[time-nuts] Stepping up the output of an OCXO
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Fri Feb 2 17:27:22 EST 2007
Didier Juges wrote:
> ---- Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
>> The RF amp IC's generally have inferior noise figures and reverse
>> isolation (20dB for RFIC, > 40dB for common base stage) than a well
>> designed discrete common base amplifier. In fact by stacking common base
>> amplifiers the reverse isolation can be made very high, at least at low
>> frequencies like 5MHz or 10MHz, without a severe impact on the noise
>> figure. HP used a dc coupled common base + common gate + common base
>> cascade in their 8554 RF signal generator.
>> NBS used cascaded common base stages in their high (120dB) reverse
>> isolation amplifiers.
>> If an OCXO buffer amplifier has poor reverse isolation then a variable
>> load impedance will have a measurable effect on the OCXO frequency.
> In cases where the output signal does not need to be a sinewave, how would a common base amplifier compare to a fast comparator and if necessary a digital buffer as necessary to deliver the necessary power level? If this is designed to drive a mixer, a square wave may actually be better than a sine wave (lower conversion loss), and I would venture that the isolation (at least load independence) in most comparators is much better than that of a monolithic amplifier.
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In general comparators tend to have excessive phase noise floors and
will degrade the phase noise of the mixer system.
However if the RF signal phase noise floor is already high this may not
There is little data on the phase noise of modern comparators, it would
be useful if someone actually made some measurements of this both for
comparators and for simpler devices used to square up sinewaves such as
long tailed pairs and logic gates.
However I would expect a comparator to be somewhere around 20dB or more
worse than a common base stage, or for that matter any amplifier
employing RF negative feedback.
Its not the choice of a monolithic implementation rather than a discrete
component implementation that limits the reverse isolation but rather
the circuit topology.
A series shunt feedback amplifier as used in most RFIC amplifiers has
inherently poor reverse isolation. Other factors such as the inductance
of bond wires and mutual coupling between them can also be a factor
especially at high frequencies.
If one cascades enough stages with relatively low reverse isolation one
can, at the expense of increased phase noise, increase the effective
isolation between the input and output of the entire circuit. This is
what can give multistage devices like comparators reasonably high
reverse isolation. However one must be mindful of the effects of the PCB
layout. For example a hybrid LM733 (100x 100MHz diff amp) cascaded with
a AMD685 ECL comparator had problems with coupling between the amplifier
input and comparator output until a closely spaced grounded shield plate
was mounted above the pins. The effect was subtle in that it distorted
the expected Gaussian shape of the plot of output pulse rate versus
comparator threshold with zero input. The circuit was triggering on the
amplifiers output noise.
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