[time-nuts] Stepping up the output of an OCXO
didier at cox.net
Fri Feb 2 14:45:29 EST 2007
---- 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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