[time-nuts] Advice on 10 MHz isolation/distribution amplifier

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Feb 10 23:30:09 EST 2010


Circuit schematic for a more recent JPL isolation amplifier design is 


life speed wrote:
> Avoiding transformers and inductors will make it virtually impossible to
> achieve very low phase noise as the dc gain from say the base of any
> transistor in the chain to the output will degrade the flicker phase
> noise. Using transformers or using an inductor to shunt any collector
> resistors reduces the flicker phase modulation to low levels.
> JPL in the past has built capacitively coupled complementary symmetry
> isolation amplifiers that avoid transformers but suffer from dc loop
> gains of around 3 or so.
> Using complementary symmetry can be a good way of keeping the dc current
> down.
> How much reverse isolation do you need?
> How low does the phase noise floor need to be?
> What about flicker phase noise, how low does that need to be?
> Bruce
> Right, what do I really need? I only have a really good 10 MHz OCXO crystal oscillator to distribute, so about -120 dBc at 10 Hz, -140 dBc/Hz at 100 Hz, - 150 dBc/Hz at 1KHz, and -155 dBc/Hz noise floor.  No maser or cesium clock, living in the world of practical realities here.  Of course I would like to be 3 - 6 dB better than the OCXO numbers.
> Reverse isolation is my primary interest in the distribution amplifier approach, although the OCXO is good enough that a sloppy approach could contaminate the phase noise also.  I would like to accomplish at least 100 dB reverse isolation at frequencies below 20 MHz, but more is better in this case.  The 10 MHz is running all over a noisy aircraft, to potentially noisy receivers.
> In reading up on the subject, I have come to understand that DC gain is the bane of close-in phase noise.  Given that flicker noise is such a headache for we frequency synthesizer designers, I guess this should come as no surprise.
> Clay (AKA Lifespeed)
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