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

Bob Camp lists at cq.nu
Thu Feb 11 03:10:36 UTC 2010


Hi

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.

Bob

--------------------------------------------------
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
> 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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