[time-nuts] Ultra High Stability Time Base Options for 53132A

Magnus Danielson magnus at rubidium.se
Mon Apr 13 15:14:35 EDT 2015


On 04/13/2015 01:48 AM, Richard (Rick) Karlquist wrote:
> On 4/12/2015 2:22 PM, Magnus Danielson wrote:
>> Hi,
>> The buffer transistors has not AC-bypass of the emitter resistance, so
>> that the DC current becomes large and thus contributes flicker noise.
>> The comparator at the bottom isn't doing a beutifull work of squaring
>> things up without contributing noise, considering the sine output of the
>> 10811.
>> Was that it, Rick?
>> Cheers,
>> Magnus
> The resolution of page 13 is poor, and it seems to be a bitmap instead
> of a vector file.  The fuzzy thing in the lower right corner looks
> like it might be a comparator.  I think this was the smoking gun.

I checked the component listing, which provided very good hints.

> There was a saying by H.L. Menken to the effect that for every
> complex problem, there is a simple, obvious, invalid solution.

Oh yes. Some people say that you should not overcomplex things. My 
experience is that oversimplifying them can cause a long stretch of 
complex problems and complex workarounds making the total solution more 
expensive in development, customer relations and more complex than 
starting with a more advanced solution, that actually attempts to 
address the design issues. Ah well.

> Squaring up a 10811 with a comparator is a perfect example of this
> principle.  Non-time-nuts always seem to gravitate to this design.

Oh, they defend their choice with that they use a schmitt-trigger.

> Of course you're right, any comparator will add jitter to a 10811.
> The faster they are, the more jitter they add.

Indeed. Hello Noise-bandwidth.

> I noticed that the standard 10 MHz oscillator is built with
> an ECL line receiver.  Another example of Menken's saying.
> This is a TERRIBLE oscillator design, but one that would appeal
> to the non-initiated.  I built one of these oscillators in 1976
> at the suggestion of my boss.  After seeing how bad it was, I
> quietly designed it out and never used it again.

Go and check the HP5370A/B reference amplifier board. It has an ECL 
circuit to detect the presence of 10 MHz. It does this by producing a 5 
MHz 25% PWM signal... with ECL... with very good rise-time. ECL have 
never been known for its speed and rise-time (irony might have been 
used). So, it turns out that the board spewes out a wide spectrum of 5 
MHz spikes. All this to drive a LED that goes green if there is a 10 MHz 
to aid the fault analysis once you lift the lid. A bad design. Disabled 
the detector by grounding a base on a transistor, and got a much more 
quiet box. The Motorola ECL handbook warns about the rise-time issue, 
it's a standard signal integrity and EMC issue.


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