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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Feb 11 01:48:49 UTC 2010


life speed wrote:
> Hello everyone,
>
> I am new to this list, happened across it while searching on distribution amplifiers.
>
> I need to design a 10 MHz isolation/distribution amplifier with two outputs for a high-vibration wide temperature range environment.  I was considering using a design based on the NIST article 'A new 5 and 10 MHz distribution amp', most likely the third incarnation, as I cannot tolerate modulation of the signal via the transformer.
>
> I have looked at the work by Gerhard Hoffman, which is impressive.  However, the transformer remains.  I should mention I am also limited to 15V power supply, and need to output into 50 ohms at 10 dBm.  Ideally DC current will be minimized, preferably 20 - 30 mA per channel.  I can generate a negative power supply if necessary, but would prefer not to.
>
> If anybody can offer tips so I don't undertake to reinvent the wheel, it would be appreciated.
>
> Thanks in advance,
>   - Lifespeed
>    
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





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