[time-nuts] Stepping up the output of an OCXO

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Feb 4 02:32:04 EST 2007

John Ackermann N8UR wrote:
> I had a chance to measure a TADD-1 using an HP-3048 phase noise system
> last year.  I've attached a screen shot of the results; in short it was
> below -140dBc/Hz from 100 Hz on out (by the way, I'm not sure I would
> trust the noise floor shown in this test; I am not certain we had the
> best test set-up).  That's within a few dB of what HP said their 5087A
> amplifier would do.
> I'll soon have access to a new phase noise measurement system and will
> relook at the TADD-1 performance with it.
> John

My calculations using the component values in the circuit indicate that 
the TADD-1 noise figure should be about 19dB and its corresponding phase 
noise floor should be lower than -160dBc/Hz. when the input signal is 
+7dBm or greater. So unless you had an unusually noisy set of opamps 
and/or resistors the measurement setup phase noise floor probably 
exceeds that of the TADD-1 by around 20dB.

With a 10V supply instead of 9V and a +13dBm input the phase noise floor 
should be about -168dBc/Hz which is respectable but a few (~10) dB shy 
of the state of the art.

The only way to achieve the state of the art performance with opamps 
using a 9 or 10V supply is to use lower noise (2nV/rtHz appears 
feasible) opamps pairs driven in push pull by an input step up (1:2 
turns ratio) transformer with an output stepdown transformer driven in 
pushpull by the 2 opamp outputs. The noisy 1K input protection resistor 
would also have to be reduced in value as would the opamp feedback 
network resistors.

NIST's 1995 distribution amplifier claims a phase tempco of 0.3ps/K and 
120dB isolation between channels.


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