Rick,
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.
*facepalm*
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.
Cheers,
Magnus
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