[time-nuts] RF mixers for oscillator characterization, some questions
bruce.griffiths at xtra.co.nz
Mon Apr 6 23:40:41 UTC 2009
Magnus Danielson wrote:
> 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
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>> 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 effect of reflecting the sum frequency back into the mixer is
documented in some Watkins Johnson and HP/Agilent appliction notes.
It can be very effective even at microwave frequencies.
>> 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.
The best combination is a series resistor plus an attenuator pad.
>> 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.
The B versions intended for through hole PCB mount have separate grounds
for all ports.
These don't seem to be as widely available as they once were although I
have an HP10534B one.
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