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

Tim Shoppa tshoppa at gmail.com
Thu Jan 7 12:09:44 EST 2016


Gilbert cells are very useful in many "compromise" circuits. If you have a
fixed power budget on the very low end, Gilbert cells may be the best
choice because they make a balanced mixer stage that has substantial gain
with miniscule LO drive requirements.

Contrast that with a DBM diode mixer where they take substantial LO drive
and operate some loss.

In the no-compromise time-nuts context, there may be a good place for a
Gilbert cell. Maybe in a circuit where the Gilbert cell is getting hit with
a well-defined signal with low dynamic range that dwarfs the noise
contribution. The low LO requirement of a Gilbert cell also makes it easier
to isolate the LO from the rest of the circuitry and that can reduce
shielding needs. But it's not gonna go where you need both low noise and
high dynamic range.

Tim N3QE

On Wed, Jan 6, 2016 at 9:30 PM, Richard (Rick) Karlquist <
richard at karlquist.com> wrote:

>
>
> 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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