[time-nuts] RF mixers for oscillator characterization, some questions
magnus at rubidium.dyndns.org
Mon Apr 6 22:58:56 UTC 2009
Bruce Griffiths skrev:
> Pete wrote:
>> I have had good experience with the SYPD series
>> from Mini-circuits. I have not seen any used, but
>> their new cost is reasonable. They do several things
>> well e.g. the DC offset on the units I received is
>> <1mV & they produce >2V p-p when driven @
>> Pete Rawson
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> These are OK except when one needs to isolate the IF ground from the RF
> grounds to avoid low frequency ground loop problems.
> The RPD and MPD through hole series are better in this regard as they
> allow the IF ground to be isolated from the RF grounds.
It should be noted that several of their SMB mixers have separated
grounds, but it is not documented in their datasheets.
> The flicker phase noise characteristics of the mixer/phase detector
> should be measured as some mixers/phase detectors have lower flicker
> phase noise than others.
> The termination of the IF port will affect the mixer phase noise. For
> offset frequencies < 100kHz a capacitive termination of the IF port
> which reflects the sum frequency back into the mixer reduces the mixer
> phase noise.
It essentially sees a very low impedance at those frequencies.
My experiments with capacitive loading of mixers basically indicates
that the actual low-frequency slope of an unloaded mixer does not
change, but the capacitor load filters the sum-frequency (with
overtones) while a resistive 50 ohm load just loads the amplitude down
and gives no significant change to performance. Optimum performance out
of a mixer in my experience comes from fairly high-impedance load at low
frequencies with a direct capacitive loading for filtering effects.
A non-filtered response is quite interesting to see with a fairly slow
beating frequency occuring. Kind of soothing waveforms floating slowly
as waves over the scope.
> The tradeoff is that the mixer output at higher offset
> frequencies is attenuated by the IF port termination.
> Terminating the IF port in a capacitor reduces the RF port impedance, so
> that a low value series resistor (22 to 39 ohms - select for lowest
> VSWR) is then required to improve the RF port VSWR.
Which in improves phase-stability as reflected waves has less impact.
-3 dB pads have also been used by the good folks over at NIST at one time.
> Terminating the IF port with a capacitor also alters the mixer gain (as
> a phase detector) so this needs to be measured in conjunction with the noise
Well... the normal 50 Ohm loading alters the mixer gain... not the cap.
But since the normation is towards 50 Ohm... ah well...
> 10514 and 10534 mixers using discrete diodes supposedly have lower
> flicker noise than mixers using integrated quad diodes.
They however has common ground in their every day laboratory variants.
There exist variants meant for production. However, they are not made
out of that exquisite components, so their performance should be
replicable, as have been pointed out before.
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