On Monday, 22 February 2016 02:44:32 UTC, ChesterW wrote:> On 2/21/16 5:12 PM, John Larkin wrote:> > A single-transistor multiplier would probably be awful. > > I was afraid I might hear that.why is it awful? NT
Single transistor mixer
Started by ●February 21, 2016
Reply by ●February 22, 20162016-02-22
Reply by ●February 22, 20162016-02-22
On Sun, 21 Feb 2016 20:46:00 -0600, ChesterW wrote:> On 2/21/16 5:49 PM, John Larkin wrote: >> On Sun, 21 Feb 2016 17:23:43 -0600, Tim Wescott >> <seemywebsite@myfooter.really> wrote: >> >>> On Sun, 21 Feb 2016 15:12:58 -0800, John Larkin 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 >>>> >>>> A single-transistor multiplier would probably be awful. >>>> >>>> You don't need to multiply sine waves. Just multiply the signal >>>> alternately by +1 and -1 and then lowpass filter. >>>> >>>> That can be done, for example, by alternately multiplexing between >>>> SIG+ >>>> and SIG- with a good fast analog mux. That will always have residual >>>> errors, but could be pretty good. >>>> >>>> I guess the best lock-in would use a good ADC and do the multiply >>>> digitally, in an FPGA. Noise dither maybe. >>> >>> With today's parts you may be able to do this with a set of fast >>> analog switches and an op-amp. Perhaps even a 74<something fast>4051 >>> or whatever? >> >> 74HC4051 would work, but the 1 MHz requirement is tricky. >> >> Something like FSA3157 or ADG604 or one of those T3USB gadgets might be >> better at 1 MHz. >> >> The classic single-opamp +1/-1 circuit might work with a small phemt as >> the grounding switch. >> >> But no single transistor! >> >> > I certainly like the price of that FSA3157. > > To be clear, 1 Mhz is not a requirement, I can pick the frequency. I was > planning to look for a clean spot in the spectrum maybe around 40 kHz - > something where I can get a reasonable over-sample on the A to D if I go > with signal processing in an inexpensive micro.At 40kHz a jelly-bean op-amp for a few pennies, and John's classic* +1/-1 circuit would almost certainly work, with whatever is cheapest for an analog switch. DigiKey lists a 74VHC1GT66 at 7 cents each in qty 1000, which is pretty cheap and should do. * I invented one of those when I was in grad school. I tend to invent a lot of stuff that's been around for decades. It's trying. -- www.wescottdesign.com
Reply by ●February 22, 20162016-02-22
On 02/21/2016 05:22 PM, ChesterW 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 comWhat about the LM13700 OTA? I've used them to make a mixer at 500kHz or so - works great! They should have enough bandwidth to do 1MHz no problem. And you also get a Darlington transistor with each section, which is nominally intended for use as a buffer, but which I used to build a single transistor Sallen-Key filter to low pass filter and get the difference signal.
Reply by ●February 22, 20162016-02-22
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?A single transistor single ended mixer will have a poor performance. In addition to the sum and difference frequencies (select either with a filter on output side) it will also pass through the original tones. It will probably also cause mixing with the harmonics of both tones. If the input signals are not clean sine waves but also contains some other spurs, these spurs will also be mixed with the other frequency. With _very_ careful selection of frequencies, you might be able to (low pass) filter the desired frequency and reduce the spurious frequencies. A better solution is to use a double balanced mixed such a 1496 or a newer mixer. It quite effectively blocks the original frequencies and only produce the sum and difference tones. You still need to be careful with any odd harmonic products if hard driven. With a double balanced mixer the post mixer filtering is much simpler.
