One problem with this circuit is that tolerances in the resistors
produce an offset between the 2 transistor bases in addition to any
transistor mismatch,
It's better to share a single divider and short (eg a transformer
winding. Or at least connect them with a low impedance at low
frequencies) the transistor bases together.
Capacitively coupling the emitters can also be useful.
Bruce
Robert Darlington wrote:
Just an FYI guys, I'm pretty sure the 3rd circuit down on the Wenzel page is
identical to the input circuit for the TAPR TADD-2 frequency divider. The
TADD-2 adds a transformer and load resistor. Schematic is in the manual
here:
http://www.tapr.org/~n8ur/TADD-2_Manual.pdf
-Bob
On Wed, Dec 8, 2010 at 8:46 AM, John Ackermann N8UR<j...@febo.com> wrote:
Wenzel has some discussion and circuits at:
http://www.wenzel.com/documents/waveform.html.
The Shera GPSDO made clever use of the input circuit of a 74HCT4046 PLL
chip for squaring.
John
----
On 12/8/2010 10:31 AM, jimlux wrote:
I'm looking for suggestions on a general circuit that can be used to
receive an external frequency reference (nominally a real clean sine
wave at, say, 10 MHz, although up to 100 MHz is possible) and turn it
into a "real clean" square wave. Galvanic isolation is a plus (a
transformer or capacitor would probably do that).
I was thinking about rummaging through the schematics for test equipment
reference inputs (since they've already "solved" the problem, eh?), but
any other ideas would be welcome.
I've scanned the archives of time-nuts, and while we have a fair amount
of discussion on how to square up the 1Hz (or 100Hz) in a phase
noise/ADEV setup, not so much on what to do with the 10 MHz. Rick has
commented that you don't want to use a comparator. I have the papers by
Dick, et al, and Collins, as well as all the others.. they tend to be
looking at the low frequency problem, although the analysis is certainly
applicable.
I don't know that I'm looking for the whole multiple limiting stages
scheme in any case.
Oh, as far as performance.. Say the need is to not horribly degrade a
good quality crystal oscillator... here's a typical set of specs:
76 MHz
1Hz<-90dBc
10Hz<-110dBc
100Hz<-120dBc
1k-100k<-125dBc
Adevs of the oscillator run from 5E-12 at 0.1 sec, down to 1E-12 at 10
sec, and back up to 2 E-12 at 1000sec.
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