[time-nuts] homebrew 13 dBm distribution amplifier based on NIST design 5 to 100 MHz

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Sep 25 04:45:01 UTC 2008


John
>> It (LTSpice) has some severe limitations for most of the simulations I
>>     
> have done.
>
> You might bring those up with Mike Engelhardt (the author).  He doesn't miss
> many tricks.
>
>   

>> These need to be supplemented with on board filtering as they aren't
>> quite as quiet as you need.
>> Either the NIST style darlington buffered RC low pass filter (one per
>> amplifier) and/or a modified (stabilises the shunt transistor re by
>> making its collector current approximately PTAT) version of Wenzel's
>> active power supply noise filter can be used.
>>     
>
> The Jung article at Gerhard's link claims 3 nv/root-Hz at 1 kHz.  Wenzel's
> page claims 20 nv/root-Hz at 1 kHz.  What figures would be expected from the
> modified version you're talking about?
>
>   
The amplifiers can couple via the power supply impedance if one isnt 
careful.
When one is trying to achieve crosstalk below -100dB this can be an 
important path.

The transistor version of Wenzels circuit (without the opamp) is much 
better than that.
The real issue is the noise below 1kHz.
Extending the low frequency cutoff sufficiently low is difficult but not 
quite impossible.
>> You can easily measure the phase noise for low offset frequencies using
>> a low noise mixer with appropriate (not 50 ohm) IF termination followed
>> by a low noise (audio frequency) preamp driving a sound card. A 24 bit
>> sound card is ideal, however 16 bit sound cards just need a little more
>> preamp gain. No need for a PLL just split the output of a low noise OCXO
>> or similar source drive the mixer LO port with one output and the
>> isolation amplifier with the other whilst the isolation amplifier output
>> drives the mixer RF port. You will need to adjust the phasing between
>> the LO and Rf signals so that they are approximately in quadrature by
>> using a suitable length of coax or other means. You can even take
>> advantage of the 2 channel (stereo) sound card inputs to do get well
>> below the mixer noise and/or sound card noise floor by using cross
>> correlation techniques.
>>     
>
> What's the current thinking re: FFT window functions for noise measurement?
> Does it matter what you use, as long as the window's equivalent noise
> bandwidth is factored in?
>
>   
Yes it does matter particularly at low offset frequency where the noise 
bandwidth of the window approaches the offset and the noise process is 
sufficiently divergent.
At higher offset frequencies the window function is much less critical see:

http://tf.nist.gov/timefreq/general/pdf/112.pdf
> -- john, KE5FX
>
>
>   
Bruce






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