Reply by ●February 22, 20162016-02-22
On 2/21/2016 9:48 PM, ChesterW wrote:> On 2/21/16 6:28 PM, mixed nuts wrote: >> On 2/21/2016 5:22 PM, ChesterW 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? >> >> Single diodes have been used as mixers for nearly a century. A >> transistor biased near cutoff with a strong signal applied to the base >> at one frequency and a weak signal at another will produce sum and >> difference frequencies with some gain over a simple diode. >> >> The conceptual information and enough math to give you a handle on >> behavior are here: >> >> https://en.wikipedia.org/wiki/Frequency_mixer >> https://en.wikipedia.org/wiki/Superheterodyne_receiver >> >> A CMOS exclusive-or gate (74HC86) is useable as a mixer as well. >> > Thanks for the idea. I think using a diode will cause a lot of my > desired mixing products to be well removed from DC and so not available > to add to my signal. Also, noise in the spectrum of the input signal far > removed from the carrier frequency will end up translating down to DC > and further decrease my SNR.If those are your concerns, you'll need a double balanced image reject mixer. If you can sample and process fast enough, you can do it in software. Elsewise, you'll need a quadrature local oscillator, two balanced mixers and possibly a Hilbert transformer to sum the I and Q signals so you can gain a sqrt(2) improvement in s/n. -- Grizzly H.
Reply by ●February 22, 20162016-02-22
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. -- Grizzly H.
Reply by ●February 22, 20162016-02-22
On a sunny day (Sun, 21 Feb 2016 16:22:20 -0600) it happened ChesterW <iamsnoozin@yahoo.com> wrote in <oMqyy.140627$OT2.139745@fx39.iad>:>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?No. but I have used dual gate MOSFETs any time I needed just a simple mixer. Oscillator on gate 2 biased with a trimpot to get the best conversion point. But there are great 4 quadrant multiplier chips that work into the GHz range with a good large signal performance (google). Anyways, in the dual gate scenario gate 2 controls the gain to put it simply. Even cheaper use a normal BJT, signal on base, oscillator input on emitter. The classic diode ring modulator is what I have used many times in large signal cases, but likely needs transformers in your case. https://en.wikipedia.org/wiki/Ring_modulation As somebody in some other group once mentioned: 'almost anything that is non -linear will 'mix'' A single diode works too. So, not sure about your setup, get a 4 quadrant multiplier chip? Or maybe in a 64 MHz PIC in software with sine lookup tables, but I have not tried, well actually I did to about 200 kHz for FT display http://panteltje.com/panteltje/pic/scope_pic/index.html Here you see me using XOR gates as multiplier in a quadrature modulator: http://panteltje.com/panteltje/raspberry_pi_dvb-s_transmitter/ Also note the GHz RMS42-N diode ring modulator, will not work at low frequencies, but you can use cheap audio transformers (like from a transistor radio) for that. And note the AD8347 QAM modulator on board... That should? work for LF too. There are so many of these type of chips. This would become a very long posting, as again, anything non-linear will mix, even a bad contact.
Reply by ●February 22, 20162016-02-22
On Sunday, February 21, 2016 at 9:38:46 PM UTC-8, bitrex wrote:> On 02/21/2016 05:22 PM, ChesterW wrote: > > I need to make a mixer (a multiplier, not an adder) for sinusoid signals> What about the LM13700 OTA? > > I've used them to make a mixer at 500kHz or so - works great! They > should have enough bandwidth to do 1MHz no problem.Yep, should work fine; connect signal to (+) input on one section, and (-) input on the other, grounding the unused inputs, and feed the current source inputs differentially with the local oscillator. Wire both outputs together, of course. You can also do it with a single section, but that doesn't balance out the LO.
Reply by ●February 22, 20162016-02-22
If your microcontroller has a sigma-delta ADC covering the audio range, you may find that it responds at multiples of the oversampling clock and nearby frequencies. All you need to do is to remove the RC anti-aliasing filter from the ADC input. If the ADC does respond in this way, it will behave like a very good quality digitising mixer. I have made this work with Cirrus sigma-delta audio ADCs in the past. John
Reply by ●February 22, 20162016-02-22
On 22/02/2016 02:47, ChesterW wrote:> 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. > > > ChesterWIf the DUT doesn't like a square wave then what about just reducing the amplitude of the square wave until the DUT treats it linearly? Or how about a triangle wave - that would be easier than a sine for your micro to generate? Won't you need a low pass filter or anti-alias filter between DUT and demod anyhow, if so then that would take care of 3rd order harmonics and allow use of a simple commutating switch mixer, in its simplest incarnation just one transistor as you wanted? piglet
