volume 39 page 565-67 (1967)
>
> Both references are from my Ph.D. thesis.
>
> The original Faulkner and Harding phase sensitive detector turned the - off-set - signal into a modulated direct current at the collector of a transistor constant current source, and fed that into a long-tailed pair of transistors whose bases were driven by the demodulating signal - ideally a pair of anti-phase square waves at the frequency of interest.
>
> The integrated difference in the currents coming out of the two collectors was the demodulated signal - an operational amplifier subtractor could refer that to 0V. I just stuck a moving coil ammeter across the collector resistors.
>
> You have to add enough voltage offset to the original AC signal to make sure that the current going into the long-tailed pair never drops to zero, and the mark-to-space ratio (which should be exactly 50%) of the demodulating drive determines how effectively this offset is rejected.
>
> It's three transistors rather than one, but it's a remarkably good circuit - even better if you used a matched transistor pair for the long-trailed pair.
>

I estimate using a sine for demod would give only about 1 dB improvement 
over square wave demod for uncorrelated noise. For correlated noise it 
should be in the neighborhood of 7 dB. I think you're right that the 
lions share of the improvement is getting out of the baseband.

My newsreader seems to have lost some of my replies to everyone. Anyway, 
thanks for the help.

ChesterW