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

Richard (Rick) Karlquist richard at karlquist.com
Thu Jan 7 11:48:23 EST 2016


On 1/7/2016 4:35 AM, Attila Kinali wrote:
>
> How about using the Gilbert Cell as "digital" mixer,
> ie driving the currents hard from one branch to the other
> and replacing the current sources by resistors?
>
> How much would that improve the noise? Would it be still much
> worse than the diode mixer?
>
> 			Attila Kinali

You can drive the Gilbert cell as hard as you want, but
the active region is only about 100 mv so the extra
drive voltage doesn't help.  It is the same as if you
drove it with a 100 mV square wave.  Somewhat better
than a sine wave, but not a game changer.

You can of course try to replace the emitter current source
with a resistor, which works to the extent that you can
afford to throw away voltage across the resistor, but
you will never get a very high impedance.  No OTS IC's
are designed this way.  What would be better would be
to use an inductor.  A true noiseless current source.
Again no OTS IC's are designed this way.

You would have to homebrew the whole mixer from discretes.

Rick Karlquist N6RK



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