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From: "Talbot Andrew" <ACTALBOT@dstl.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: Reference Osc. Waveform
Date: Thu, 24 Jan 2002 08:34:51 -0000
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The AD9850 wants a (nominal) 0 - 5V squarewave waveform, the duty cycle is
not important.  You haven't specified the absolute voltage levels in your
diagram below, but if it is nominally a TTL type of signal it may work.  In
practice though, I've found the AD9850 will work with input waveforms a lot
worse than the specs call up. In some cases a 3Vpk-pk waveform will operate
it provided som DC bias is added.

The best way to correct an arbitrary shaped waveform to suitable levels is
to pass it through a buffer, such as a 74AC14 Schmitt trigger gate.  This
particular logic family will go to about 150MHz.  If you DC bias the input
at a point between its two hysteresis levels then AC couple the signal, the
buffer will respond to signals of less than 1V pk-pk.   For higher gain,
couple to a 74HC04 inverter, biassed into linear operation with a resistor
of a few 10s or 100s of k ohms between output and input.

In retrospect it would have been a good idea to add a clock buffer on that
DDS module, and space for a TTL oscillator module,

Andy  G4JNT

-----Original Message-----
From: Larry Kayser [mailto:kayser@sympatico.ca]
Sent: 23 January 2002 16:07
To: rsgb_lf_group@blacksheep.org
Subject: LF: Reference Osc. Waveform


Greetings All:

This discussion of DDS chips reminds me of a problem I have here.  I have 
two, very nice TOYOCOM TCO-512K oscillator modules.  I want to put these to 
work here.  The output waveform looks like this on the Scope I have here

x             x                x
  x            x  x            x
   x           x   x           x etc.The waveform looks almostexactly like a
right angle triangle.
    x          x    x          x
     xxxxxxx     xxxxxxx
       ^       ^        ^
The up arrows are the beginning of a cycle, the middle of the cycle, the 
end of the cycle.

What will this type of waveform do to the 9850?  Does the 9850 want a 50% 
duty cycle? or can I put this kind of wave form into the 9850 without risk?

These are very high quality units, they were the frequency determining 
elements in an early analogue cellular radio system (Swedish system), but I 
am at a loss to understand this waveform on the output.  I have tried 
different loadings on the output (10K to 50 ohms) and it seems to stay very 
close to the same,

If this waveform is not suitable is there a simple way to add something to 
make the waveform a square wave with 50% duty cycle?

Larry
VA3LK


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