[time-nuts] how to find low noise transistors

[KA2WEU at aol.com]

Good questions , I am looking forward to the answers, this is  my area of 
work ... Ulrich N1UL 
 
 
In a message dated 7/28/2015 1:45:55 P.M. Eastern Daylight Time,  
dk4xp at arcor.de writes:

Am  20.07.2015 um 01:57 schrieb KA2WEU--- via time-nuts:
> Good evening,  this turns out to be a good discussion...
>   
> Any  more inputs ?  73 de Ulrich
>   
>

1. To get  a gut feeling about the virtues of nonlinear noise simulation: 
how much  phase noise will we typically lose if we stay with linear 
simulation? I  mean, we have been told so often how important it is that 
the amplifier  offers a constant (low) impedance to the crystal and that 
the smallest  nonlinearity would be an invitation to noise up conversion. 
It does not  take a lot of conversion gain when one looks at -150 dBc. 
So, even if we  use a HB simulator, the DUT will have to be pretty linear.


2. What  do you consider the optimum AGC for, say, a Driscoll or Butler 
at 100 MHz?  In my current work, most of the logic is triple module 
redundant and the  oscillator is a single point of failure. Stopping 
oscillation at an EFC  extreme would be a nightmare, but phase noise 
performance still cannot be  ignored.


3. Is there any work on AGC vs. post tuning  drift?


4. In [1] there are is some treatment about removing 1/f  noise of a RF 
transistor by active LF feedback. It is applied to a BFR93A  and the 
effect can be seen clearly. There are other faster transistors  that 
would need that much more urgently, and for > 40 dB of 1/f noise  
probably more loop gain would be required. I can see a place for an  
ADA4898 here… Also, there are 1K resistors in the bases of both the RF  
and the AF transistors while we are discussing here replicating the  
transistors to shrink the effective base spreading resistance. It seems  
that the improvement could be much larger.


BTW I got -145dBc  @100MHz @100Hz with mass production BFR93 transistors 
in Butlers, and the  limit seemed to be ONLY the crystal; most crystals 
were much worse, even  when they had comparable parameters and were from 
the same  batch.


5. One must always find a balance between optimum close-in  or far-out 
noise. The emitter input impedance of a 2 stage Butler  sustaining 
amplifier may serve as an example. Make it small and there will  be only 
a slight operating Q degradation - but less power available to the  input 
of the sustaining amp. with a given crystal current; needing more  gain 
and raising the floor.
Make it larger, and you get less operating  Q and better floor.

Only 10% of a crystal batch seem to provide  excellent close-in noise, 
the others being easily 10 dB worse. These  others are more or less free 
(at least already paid for). They still could  be used as a post-filter 
to shrink the noise floor. It would be necessary  to de-Q them with 
resistors so that they can withstand the power and that  they do not 
spoil the close-in noise.

Or use a bridge xtal filter  that has no crystal resonance on the center 
frequency. That would require  some discipline when tuning the oscillator 
to avoid blowing the filter  crystals. Far out the noise still would 
decrease by 6 dB/oct  Fourier-frequency-wise. 20 dB better makes the 
difference between OK and  excellent.

[1] Rohde/Newkirk: RF/Microwave Circuit Design for Wireless  
Applications, Wiley

very short excerpt for a few days on <  
https://picasaweb.google.com/lh/photo/XUfeAuD8TvNqBOMuJiPtltMTjNZETYmyPJy0li
ipFm0?feat=directlink>

73,  Gerhard, DK4XP
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