John said:
[...]
I think the 7000 series goes up to 12 or 14GHz, depending on the
sampling head. Yes, they are very cheap now, thanks to EBay.
As shown on my web site, the 100EP52 will track a 200 ps
risetime. The GigaComm part will probably do much better. As I
mentioned on my web site, the Binary Sampler will never be as
fast as conventional sampling scopes.
All I saw on the web site seemed to be simulations. Was Fig 2 an
actual measurement?
The Binary Sampler is now mirrored at
http://www.smg.uni-karlsruhe.de/add.automation
Where I use simulated data in the description, I clearly mark the
figure and show in the text it is simulated. In the "Binary Sampler
vs Conventional Sampling" page at
http://smg.iwk.uni-karlsruhe.de/add.automation/binsamp.htm
Figs. 1 and 2 are simulated noise with a conventional sampler. Figs.
3 and 4 are actual data from the Binary Sampler.
In the "Smoothing and Slew Rate Detect" page at
http://smg.iwk.uni-karlsruhe.de/add.automation/smooth.htm
Figs. 1 and 2 are actual data from the Binary Sampler.
Handheld battery-powered 150-200 ps TDRs use conventional diode
samplers and are in everyday use; they look like calculators and
are commodities these days. It takes very little power to run a
diode sampler, even a fast one, certainly less than what a couple
of EL chips burn up.
Yes, they are a marvel of low cost design. Any idea what the
bandwidth is? I'd guess perhaps 1 GHz or so. Perfectly adequate for
the application.
For other applications, such as OTDR, averaging is needed to improve
the SNR. I watched a line crew using a commercial unit, and it had
to average 1,000 waveforms to get a result. This took a long time,
and they had to repeat it each time they changed fibres.
The only time I tried making my own half-bridge sampler I got
about 5 GHz, 70 ps risetime, using a mediocre SOT-23 dual schottky
and a flea-market SRD.
Interesting. Got the part numbers? Of course, you have the same
problem with noise and low sample rate.
You keep forgetting how bad it is. Median-seeking simulates well
with perfectly symmetric noise, but sucks in real life. True
averaging is correct. Besides, I'd rather have averaging be a
option (as for eye diagrams or jitter measurements) than be
mandatory.
The Binary Sampler does not respond to the amplitude of the error,
so it cannot respond to the median. It only reponds to the direction
of the error, which tends to zero in Gaussian noise.
As Souders points out in "The effects of timing jitter in sampling
systems," IEEE Trans. Instrum. Meas., vol. IM-39, Feb. 1991,
averaging causes significant errors at slope changes.
Since the Binary Sampler only responds to the direction of the
error, and not the magnitude, it gives greater accuracy than
conventional sampling under these conditions.
Yes, you cannot measure noise with the Binary Sampler, as I mention
on my web site. You need a conventional sampler or digitizer. But
for many applications, the noise is well-defined and does not
change.
No human-readable waveform display needs more than 512 data
points, 1024 to ensure overkill. At 200 KSPS, like an older
conventional sampler, that's 200 waveforms a second, more than
anybody can use. 512 points, 32x averaging still gives 12
waveforms per second. The reason sampling scopes don't sample
faster is certainly not because they can't be made to do it, but
rather because nobody cares.
The required number of data points depends on what you need to
measure. The 200KSPS is a real problem when you have to use
averaging. Heterodyne sampling can be 1,000 times faster, so you get
higher precision and faster throughput.
The heterodyne timebase *must* sample the entire period of the
waveform, even if just one region is of interest, which is the
common case. Zooming a region becomes a huge PITA. And 'sample' is
a misnomer, since each 'point' must be, effectively, sampled
thousands of times by the nature of the 1-bit information stream.
You can use heterodyne sampling or a conventional delayed trigger,
depending on the application. The conventional trigger will probably
have more jitter and poor linearity.
The point is you have a choice with the Binary Sampler. With a
conventional sampler, you have no choice and are stuck with the low
sample rate.
And jitter is a real bear for a heterodyme timebase... your claim
of 10 ps jitter between two plain vanilla crystal oscillators,
over one second, is incredible: 1 ns would be pretty good.
The jitter spec is 25 picoseconds rms, and is pretty typical for
these oscillators at 1MHz. I measured 24.7 ps with the HP5370A. I
think the measurement interval was 10 seconds.
I wrote the manufacturer and asked what the jitter spec was over a 1
second interval. He did not have that information, but another
similar vendor said the 1 second jitter was about 1/2.5 of the spec.
I used a factor of 3.5 in the calculations to be conservative when
comparing the Binary Sampler to the conventional sampler. This gave
the conventional sampler a significant advantage, but the Binary
Sampler still beat it.
'Binary sampling' is a cute trick, is at least 40 years old, and
isn't especially useful. I wish I'd never told you about it.
Sorry John, this is simply not true.
You sent me the GE tunnel diode sampler gif via email after I had
posted the description of the Binary Sampler to the web. You did not
include a description of how the GE circuit worked, and you did not
provide that information until much later.
When you saw the information I had posted to the web, you claimed it
would rail. Clearly, you did not understand it.
Yes, as I show in the references, the basic idea has been around a
long time. But nobody noticed the noise-rejection properties, or
that the pure integrator would oscillate.
Best Wishes,
Mike