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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Feb 11 08:39:56 UTC 2010


If the first stage has a voltage gain of 2x then the total dc current 
can be reduced as the output stages no longer need to drive the feedback 
100 ohm resistor connected to ground at RF. The attached circuit 
schematic also includes faster input transistors in each 3 transistor 
feedback circuit to improve stability and increase the reverse isolation 


Bruce Griffiths wrote:
> Clay
> You could try something like the attached circuit schematic.
> Austron used buffer amplifiers like this albeit without the 
> complementary symmetry output stage.
> There are no transformers and the dc gain is low.
> Simulated reverse isolation at 10MHz is around 120dB.
> Simulated crosstalk between the 2 outputs is around -100dB at 10MHz.
> The transistor models used usually predict reverse isolation 
> reasonably accurately at 10MHz.
> The phase noise floor should be around -170dBc/Hz or less at 100kHz 
> offset.
> V1 is the input signal.
> The 50 ohm sources  V6, V7 shown at the outputs are used for 
> simulation purposes (reverse isolation and crosstalk).
> Off course, more elaborate power supply decoupling will be necessary 
> to avoid degrading reverse isolation and crosstalk.
> If you are really desperate to reduce the dc current the output 
> transistors could be operated in class B.
> However the distortion will increase a little.
> 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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