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

[Bob Camp]

Hi

Implementing that circuit without using a hybrid would be a bit of a challenge. 

Bob


On Feb 10, 2010, at 11:30 PM, Bruce Griffiths wrote:

> Clay
> 
> Circuit schematic for a more recent JPL isolation amplifier design is attached.
> 
> Bruce
> 
> 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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