[time-nuts] HP10514B Mixer Terminations

[Bob Camp]

Hi

In general, you terminate the mixer in 50 ohms at the RF frequencies (say 10 and 20 MHz). Termination at the "IF" (in this case audio) frequencies is what turns out to be tricky. Any time you terminate a source in a high impedance, you get a higher output voltage. Reactance rarely adds noise to a termination. With a DMTD, slew rate is the issue, generally mixer noise goes up less than the slew rate increases with a "non-50 ohm" termination. 

Bob 



On Sep 9, 2012, at 4:41 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> David Kirkby wrote:
>> On 9 September 2012 18:28, Pascual Arbona<p.arbona at securimar.com>  wrote:
>>   
>>> 
>>>             Hello Brian,
>>>              I am a radio amateur and and also in the Time nuts list,  At the moment I am planning to bild a DMTD for experimentation. and as you have a nice experience in this field , for me will be very wellcame your help. My ask is ¿whitch is te best temination for the mixer? (FI 10Hz or 100Hz)  ¿what about the amp, limiters and zero crosing detec.)
>>>     
>> Mixers should be terminated in 50 Ohms - at least at all frequencies
>> where there is an output. Someone mentioned minicircuits. They sell
>> constant impedance filters, where the impedance in the both the
>> passband and stopband are 50 Ohms. Most filters have a impedance far
>> away from 50 Ohms in the stopband.
>> 
>>   
> This is an often repeated fallacy.
> For low IF frequencies such as in a DMTD, a reactive IF termination can have the advantage of lower noise and larger IF signal slew rate.
> The resultant reduction in RF and LO port VSWR is easily corrected (at least for low RF and LO frequencies) with a series resistor and/or resistive pad.
> There are a number of NIST papers on the effect of mixer termination for DMTDs and phase noise measurements.
> The minicircuits phase detectors (actually specialised mixers) are specified for use with 500 ohm IF terminations.
>> I don't know the details of what you are trying to do, but keep in
>> mind what I said.
>> 
>> Dave
>> 
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>>   
> Bruce
> 
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