Forums

Precision synchronous demodulator

Started by Spehro Pefhany September 23, 2013
What's a good approach for use at 1MHz, give or take 2:1? 

Things like the AD630 are slow, and my usualy go-to approach (Gilbert
Cell balanced demodulator with LO >> Vt) has input referred drift and
offset typically in the ~1uV/K and a couple mV. I'd like both to be
better by at least an order of magnitude, and preferably with some
gain like the Gilbert cell things, which have 15-20dB of gain.  

Analog switches have negligible offset- how much trouble will all that
charge injection (~4pC for a good one) cause at 1-2MHz? 


Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:

> What's a good approach for use at 1MHz, give or take 2:1? > > Things like the AD630 are slow, and my usualy go-to approach (Gilbert > Cell balanced demodulator with LO >> Vt) has input referred drift and > offset typically in the ~1uV/K and a couple mV. I'd like both to be > better by at least an order of magnitude, and preferably with some > gain like the Gilbert cell things, which have 15-20dB of gain. > > Analog switches have negligible offset- how much trouble will all that > charge injection (~4pC for a good one) cause at 1-2MHz? > > > Best regards, > Spehro Pefhany
How about a AD8551? 1uV offset and 5nV/C. $1.84 at Arrow: http://octopart.com/partsearch#search/requestData&q=AD8551 Get the signal up out of the noise then use any analog switch to suit. JK
John K <spam@me.not> wrote:

> Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote: > >> What's a good approach for use at 1MHz, give or take 2:1? >> >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >> Cell balanced demodulator with LO >> Vt) has input referred drift and >> offset typically in the ~1uV/K and a couple mV. I'd like both to be >> better by at least an order of magnitude, and preferably with some >> gain like the Gilbert cell things, which have 15-20dB of gain. >> >> Analog switches have negligible offset- how much trouble will all that >> charge injection (~4pC for a good one) cause at 1-2MHz? >> >> >> Best regards, >> Spehro Pefhany > > How about a AD8551? 1uV offset and 5nV/C. $1.84 at Arrow: > > http://octopart.com/partsearch#search/requestData&q=AD8551 > > Get the signal up out of the noise then use any analog switch to suit. > > JK
Oops - thought you were interested in DC. Same principle though. If you are demodulating a weak 1MHz signal, use a low noise amplifier to boost it, then chop. OTOH, the Tayloe demodulator might be interesting. Using a LT1115 preamplifier at the output, it can get down to 17.3 nV or -142.1 dbm for 3dB SNR in 1KHz bandwidth: http://www.norcalqrp.org/files/tayloe_mixer_x3a.pdf I don't know how long until his patents expire, but you might ask. Since he has promoted it so widely in ham use, he might be willing to let you use it for a nominal fee. Another alternative is the H-Mode mixer, invented by Colin Horrabin, G3SBI. Martein Bakker, PA3AKE, has done extensive research on it. The site is http://martein.home.xs4all.nl/pa3ake/hmode/ JK
On Mon, 23 Sep 2013 10:37:09 GMT, the renowned John K <spam@me.not>
wrote:

>Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote: > >> What's a good approach for use at 1MHz, give or take 2:1? >> >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >> Cell balanced demodulator with LO >> Vt) has input referred drift and >> offset typically in the ~1uV/K and a couple mV. I'd like both to be >> better by at least an order of magnitude, and preferably with some >> gain like the Gilbert cell things, which have 15-20dB of gain. >> >> Analog switches have negligible offset- how much trouble will all that >> charge injection (~4pC for a good one) cause at 1-2MHz? >> >> >> Best regards, >> Spehro Pefhany > >How about a AD8551? 1uV offset and 5nV/C. $1.84 at Arrow: > >http://octopart.com/partsearch#search/requestData&q=AD8551 > >Get the signal up out of the noise then use any analog switch to suit. > >JK
Hi, John:- The demodulator (and following LPF) will eliminate low-frequency noise (including DC offset). We'll be operating well above the 1/f corner on the power spectral density plot of the amplifiers, so amplifier TCVos and Vos really don't matter until you run out of headroom. The purpose of the demodulation is to accurately recover a very small signal buried deep in noise. Think old-school analog lock-in amplifier (no modern ADC->DSP/FPGA digital signal processing) with a relatively large 'dynamic reserve'. Before demodulation we can only amplify signal+noise, and you can't let that clip or you lose the signal (and it shouldn't distort too much or accuracy is lost). Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:

