[time-nuts] DMTD - analog multiplier vs. diode mixer ?

Richard (Rick) Karlquist richard at karlquist.com
Wed Jan 6 21:30:07 EST 2016



On 1/5/2016 12:07 PM, Bruce Griffiths wrote:
> The noise of such Gilbert cell based analog multipliers far exceeds that of the traditional mixer.
> Bruce
>
Read Gilbert's paper or Gray and Meyers analog IC textbook and
you will see that the whole theory of operation of these
depends on keeping the signal levels in them very small,
especially if linearity (actually translinearity) is
important.  They always have current sources in the
emitters that contribute a lot of noise.  So you have
small signals and large noise.  The IC's that are
designed to be DC coupled have even more sources of
extra noise.

IMHO, they only make sense in low performance applications
where the lack of transformers is important or in DC
coupled applications.  The only time I have used an
analog multiplier IC was in Costas loop to demodulate
QPSK from weather satellite.  It needed to be DC coupled.

Rick Karlquist N6RK




>      On Wednesday, 6 January 2016 9:01 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
>
>
>   My little HP5065 project is continually running into the jitter of
> my HP5370B counter which is annoying me, so I'm looking int DMTD.
>
> Everybody seems to be using traditional diode-mixers for DMTD,
> and to be honest I fail to see the attraction.
>
> Why wouldn't a analog multiplier like AD835 be better idea ?
>
> What am I overlooking ?
>



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