Re: [time-nuts] Squaring Tbolt 10Mhz output

[Bruce Griffiths]

One problem that is evident when a simple longtailed pair (differential 
amplifier) is used to convert a sine wave to a square wave is the tilt 
that is evident in the waveform when the output transistor is 
conducting. This is due to feedthrough from the input signal via the 
emitter base capacitance of the input transistor to the emitter of the 
output transistor.
The attached circuit schematic illustrates one classical method of 
minimising this tilt.
The value of C5 is selected/adjusted to achieve nominally zero tilt.
Compensation isn't perfect due to the voltage dependence of the emitter 
base capacitance but the tilt can be significantly reduced,

Using transistors with lower emitter base capacitances also helps as 
does reducing the value of the output collector load resistor.
This requires increasing the tail current to maintain the output swing.
The attached circuit produces a 3.3V pp output signal.
For lower amplitude input signals capacitively coupling the emitters of 
the differential amplifier and splitting the current source so that each 
transistor has its own current source can be useful in minimising the 
effects of Vbe mismatches between the transistors.

Bruce

Charles P. Steinmetz wrote:
James wrote:

I'm trying to square the 10Mhz output from a Thunderbolt GPSDO, and 
failing
miserably. I'm still very new at this so please forgive my ignorance. I
thought it was going to be easy. I have tried diodes, comparators,
op-amps, Schmidt trigger buffers,regular buffers, inverters, and
differential amplifier circuits. I must be missing something simple 
because
i just can't shape the wave into a square wave. Has any one done this?

I concur with Bruce that most of these should work.  More details 
about what you have tried (i.e., schematics), and what results you 
have gotten, will be necessary for us to be able to help you.  (The 
list accepts small attachments.)

Meanwhile, here are two links you can peruse that give several shaping 
circuits (the second link is Bruce's work):

http://www.wenzel.com/documents/waveform.html

http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:clock_shapers

Best regards,

Charles







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