[time-nuts] Close-in phase noise question...

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Dec 16 10:29:18 UTC 2008


Grant

The trouble with most MMICs is that they usually have too much gain and
insufficient reverse isolation for use in an isolation amplifier.
However when the available signal level is low using an MMIC to boost
the signal to around +13dBm or more can be useful.
They can have quite low close in phase noise if the right technology is
employed.
Bipolar MMICs (Si, SiGe, HBT) usually has low close in phase noise (see
Wenzel's sight for some low frequency (5Mhz, 10MHz) MMIC close in phase
noise data).

Wide band voltage feedback opamps like the now obsolete MAX477 don't
usually specify the low frequency noise and have higher phase noise than
a good discrete design.
Some current feedback opamps can have low input voltage noise at low
frequencies, however the relatively high low frequency input current
noise flowing through the feedback resistor more than negates this.
If only it were possible to shunt the feedback resistor with an inductor
whilst maintaining stability the effect of this noise source could be
reduced.

To minimise the effect of opamp input noise on the phase noise, the
signal amplitude at the opamp needs to be large (use an input
transformer to increase the signal amplitude) and the opamp gain kept low.
The dc noise gain of the opamp should be low (ie 1) to keep the close in
phase noise low.

There are very few opamps that are useful at 630MHz.
The few that are available tend to have high flicker noise.

Another approach at higher frequencies is to use feed forward techniques
to reduce amplifier phase noise by largely canceling it.

Bruce

Grant Hodgson wrote:
> Brian
>
> Could you give some more details?  I think alternative solutions to 
> MMICs at 5 and 10MHz would give much lower phase noise.
>
> Bruce's web pages have a number of designs that will get the 5MHz 
> reference up to 20MHz in 2 stages :-
>
> http://www.ko4bb.com/~bruce/FrequencyMultipliers.html
>
> Another approach would be to use op-amps instead of MMICs, op-amps with 
> a low flicker corner frequency should give lower close in noise than a 
> MMIC, but probably won't be as good as a discrete solution.
>
> regards
>
> Grant
>
>   
>> 	
>>
>> Looking for comment here...
>>
>> The background:
>> I'm working on a sub mm-wave LO chain for
>> a ham radio application. While chasing issues
>> of close-in phase (ie: within 1KHz of RF
>> carrier) by peeling the "layers of the onion",
>> I'm starting to question the performance of
>> the MMICs that are used as buffers and amps
>> following my Wenzel reference OCXOs.
>>
>> Question(s):
>> Should any MMIC be allowed to be driven
>> close to compression or into compression
>> when striving for best close-in noise?
>>
>> I know and have seen the NF of a MMIC
>> degrade while in compression, but my
>> target right now is close-in noise rather
>> than broadband noise.
>>
>> My design, in summary, takes 5MHz up to 630GHz
>> via several multipliers and PLL stages.
>>
>> -Brian
>>     
>
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