[time-nuts] DMTD - analog multiplier vs. diode mixer ?

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jan 9 17:01:31 EST 2016


Attila,

On 01/09/2016 09:25 PM, Attila Kinali wrote:
> On Sat, 9 Jan 2016 10:19:05 +0800
> Li Ang <lllaaa at gmail.com> wrote:
>
>>      In some article, I see people use a D-flipflop to sample the input
>> signal with reference clock. When you want implement a mixer what's the
>> difference between D-flipflop and XOR gate?
>
> A D-Flipflop is a rather weird mixer. I have not done the calculation,
> but i'm pretty sure that the output is not exactly what you'd expect
> it from a normal mixer (namely having half the energy at the frequeny
> difference and half at the sum).

It's not that wierd. It's a sampler, and thus it acts like a mixer as if 
the signal is spikes, which is just another interpretation of the 
Nyquist frequency aliasing. Meta-stability however creates an 
"interesting" aspect.

> An XOR gate on the other hand, produces a very nice spectrum, given
> you input two clean square wave signals.

Indeed.

An interesting variant of the XOR gate as being used as a mixer is when 
you build a rubidium. One synthesis approach being used is to divide the 
5 MHz OCXO signal with 16 to get 312,5 kHz. Then XORing it with 5 MHz 
produces as one of it's mirror signals 5,3125 MHz which is then fed with 
a step-up signal of 60 MHz or 90 MHz into the SDR diode in the cavity.

A third digital phase-detector is the SR flip-flop. It avoids the 180 
degree phase property (really a triangle wave signal) of the XOR, but 
give a 360 degree phase sawtooth. This can be helpful in certain lock-up 
conditions.

The phase-frequency detector of the 4046 and the like has additional 
flip-flops to remember slipped cycles and forcing the frequency to 
regain that. Those provide a strong frequency lock mechanism with a 
phase detector in one.

>>      When you refer high speed CMOS XOR gate, do you mean 74LVC1G86?
>
> Generally speaking: Faster CMOS better than slower CMOS in terms of phase noise.
> (Though, I have yet to see actual measurements of this)
>
> Single gate chips better than multi gate chips.
> (no interference through the power supply of the different sub-parts)

Well, you should wire the other parts into passive mode.

Cheers,
Magnus



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