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The Rigol DS1052E
rossmoffett:
Those are in-circuit measurements - I just measured the caps (I kept them) out of circuit and mine are ~160 pF too. Doh! There must be something else around to limit bandwidth.. unless you have different size inductors, or maybe there is something behind those vias.
That ADC driver could be a bandwidth limiter, if I could find a match for it. It's not an 8-lead TSSOP as I posted earlier, I looked closer in zoom images and it seems to be a 10 pin no-lead chip of some sort. It's so hard to tell, they're tiny! I still can't find it from Analog Devices, though it must be one of theirs based on the Analog sandwich around it. I imagine it's classified as a differential op-amp, but I can't find it in that category yet. It's approximately 2.6 x 2.9 mm (+/- .1 mm) based on the photographs, I found that by comparing to known parts data.
Don't worry about re-installing the filter - if you read the datasheet on the page that I mentioned it notes that the filter is only necessary depending on application. At least, I think. The only use those capacitors could have is to hold a voltage stable, but I don't see how that could be necessary.
Given that you have a 100 MHz scope, you should only see marginal improvements at 125 MHz. There will be a sharp dropoff following that, though. If you have anything that will generate 100 MHz, check to see what V1/V2 is there. It should be .7 if channel 2 is of a sufficiently higher bandwidth, as you can see where I posted V1/V2 at 50 MHz. From my graphs above, you can also see that my good luck starts to end at about 130 MHz. That's where the modified channel gain starts losing ground to the unmodified channel. In the app notes for the ADC there's nothing else stopping the signal but the ADC driver, so that may be the next place to look, unless someone finds some filter before the variable gain amplifier.
jahonen:
--- Quote from: rossmoffett on February 15, 2010, 06:31:20 am ---I'm not sure what's going on here, Channel 1 seems to be picking up the transients now. Maybe I had a poor connection when I took the earlier measurement.
--- End quote ---
That looks like spurious oscillation. For easy bandwidth check, a high-edge rate digital (repetition frequency can be set to very low frequency, say, 1 kHz) signal is a easy check. One should see rise or fall time of about 0.35/BW if everything is ok. For 100 MHz that translates to 3.5 ns etc. If the rise time is not reduced, then the effective bandwidth for practical measurements is not going to be very significant.
You can get fast edges from just about any digital chip, except from that old 4000-series CMOS. Even old PIC16F877 seems to have about 160 MHz bandwidth (without any other additional loading than scope probe), see here. That contains some probing hints also :)
Regards,
Janne
rossmoffett:
I understand spurious oscillation, it's just that the earlier post I made was observing a 1 MHz square wave under identical conditions and the unmodified channel failed to pick up all of the intricacies. I believe I must have not grounded the probe properly the first time around. Thanks, though!
OK, I think I have narrowed down what kind of chip the ADC driver is. I thought I saw an extra pad on one of Dave's close-ups, which would have explained why I couldn't find an 8-lead package, but upon close inspection from top views I can't see that it would be the case. Browsing the ADC drivers at the Analog Devices website seemed futile, until I checked a supplier. These chips are available in packages not described on the AD website. I noticed that the screen print around the part seems to be 3x3 mm. This leads me to believe they must have some oddball part, but the PCB still looks like it would fit these two parts, which come with the squared edges.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=AD8350ARMZ20-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=AD8132ARMZ-ND
They are 900 MHz and 350 MHz respectively. I'll have to crack it open again and take some measurements on gain and voltage to be sure which part it is, still could be any of the other 8 pin parts. The version with the worst (feasible) bandwidth is the AD8137, with a bandwidth of 110 MHz. The only lower ones clock at 38 and 30 MHz. I can find it in a package with bevelled edges in 3x3 mm, but not with the square ones.
and.. here's the AD8138 in the package that's in the scope:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=AD8138ARMZ-ND
A 320 MHz part.
rossmoffett:
--- Quote from: flolic on February 15, 2010, 07:35:05 am ---Yes, I modified second channel on my 1102E last night. I removed both (HI & LO) caps, btw. they are 160pF on my scope. I don't have any function/signal generator that go beyond 100MHz range, so I used old FM tuner and pick up signal from local oscillator set up at 125MHz.
It seems that mod did not do anything to my scope, signal amplitude on modified channel is just marginally better than unmodified. I used standard probes. Today I will buy some BNC connectors and coax and make high bandwidth 50 ohm terminated probe.
But I noticed another thing when measuring complex signal (square wave, 32MHz xtal osc. and 200kHz osc. with joined outputs). On modified channel you can clearly see higher components of a signal, in a 250MHz range! So it seems that mod is actually working ;)
Just hope that that is real signal and not some kind of aliasing or ringing... ;D
--- End quote ---
I took mine apart again in light of your not noticing much difference. My digital camera still has a broken screen but I used it to take some pictures anyway, they're of pretty low quality or I'd post them. I made a quick-flash gif between my scope and yours, I can't find a single component difference! Those inductors hanging off of the caps that we removed are different values on Dave's scope, but not on mine.
I flipped to the underside, flabbergasted, and realized that those vias drop down to a couple more caps on the underside! In addition to those, there are two other sets near the signal input jack and the input of the variable gain amplifier.
So I think I might still have found it, those caps on the underside might be the key difference. There may even be additional filtering on the input side of the variable gain amplifier too. My curve seems to lack that dramatic filter swoop, seems a little too linear. I don't know what the underside of the DS1102E looks like though, so I can't say for sure. All I know is that it seems like my scope is working fantastic with this mod, I wish I had another reference to compare to besides my unmodded channel. The DS1102C scopes at school are only 400 MS/s with no sinx/x feature, so they aren't good for comparison. Unfortunately all of our other 100 MHz scopes are analog and long separated from their last cal date.
Shine a light underneath the board and you will see everywhere that signal passes down. They cut out chunks of the inside layers around those areas.
flolic:
Ok, I modified both channels. I don't know if that was wise idea but it's over now ;D
Few thoughts:
-First, original probe (for 1102E) is declared as 150MHz bandwidth and WILL attenuate higher freq. signals.
-Second, my modified scope (with homemade high bandwidth probe) easily picked up 446MHz PMR radio transmission from few meters away. It is easily seen on FFT display. Unmodified scope can show that transmission only if I put radio right next to the probe.
From that it is obvious that mod is working. But I am still worried about spurious/false readings...
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