[time-nuts] DMTD using Active Mixers

Mario Sanchez mfsdlr at gmail.com
Tue Nov 24 16:35:51 UTC 2009


Hi Bruce...

Thanks for your answer...

I have a couple of questions:

1. I am using a 1kHz beat frequency, and measure back2back period using a
Pendulum CNT-91 high resolution frequency counter. Does the DC stability of
the mixer still matter?

2. I have been comparing the AD831 with some passive mixer, but i can not
see any great improvement. I am basically following this procedure:
a. As I just want to compare the mixers, using only one of them I mix 10MHz
from a Rb with 10MHz+1kHz generated by a R&S signal generator referenced to
the same Rb.
For passive mixer, I amplify before the mixer, I have also tested different
input levels at both LO and RF signal.
b. Filter the high component.
c. ZCD - Hard limitter, using Collins-type amp.
d. Calculate ADEV at 1s.

Any approach on how to test the mixer?


My goal is to reach an accuracy of 5E-14 @ 1s


Regards,
Mario


On Thu, Nov 12, 2009 at 10:35 AM, Bruce Griffiths <
bruce.griffiths at xtra.co.nz> wrote:

> Mario
>
> No there is no diode mixer in the AD831,  it uses a conventional Gilbert
> cell mixer with preceding LO and RF amplifiers.
> Compared to a diode mixer it has high flicker noise and poor dc stability.
>
> Bruce
>
>
> Mario Sanchez wrote:
>
>> Thanks for your inputs...
>>
>> I dont know exactly which are the microcircuits ActiveMixers you refer to;
>> just to make it clear, the IC that I am using is Analog Devices AD831,
>> which
>> basically has a conventional diode mixer, a limiter amplifier in the LO
>> input and a low noise output amplifier.
>>
>> Regards,
>> Mario
>>
>>
>> On Thu, Nov 12, 2009 at 1:39 AM, Magnus Danielson<
>> magnus at rubidium.dyndns.org>  wrote:
>>
>>
>>
>>> Bruce Griffiths wrote:
>>>
>>>
>>>
>>>> Hej Magnus
>>>>
>>>> To confuse matters Minicircuits use the term active mixer for a
>>>> conventional diode mixer that uses amplifiers on the LO and/or RF ports
>>>> to
>>>> boost signal levels.
>>>>
>>>>
>>>>
>>> While it confuses matters, it is usefull to know about them, since they
>>> may
>>> be good for some designs, but maybe not for DMTD designs.
>>>
>>> I have argued that keeping a buffer-amplifiers (not to be confused with
>>> the
>>> -120 dB isolational amp) just at the mixer will relief the issues of VSWR
>>> to
>>> tempco. Also, getting that extra boost is also good. Needs to have
>>> meaningfull phasenoise to be usefull thought.
>>>
>>>
>>>  Whereas I was referring to mixers like gilbert cell mixers as active
>>>
>>>
>>>> mixers.
>>>>
>>>>
>>>>
>>> I was making the same distinction. Passive JFET mixers is just another
>>> variant to using schottkydiodes.
>>>
>>>
>>>  With the Minicircuits so called active mixers one needs to measure their
>>>
>>>
>>>>  phase noise in order to make meaningful comparisons.
>>>>
>>>>
>>>>
>>> Indeed. Their target is probably not considering cutting edge DMTD
>>> systems.
>>>
>>> Cheers,
>>> Magnus
>>>
>>>
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