[time-nuts] Advice on 10 MHz isolation/distribution amplifier
Bob Camp
lists at cq.nu
Thu Feb 11 12:54:40 UTC 2010
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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>
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