[time-nuts] Re: Low 1/f transistors for oscillator

[Magnus Danielson]

Hi Rick!

On 2023-01-17 02:12, Richard Karlquist via time-nuts wrote:
> Generally speaking, the 1/f noise at, say 10 Hz, for a crystal oscillator is determined entirely by the intrinsic noise of the crystal.  It's really hard to screw up an oscillator circuit so badly that it actually contributes anything to the oscillator 1/f noise.
>
> For instance, I'm sure we will all agree that the HP 10811 is a pretty decent oscillator.
>
> FYI, essentially the same crystal is used in the E1938A that I designed. Those two oscillators have ENTIRELY different circuits.  Yet both achieve the same close in phase noise as what I measured for the intrinsic noise of the crystal. By the way, the HP 10811 uses a selected 2N5179 transistor.  But is NOT SELECTED FOR 1/f NOISE.  Instead it is selected for good gain at high DC collector current, because the HP engineer who designed it had trouble with the oscillator starting up correctly, in conjunction with the ALC circuit.
>
> If you use the same output circuit for your oscillator as the 10811, you will also have no oscillator transistor contribution to far out phase noise. This is all very well documented if you want to read the details.
>
> In a previous life, I designed many many 5th overtone oscillators.  Your crystal like the Croven ones I used in those days is not of the same caliber as the 10811 crystal, so therefore it is even more unlikely that the transistor you choose is going to make any difference.  I always locked my 5th OT crystals to something like a 10544 oscillator to clean them up close in using PLL synthesizers.  The 10811 hadn't been invented yet.

Good point raised here! The clean-up lock to solve close-in noise for 
higher frequencies I think some may not have considered. As one has an 
oscillator at higher frequency, the same loaded Q of the resonator will 
mean higher cut-over corner for the noise. To keep 10 Hz or so to be 
quiet, lock the high frequency oscillator to a lower frequency 
oscillator with a narrower noise, using PLL. For odd frequencies 
synthesis may be needed also. The PLL will high-pass filter the locked 
oscillators close-in noise while lowpass the reference oscillators 
close-in, so a nice cross-over.

I think that is a much more reasonable approach than trying to cram 
everything into one signle oscillator.

> I hope this gets you back on the right track.

Very good read. Thanks for sharing!

Cheers,
Magnus

>
> ---
> Rick Karlquist
> N6RK
>
> On 2023-01-16 14:26, glen english LIST via time-nuts wrote:
>
>> Hi
>>
>> I am embarking on building up a block - a common collector Butler oscillator with an overtone  crystal at 98.304 MHz
>>
>> on on FR4 , I've used the Philips NPN type  BFS17.
>>
>> The results <=10 Hz phase noise were quite disappointing. (-69 @ 98 MHz)
>>
>> Does anyone have any good suggestions. Maybe something slower (larger) like a 2N5179 (or its SMT counterpart MMBT5179) will be better.
>>
>> regards
>>
>> glen
>>
>> On 13/12/2022 11:14 am, Lux, Jim via time-nuts wrote:
>>
>>> On 12/12/22 2:43 PM, Alex Pumm
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