[time-nuts] Advice on 10 MHz isolation/distribution amplifier
lists at cq.nu
Thu Feb 11 03:10:36 UTC 2010
Is your OCXO vibration isolated?
If not and it's got "typical" g sensitivity, your phase noise in an aircraft
may be much worse than the static numbers.
If you are sending the signal a distance to your systems, a balanced feed
may be the only way you will deliver a clean signal at the far end.
From: "life speed" <life_speed at yahoo.com>
Sent: Wednesday, February 10, 2010 10:03 PM
To: <time-nuts at febo.com>
Subject: Re: [time-nuts] Advice on 10 MHz isolation/distribution amplifier
> 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
> 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?
> 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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