[time-nuts] sine to square wave converter

[EWKehren at aol.com]

Bruce, 
please make those tests it will help me to how to proceed and probably many 
 other time nuts.
Bert Kehren
 
 
In a message dated 7/11/2014 3:50:36 A.M. Eastern Daylight Time,  
bruce at ko4bb.com writes:

The  Achilles heel of that biasing technique is that the emitter currents 
of  the
pair of pnp's is affected by the noise on the 20V supply.
The 20V  supply noise is only attenuated by a factor of 5 or so when  both
transistors have equal collector currents.

I have both an LTC  evaluation board for the LTC6957 and a Holzworth sine 
to CMOS
converter as  well as a Timepod so I could measure the phase noise of both 
of
these for  various 10MHz input signal levels.

Bruce

> On July 10, 2014  at 8:21 PM Charles Steinmetz <csteinmetz at yandex.com>  
wrote:
>
>
> Bruce wrote:
>
> >Currently  Linear Technology's sine to square wave devices with 
selectable
>  >filtering (LTC6957 series) are better in that they are a closer
>  >approximation to
> >the ideal zero crossing detector.
>  >Failing that the next best is perhaps an AC coupled (both at input
>  >and between
> >emitters) differential pair of 2N3906's or  similar.
>
> My initial results with the LTC6957 did not produce  lower phase noise
> at 10MHz than an optimized Wenzel two-PNP circuit  (it may be possible
> to do better than my initial experiments with the  6957).
>
> Here is the circuit I use:
>
>  Emacs!
>
>
> Using a 20v supply reduces the input  feedthrough due to Q1's B-E
> capacitance, which tends to give the  output square wave a sloping top.
>
> Using MPSH81s rather than  2N3906s helps with feedthrough, also, as
> well as reducing the rise and  fall times (both about 2-4 nS with this
> circuit, depending on how hard  it is driven, if it is built with
> proper attention to layout and stray  capacitance).
>
> Some will insist that the LM329 is overkill, but  the base bias can be
> a significant source (even the dominant source)  of phase
> noise/jitter. The stability and low noise of the 329  improve
> performance materially -- even a TL431 or 1N829 is  measurably
> inferior. An LM399 is somewhat better than the 329, but I  have not
> found it necessary in practice. (Note that the pullup  resistor is
> not shown -- 1.5k to 10k metal film from the 329 to +20v,  not critical.)
>
> Some additional improvement can be achieved by  using the PNP devices
> in an HFA3096 or HFA3128 array, but I have  generally not seen the
> need for this in practic. As drawn, this  circuit has lower residual
> PN than any 10MHz oscillator I have  measured.
>
> Works best with input levels from 1 to 10Vpp (350mV  to 3.5Vrms sine
> wave). There is a small duty cycle asymmetry (high  longer than low),
> which depends on drive level. Using faster devices  (such as HFA3096
> or HFA3128) reduces the asymmetry. If this is a  problem, a resistor
> can be added from the base of Q1 to ground to trim  out the asymmetry
> if the input level is well controlled. Otherwise,  the mean output
> voltage can be detected, compared to a reference, and  used to adjust
> either base voltage with a servo loop.
>
>  Best regards,
>
> Charles
>
>
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