[time-nuts] Spectrum Analyzer Specifications --> phase noise test set

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Mar 25 16:04:28 EDT 2016


On Friday, March 25, 2016 10:55:59 AM jimlux wrote:
> On 3/25/16 5:07 AM, Bob Camp wrote:
> > Hi
> > 
> > The reverse isolation issue is indeed one of the weaknesses of this setup.
> > For testing OCXO’s isolation is not a big deal. A normal OCOX has very
> > good output buffering to give it the stability you are after. If you are
> > running (maybe) a VCO with no buffering, that assumption falls apart. The
> > VCO will  / can injection lock through the mixer. In that case you *do*
> > need an amp to provide enough isolation to prevent the injection lock.
> But if someone were building a little module for a cheap and cheerful
> noise analyzer, then the buffer amp would be a separate module.
> 
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Yes that way one can tailor the amp gain and/or operating frequency to the 
task at hand.
As a general guide for amps employing 1-2 transistors if a reverse isolation 
of 40dB or more is required:

For a gain of 0-3dB a CB stage offers the lowest noise.
For higher gain and/or output push pull CB stages  are required.

For a gain of 3-10dB a series shunt feedback amplifier can be effective 
particularly if the output is taken from a transformer connected between the 
collector and the shunt feedback resistor. This configuration is useful in that 
one can employ the same output transformer for a wide range of gains.
The gain is changed by changing the value of a pair of resistors.

HP used discrete series shunt feedback stages in some of their PN measurement 
gear. However these suffered from relatively poor close in PN which can be 
cured by substituting a lower noise biasing system.

As long as the amp uses low noise biasing and low noise supplies together with 
sufficient  RF negative feedback then its flicker noise wont be an issue.

With a cross correlation system employing independent amps for each channel an 
amp with a noise figure < 6dB will usually suffice in that the added noise 
penalty can be traded off against a longer averaging time to reduce the system 
PN noise.

Bruce


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