[time-nuts] sine to square wave converter

Charles Steinmetz csteinmetz at yandex.com
Thu Jul 10 23:12:14 EDT 2014


The schematic of the converter I attached to my previous message did 
not make it through in my copy of the message, so it may be missing 
from other people's copies, as well.  If anyone wants it and did not 
receive it, it is available at:

<http://www.febo.com/pipermail/time-nuts/attachments/20140710/9fb493f1/attachment-0001.jpg>

Best regards,

Charles



>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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