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

[Magnus Danielson]

Bruce Griffiths skrev:
> Hej Magnus
> 
> Magnus Danielson wrote:
>> Hej Arnold,
>>
>> Arnold Tibus skrev:
>>> Bruce,
>>> a very good summary!
>>> I do not know where I can get the proposed MinCircuits DBM here, I
>>> looked to all the big and international well known supplier of
>>> electronics parts with negativ result.
>>> If I have to import them from USA they could end up even above 100 $
>>> ea. we have in Europe to take the high shipment costs and the 19% VAT
>>> into account.  
>> It was not too hard to find their official list of representatives.
>> That is what I usually do besides the top catalog firms.
>>
>> Prices does not seem too bad...
>> http://www.minicircuits.com/cgi-bin/modelsearch?search_type=model&model=ZP-10514&tb_no=
>>
>>
>>> Is it likely to get used mixers with damaged diodes?
>>>
>>> Did somebody alredy look inside the metal case of the 10514A?
>> You made me remember I needed too....
>>
>>>  Will it perhaps possible to reconnect the common ground connections
>>> such a way that the mentioned loops can be opened?
>> Yes. The grounds is separated on the PCB, so it is only on the front
>> plate that they actually interconnect when the BNCs screw tight.
>>
>> What's inside then?
>>
>> Well, there is 4 small torroid cores (as compared to the 10534A which
>> has only two cores). It also has 4 diodes (marked as HP 82) and 2 50
>> Ohm resistors.
>>
>> The L and R ports have an isolational transformer each, with the 50
>> Ohm resistor loading on the secondary side. A inner transformer then
>> follows, with two secondaries, hooked up like expected for a double
>> balanced mixer. Essentially this is a double balanced mixer with
>> additional isolational transformers and 50 Ohms terminations.
>>
> Its a little strange to terminate a mixer RF or LO port with 50 ohms to
> ground as usually the IF port termination largely determines (together
> with diode on impedance) the input impedance seen at the RF and LO ports.

I was also reacting to this...

> Are you sure that the first transformer isnt a splitter with the 50 ohm
> termination being the internal splitter termination?

Yes. But looking more closely on them, they look like RF chokes, such 
that common mode is suppressed. Having an RF choke there, loading it 
with a resistor seems reasonable, as it needs to be loaded even for 
lower frequencies to be useful.

Cheers,
Magnus