[time-nuts] HP10514B Mixer Terminations

Sun Sep 9 23:00:36 UTC 2012

In the late 60s I built a VLF upconverter using a ring mixer.
I tried a few different devices for the diodes.
The base/collector junctions of germanium switching transistors
gave the best results.

On 09/09/2012 03:21 PM, Bruce Griffiths wrote:
> NIST have indicated that mixer PN noise measurements with a non 
> dissipative terminations (even RF) are intended to be made.
>
> Using a discrete mixer using diode connected transistors may also be 
> useful at least for 5MHz and 10MHz input frequencies in that their 
> flicker noise can be significantly lower than for mixers using diodes. 
> NIST used a simple diplexer arrangement that terminates the RF sum 
> frequency in 50 ohms whilst using a reactive termination for the 
> difference frequency.
> With such a termination the minicircuits phase detectors had lower 
> measured PN noise than the 10514.
>
> Another issue to consider is saturated or unsaturated (linear) 
> operation of the mixer.
> Saturated operation, for most mixers, produces lower  noise (flicker 
> and floor) however linear operation can have for some mixers lower 
> phase shift tempco which can be important depending on the environment 
> and required phase shift stability.
>
> Bruce
>
> Bob Camp wrote:
>> 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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>
>
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