[time-nuts] RF mixers for oscillator characterization, some questions

[Arnold Tibus]

Bruce, Magnus, Pete, 

sorry for being late responding, but the Easter Holydays ... 
they are over now :(

Thank you for the informations giving me some more ideas! 

I am still wondering about the specials of these mixers. 
What are their secrets, what the differences to "normal" or high level DBMs? 
Is high level a criteria?

There are some companies selling the 10514A with SMB connectors. 
I have no idea what is a good or reasonable price fore and if I am running 
into possible risks to get deaf ones, for totally around $ 100,- ?

In may 2007 there was aleady a short discours about the 10514A. 
Rick mentioned that today it could be possible to build a considerably better 
design with currently available technology. 

Does it mean it is really possible to build a DBM with currently available 
diodes, transformer cores etc. with similar or better characteristics 
as the old known commercial types? Or did he think to special chip 
designs?
Are there no real merits anymore with the famous old hp 10514A? 

Would be interesting to learn more about.

Arnold


On Tue, 07 Apr 2009 11:40:41 +1200, Bruce Griffiths wrote:

>Magnus

>Magnus Danielson wrote:
>> Bruce Griffiths skrev:
>>   
>>> Pete wrote:
>>>     
>>>> Arnold,
>>>>
>>>> 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 @
>>>> +7dBm.
>>>>
>>>> Pete Rawson
>>>>
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>>>>   
>>>>       
>>> Pete
>>>
>>> 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.

>> Cheers,
>> Magnus
>>
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>>   

>Bruce

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