[time-nuts] Stepping up the output of an OCXO

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Feb 2 18:29:56 EST 2007

Ulrich Bangert wrote:
> Bruce and Didier,
> i had the opportunity to measure the output to output as well as the
> output to input isolation of a diy MAX477 based distribution amplifier
> very similar to the TADD-1 (but not identical). The output to output
> isolation was in the order of 75 dB while the output to input isolation
> was in the order of 90 dB. This is not too bad for this simple design
> and perhaps more than adequate for most of us time nuts. I have not been
> able to measure the increase in terms of noise figure but this statement
> must be read as: The increase in noise figure was less than the noise
> floor of my equipment. Nevertheless my statement of beliefs is the same
> as Bruce's one: A single transistor stage may be more easily optimized
> than a multi-transistor-design.
> Best regards
> Ulrich Bangert   


Since the MAX477 has an input noise of around 5nV/rtHz at 10MHz compared 
to around 1nV/rtHz for a low noise bipolar transistor, the MAX477 may be 
expected to be have a phase noise floor of up to 14dB higher than that 
of an optimised discrete transistor amplifier. This however may not be a 
concern unless one has an OCXO with a phase noise floor approaching the 
state of the art (-180dBc/Hz).

There are wideband opamps with lower noise than the MAX477 available.
One could always use 4 MAX 477s inparallel to lower the phase noise 
floor by about 6dB.


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