The blow-by effect on frequency response could be tested. Grossly
misadjust it deliberately and see if the frequency response changes. I
found it to be the easiest thing to calibrate on my S-4 sampling heads
so I would not worry about temporarily misadjusting it.
I would also compare two different types of sampling heads like an S-2
and S-4 which have significantly different transient response
characteristics in the 10ns range do to design and see if they agree.
I may try the above with an S-1 and S-4 and my SG503 just to see what
kind of results I get.
On Sat, 16 Feb 2013 10:48:35 -0000, "Albert" <aodiversen@...>
wrote:
Hi David,
The step response of my S-4 is very similar to the pictures shown in the cal proc of the manual. Overshoot from 400 ps to 25 ns is specified as 10% or less. Blow-by compensation adjusts the level or trend after 25 ns. Both overshoot and blow-by might well affect the frequency response (far) below 1 GHz I think.
The awkward thing is that we always need something that's again better than what we already have. I have to trust pulse flatness of my S-52 and 264 (also Square Wave for S-2 blow-by) but in reality I have no means to check that. In S-1 and S-2 the blow-by adjustment has considerable effect, so if your S-1 shows no blow-by then you or a previous owner did a good calibration job. I'm not sure I ever re-adjusted my S-4.
I did many amplitude measurements on my 067-0532-00, 3 MHz and 60-500 MHz, using 7T11A/7S11/S-2. At 1 Vpp the amplitude changes are within 1% (that is max - min) over the whole frequency range, including the separate 3 MHz. At 3 Vpp (connected via GR 2X attenuator) still within 2%.
That looks very nice, but in theory a non-flat response of the S-2 could compensate for a non-flat output of the signal generator. I used an S-2 to eliminate a GR to SMA adapter and also because the S-2 allows for internal triggering.
Albert
--- In TekScopes@..., David <davidwhess@...> wrote:
I do not have any in dependant confirmation but I suspect given the
relatively low frequencies involved for the sampling heads up to 5%
loss that they will be very close to ideal. I do not know of a better
way to calibrate for a constant level that does not require something
else already calibrated to a better standard except for a thermal RMS
based design which itself can be calibrated at DC.
If you have even an unleveled microwave signal source, you could use
it to find the first null in the sampler frequency response which
would tell exactly what the sample gate time is.
Do you mean overshoot or blow-by? I know my S-4 sampling heads have a
lot of blow-by aberration or whatever that is at about 10ns but show
ideal pulse response as far as I can test. My working S-1 shows no
blow-by with the same input pulse but my best flat level pulse
generator while clean is not fast enough for the S-4.
On Fri, 15 Feb 2013 22:35:37 -0000, "Albert" <aodiversen@...>
wrote:
Hi David,
In my response to Ed I said something about VSWR; that crossed your message.
Your calculations (indeed needing the famous sin(x)/x) assume a perfect rectangular windowing function. Do you have any reference for how good or bad this approach might be in practice? The overshoot in step response of my S-4 is in conflict with this theoretical approach.
Albert
--- In TekScopes@..., David <davidwhess@> wrote:
I think using a sampling oscilloscope for flatness calibration is a
great idea. The sampling heads are both very high bandwidth and have
a very predictable frequency response. The weakest links will be the
SWR match and termination but that applies to any system. You can do
away with cable losses by using a sampling head extender.
If you take the 3db bandwidth numbers I posted earlier and divide by
4, that is the point where the sample head output will be down by 2%.
The second number shown is where they will be down by 1%:
S-1 260 MHz 190 MHz
S-2 1.18 GHz 869 MHz
S-4 3.60 GHz 2.61 GHz
S-6 2.99 GHz 2.17 GHz
I think this is the first time I have had a need to do math involving
a sin(x)/x function.
David Hess