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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Comb/Harmonic Generators
References: <3DF54583.49C9@Spam.Bots> <email@example.com>
Date: Fri, 13 Dec 2002 02:38:29 -0800
NNTP-Posting-Date: Fri, 13 Dec 2002 02:38:31 EST
Organization: Bell Sympatico
Steve Pepper wrote:
> Hi Mike. Vance asked me to try to answer your questions. First let
> me introduce myself: I'm Steve Pepper, Sr Design Eng. with
> Picosecond Pulse Labs. I'm a co-designer of the sampler and NLTL
> components, working with Agoston and others in Beaverton, OR.
Lucky Guy! Thank you for your help.
> First, your calculation of the RMS voltage noise of a 50 Ohm
> resistor in 100 GHz BW is correct. Vance's slide rule might need a
Maybe he included 1/F noise:)
> The 3 mV RMS noise Vance quoted is the input-referred noise for
> the combined sampler and IF charge amp operated in
> "sample-resolved" mode, in which individual samples are
> pulse-resolved and digitized the way a conventional sampling scope
> operates. I'm not sure "Noise Figure" is the appropriate metric to
> use in this case, but the input-referred 3 mV RMS noise can be
> used to calculate the "minimum resolvable signal" that you were
> looking for, when the sampler is operated in this mode.
OK, I understand this mode better, and 3 mV rms sounds reasonable.
> Another way of operating the sampler, "downconversion mode" is
> more like a conventional harmonic mixer. The sampler is strobed at
> an LO frequency that is higher than the (baseband) IF bandwidth,
> and individual samples are not resolved. Instead, many samples are
> averaged together within the IF bandwidth. Without going into a
> lot of detail, suffice to say the the noise and conversion loss
> depend on the RF/IF bandwith ratio and the LO frequency (sample
> rate). The advantage of operating in this mode (downconversion to
> baseband) is lower noise and conversion loss. The IF bandwidth of
> the sampler described on the web site is DC to about 1 GHz. We
> have other designs with a range of RF and IF bandwidths.
I need to learn more about this mode. I know HP used it in their
vector network analyzers, but switched to diode mixers for improved
> I believe you answered your own comment on the need for dynamic
> range when you referred to 10 Gb/s logic signals. These are
> volt-level signals with fast edges, and the edge speed will be
> degraded by the rise-time of the acquisition system.
You are right - waveform acquisition will be difficult. I haven't
started working with GigaCom yet, but will soon. I expect special
test point adapters will be needed. These might take the form of a
450 ohm resistor in series with a coax connector mounted permanently
on the board. Finding a suitable resistor may be fun.
From previous work with ECLinPS Lite (130 ps), I know the coax to
the sampler has to be very good quality and as short as possible.
The way you have packaged your sampler helps, since it could be
placed near the board to minimize the length of coax needed. This
would be difficult with the TEK and HP samplers.
However, the logic swing is a big problem. For the NBSG53A
differential D flop, it could be as low as 120 mV p-p. The NBSG16
Differential Receiver/Driver is spec'd at 350 mV p-p min. The
datasheets are at
With 10:1 attenuation in the test adapter, the sampler could see 12
to 35 mV p-p. With 3 mV rms noise in the sampler, the signal could
end up buried in noise. I need to learn more about your
> P.S. If you enjoy reading patents, you can get a lot of detail on
> our sampler design from our US patent applications: 20020167373
> and 20020145484, which were recently published.
Thank you! I have just downloaded them and will convert the TIFF's
to gif's to study them.
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