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From: email@example.com (Steve Pepper)
Subject: Re: Comb/Harmonic Generators
Date: 16 Dec 2002 09:42:46 -0800
References: <3DF54583.49C9@Spam.Bots> <firstname.lastname@example.org> <3DF9B8A5.585A@Spam.Bots> <email@example.com> <3DFAAFF0.35C2@Spam.Bots>
NNTP-Posting-Date: 16 Dec 2002 17:42:46 GMT
> > Mike wrote:
> 300 uV rms is much more interesting. I'm familiar with sequential
> sampling and sqrt(N). But it's not clear where and how the averaging
> takes place.
> I've got Agoston's white paper on meauring the risetime, but it's in
> Acrobat 4 and the images come out black in my Adobe 3.0 viewer. Do you
> have any other papers that talk about it more?
In "pulse resolved" mode, where individual samples are resolved and
digitized, averaging is typically done in the digital domain (like a
conventional sampling 'scope).
In "downconversion" mode, the charge from multiple correlated samples
can be accumulated on the "hold" capacitors internal to the sampler.
In this mode, the charge amp is implemented as a high-impedance
voltage follower. If the sample strobe timing is such that a single
point on the RF waveform is sampled repetetively, the hold caps will
"charge up" to the DC level of the RF waveform at the sample point.
The bandwidth of the follower amp limits how fast the strobe timing
can be "walked" through the RF waveform.
This is the operational mode of the rise time test system described in
the white paper. The RF waveform (the optically generated step)
repeats at 25 MHz (the laser PRF), and the strobe is generated from
this same laser pulse. The sample strobe is slowly walked through the
RF step waveform with an electrically-controlled delay line (an NLTL
w/ DC bias). The IF output is fed to a 70 KHz BW follower and fed to
a 14-bit, 100 Ks/sec digitizer, and displayed vs. strobe delay.
Since the strobe is generated from the same laser pulse that excites
the step generator, there is almost no jitter. The jitter of the NLTL
delay generator and LO chain were tested by "parking" the strobe on
the fast rising portion of the edge. Any jitter in the LO chain is
converted to amplitude noise by the dV/dT of the RF step. (A high
bandwidth sampler makes an exquisitely sensitive phase detector.) We
measured less than 50 fSec RMS jitter in the 70 KHz IF BW.
P.S. I have the original Word 9.0 version of the white paper, which I
can convert to a Word 2.x (or higher) .doc and e-mail it to you (and
anyone else who needs it). Will that work?
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