[time-nuts] Stepping up the output of an OCXO
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Fri Feb 2 07:15:26 EST 2007
Stephan Sandenbergh wrote:
> Hi Bruce,
> A great many thanks for all the hints and tips you gave me.
> Yes, I have actually started working my way through Wenzel's hints & tips
> pages after I posted the first message. I remembered that you pointed me
> there in a previous conversation we had.
> I find the schematic you attached in your last post very interesting - I
> happen to have a soft spot for discrete analog solutions that can still beat
> their integrated counterparts.
> However, I still tend to lean towards the "mini-circuits" type
> attenuator/Gain block idea. The reasons are all the reasons why everyone is
> leaning toward integrated solutions. (e.g. the integrated solutions might
> cost a bit more, but it uses a lot less board space; if a unit fails you
> simply replace it with another one, less components, etc, etc.)
> Nevertheless, I noted you use the word 'must' as in: if you must use the
> mini-circuits.... So I suspect there might be a down-side to using this kind
> of approach. Or is it just that the discrete solution works somewhat better?
> Do you mind elaborating a bit on this?
> Stephan Sandenbergh
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
If an OCXO buffer amplifier has poor reverse isolation then a variable
load impedance will have a measurable effect on the OCXO frequency.
It is easy to tailor the gain of a discrete amplifier to suit the
application without degrading the noise figure by using an input
attenuator as is required by an RFIC.
RF IC amplifiers also tend to misbehave (saturate) when the load is high
impedance such as when one attempts to drive a logic gate input whereas
it is easy to design a discrete amplifier to work into either an open
circuit or short circuit load without distorting the waveform whether it
is the voltage waveform for an open circuit load or current waveform for
a short circuit load. Such ICs are not used in OCXOs because of their
inferior performance and the fact that their dc power supply currents
tend to be somewhat larger than that of a well designed discrete amplifier.
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