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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