> What's a good approach for use at 1MHz, give or take 2:1? > > Things like the AD630 are slow, and my usualy go-to approach (Gilbert > Cell balanced demodulator with LO >> Vt) has input referred drift and > offset typically in the ~1uV/K and a couple mV. I'd like both to be > better by at least an order of magnitude, and preferably with some > gain like the Gilbert cell things, which have 15-20dB of gain. > > Analog switches have negligible offset- how much trouble will all that > charge injection (~4pC for a good one) cause at 1-2MHz?
Has anyone tried using a fully-balanced system to null the charge injection? -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
On 2013-09-23, Adrian Tuddenham <adrian@poppyrecords.invalid.invalid> wrote:
> Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote: > >> What's a good approach for use at 1MHz, give or take 2:1? >> >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >> Cell balanced demodulator with LO >> Vt) has input referred drift and >> offset typically in the ~1uV/K and a couple mV. I'd like both to be >> better by at least an order of magnitude, and preferably with some >> gain like the Gilbert cell things, which have 15-20dB of gain. >> >> Analog switches have negligible offset- how much trouble will all that >> charge injection (~4pC for a good one) cause at 1-2MHz? > > Has anyone tried using a fully-balanced system to null the charge > injection?
You mean like a diode ring modulator? -- &#9858;&#9859; 100% natural --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:
 
> Hi, John:- > > The demodulator (and following LPF) will eliminate low-frequency noise > (including DC offset). We'll be operating well above the 1/f corner on > the power spectral density plot of the amplifiers, so amplifier TCVos > and Vos really don't matter until you run out of headroom. > > The purpose of the demodulation is to accurately recover a very small > signal buried deep in noise. Think old-school analog lock-in amplifier > (no modern ADC->DSP/FPGA digital signal processing) with a relatively > large 'dynamic reserve'. Before demodulation we can only amplify > signal+noise, and you can't let that clip or you lose the signal (and > it shouldn't distort too much or accuracy is lost).
> Best regards, > Spehro Pefhany
Yes, I guessed you were interested in detecting a weak signal, so I appended some info on the Tayloe and H-Mode mixers. These have pretty good MDS (minimum discernible signal) and excellent dynamic range, perhaps 120dB or so. They are limited by the noise floor and large signal handling of the following amplifier stages. Charge injection does not seem to be a problem with these approaches. Your application is quite similar to sensitive receivers, except you probably don't have strong adjacent signals to cause intermodulation. But you will be sensitive to phase noise from the local oscillator which may also limit the performance. Another thing that may help is to use a transformer at the input to boost the signal before detection. You are apparently interested in a narrow band of frequencies, so a tuned input might even work. This is one of the most interesting design challenges - detecting a weak signal that can have a wide dynamic range. JK
On Mon, 23 Sep 2013 12:18:27 GMT, John K <spam@me.not> wrote:

>Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote: > >> Hi, John:- >> >> The demodulator (and following LPF) will eliminate low-frequency noise >> (including DC offset). We'll be operating well above the 1/f corner on >> the power spectral density plot of the amplifiers, so amplifier TCVos >> and Vos really don't matter until you run out of headroom. >> >> The purpose of the demodulation is to accurately recover a very small >> signal buried deep in noise. Think old-school analog lock-in amplifier >> (no modern ADC->DSP/FPGA digital signal processing) with a relatively >> large 'dynamic reserve'. Before demodulation we can only amplify >> signal+noise, and you can't let that clip or you lose the signal (and >> it shouldn't distort too much or accuracy is lost). > >> Best regards, >> Spehro Pefhany > >Yes, I guessed you were interested in detecting a weak signal, so I >appended some info on the Tayloe and H-Mode mixers. These have pretty >good MDS (minimum discernible signal) and excellent dynamic range, >perhaps 120dB or so. They are limited by the noise floor and large signal >handling of the following amplifier stages. Charge injection does not >seem to be a problem with these approaches.
Yes, it's interesting to see that circuit-- thanks very much for the link. I was thinking of something along those lines with a SPDT switch and a zero-drift instrumentation amplifier at the output.
>Your application is quite similar to sensitive receivers, except you >probably don't have strong adjacent signals to cause intermodulation. But >you will be sensitive to phase noise from the local oscillator which may >also limit the performance.
Crystal oscillator, so I don't think so.
>Another thing that may help is to use a transformer at the input to boost >the signal before detection. You are apparently interested in a narrow >band of frequencies, so a tuned input might even work.
Got all of that, and more. The input BPF can't be too narrow bw, for obvious reasons.
>This is one of the most interesting design challenges - detecting a weak >signal that can have a wide dynamic range. > >JK
"May you live in interesting times" is allegely a Chinese curse, but apparently there's no evidence of that source, according to Wikip*dia. They do list this one "May you come to the attention of important people", which is suitably ominous.
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:

>>But you will be sensitive to phase noise from the local oscillator >>which may also limit the performance.
> Crystal oscillator, so I don't think so.
A cheap crystal oscillator can have terrible phase noise at 1MHz. Low phase noise is an art. A good crystal can cost an arm and a leg. You need to know the amplitude of the noise in nanovolts per root Hz at the operating frequency, then compare that to the phase noise from the crystal at the same frequency. If the crystal noise is equal to or higher, you may need to find a better oscillator. JK
On Monday, 23 September 2013 22:18:27 UTC+10, John K  wrote:
> Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote: >=20 > > Hi, John:-=20 > >=20 > > The demodulator (and following LPF) will eliminate low-frequency noise > > (including DC offset). We'll be operating well above the 1/f corner on > > the power spectral density plot of the amplifiers, so amplifier TCVos > > and Vos really don't matter until you run out of headroom. =20 > >=20 > > The purpose of the demodulation is to accurately recover a very small > > signal buried deep in noise. Think old-school analog lock-in amplifier > > (no modern ADC->DSP/FPGA digital signal processing) with a relatively > > large 'dynamic reserve'. Before demodulation we can only amplify > > signal+noise, and you can't let that clip or you lose the signal (and=
=20
> > it shouldn't distort too much or accuracy is lost). =20 >=20 > Yes, I guessed you were interested in detecting a weak signal, so I=20 > appended some info on the Tayloe and H-Mode mixers. These have pretty =20 > good MDS (minimum discernible signal) and excellent dynamic range,=20 > perhaps 120dB or so. They are limited by the noise floor and large signal=
=20
> handling of the following amplifier stages. Charge injection does not =20 > seem to be a problem with these approaches. > =20 > Your application is quite similar to sensitive receivers, except you =20 > probably don't have strong adjacent signals to cause intermodulation. But=
=20
> you will be sensitive to phase noise from the local oscillator which may =
=20
> also limit the performance. =20 >=20 > Another thing that may help is to use a transformer at the input to boost=
=20
> the signal before detection. You are apparently interested in a narrow=20 > band of frequencies, so a tuned input might even work.
Tuned inputs can be a problem before a phase-sensitive detector. If the pha= se-shift through the input stage changes. so does the output from the phase= sensitive detector. Larsen N T 1968 Rev. Sci. Instrum. 39 1=9612 used a band-pass filter before= his phase sensitive detector, but added an all-pass network to cancel any = phase shifts through the filter. National Bureau of Standards employees can= be like that. http://ieeexplore.ieee.org/xpl/login.jsp?tp=3D&arnumber=3D5040488&url=3Dhtt= p%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5040488
> This is one of the most interesting design challenges - detecting a weak =
=20
> signal that can have a wide dynamic range.
--=20 Bill Sloman, Sydney