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

[Ulrich Bangert]

Bruce and Didier,

> > 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.

i had the opportunity to measure the output to output as well as the
output to input isolation of a diy MAX477 based distribution amplifier
very similar to the TADD-1 (but not identical). The output to output
isolation was in the order of 75 dB while the output to input isolation
was in the order of 90 dB. This is not too bad for this simple design
and perhaps more than adequate for most of us time nuts. I have not been
able to measure the increase in terms of noise figure but this statement
must be read as: The increase in noise figure was less than the noise
floor of my equipment. Nevertheless my statement of beliefs is the same
as Bruce's one: A single transistor stage may be more easily optimized
than a multi-transistor-design.

Best regards
Ulrich Bangert   

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Didier Juges
> Gesendet: Freitag, 2. Februar 2007 20:45
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Stepping up the output of an OCXO
> 
> 
> ---- 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.
> > 
> > Bruce
> 
> Bruce,
> 
> 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. 
> 
> Didier
> 
> 
> 
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