Plumber's Delight Damn Fast Pulser - Page 1
Forum section: electronics / projects

author: ccktek
author_rank: Regular Contributor
timestamp: August 24, 2017, 09:30:15 pm
content: Hello all, first post:Having recently restored two DOA Ebay Tektronix 485 oscilloscopes to full operational condition, I needed a source of a fast leading edge pulse for high frequency adjustments in the vertical amplifiers and attenuators. The Jim Williams avalanche circuit would serve the purpose, but I wanted a square step function rather than a short pulse. The circuit can be modified with a length of coax transmission line to give a square-ish pulse as described by Dave Jones.To avoid the need for the coax and a high voltage supply I instead used the Analog Devices ADCMP580 comparator (http://www.analog.com/media/en/technical-documentation/data-sheets/ADCMP580_581_582.pdf), which is capable of rise times well under 100ps. For single ended input I used the input configuration of Figure 16 of the data sheet. As in Figure 20 I used the 1K resistor to -5 volts at one latch pin and grounded the other latch pin to disable latching. I grounded the hysteresis pin, giving a level of hysteresis that works well. I bypassed the + and – supply lines with 0.1uf ceramics. Either of the two outputs can be used for single ended output; the unused output must be terminated with a 51-ohm resistor to ground. The output in use must be terminated at 50 ohms at the oscilloscope; if the scope does not have a 50-ohm input impedance option, a 50-ohm feed through terminator must be used.The I.C. is very small, 3x3mm with 16 pins at 0.5mm pitch. To avoid soldering wires directly to the pins I used an SMT to DIP adapter (https://www.digikey.com/product-detail/en/chip-quik-inc/PA0061/PA0061-ND/5014731) but without the DIP pins installed, as they would introduce too much length to the connections. Even so, the adapter board adds some undesirable connection length. I also used components with leads cut very short rather than surface mount components.The adapter board is supported by interconnecting leads on a small PCB ground plane. The assembly is contained in a short length of 3/4” copper pipe. BNC connectors are soldered onto the end caps. I cut the end caps to shorten them. In the photos the I.C. is hidden under the adapter board between it and the PCB ground plane. The scope sweeps are set to 1ns/div. Why all the extra room in the pipe? I wanted space to add input and power supply over-voltage protection some time in the future.An improvement over this arrangement would be a PCB designed with one of the on-line services that produce small quantities and to use surface mount components. Before doing that I hope to find a very fast oscilloscope to check performance of the pulser in its current state.

author: tggzzz
author_rank: Super Contributor
timestamp: August 24, 2017, 10:03:30 pm
content: Is there any noticeable difference between the ADCMP580's output and the 485's cal output terminated in 50 ohms?

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2017, 02:10:52 am
content: I would like to see the ADCMP580's output on a sampling oscilloscope. As I discovered with the MC100EL32, getting a clean output from ECL logic can be challenging and it is not even as fast as the ADCMP580.Even with a 100 MHz bandwidth, there is a noticeable calibration error between my MC100EL32 calibrator and my PG506 which is slower but produces a much cleaner edge.

author: ccktek
author_rank: Regular Contributor
timestamp: August 25, 2017, 03:38:15 am
content: tggzzz, yes there is a difference, although not a great one. In the attached photo I superimposed traces of the pulser and the internal cal source, both running at 1MHz and driving 50-ohm inputs. One of the wonderful features of the 485 is that in the alt mode and trigger set to norm both traces appear stable even if the frequencies are slightly different. Anyway, the slightly slower trace is the internal cal output, the faster one the pulser.David, I would love to see the pulser output on a very fast scope. I would expect some ripples in the 100s of MHz range due to overly long leads in my wiring. Maybe I can find such a scope in the Portland area.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2017, 04:49:20 am
content: The 485 calibrator output is pretty fast and clean but it is not intended for or fast enough to calibrate the 485's transient response. For transient response calibration, the Tektronix067-0681-01 tunnel diode pulserwas used.

author: edavid
author_rank: Super Contributor
timestamp: August 25, 2017, 05:05:36 am
content: Quote from: David Hess on August 25, 2017, 02:10:52 amThe ADCMP580 has CML (collector) output, not ECL (emitter) output, so it's not hard to get a clean output pulse.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2017, 01:45:46 pm
content: Quote from: edavid on August 25, 2017, 05:05:36 amYou are of course correct. Sometimes I wonder if I have forgotten more than I have learned.The common NBS type of reference level pulse generator design like the Tektronix PG506 uses a CML output but they did not call it that at the time and they added a fast low capacitance diode or a cascode stage to isolate the termination from the Miller capacitance of the output transistor. The ADCMP580 lacks that which is why I am suspicious and want to see its output on a sampling oscilloscope.Unfortunately the same diode or cascode trick cannot be used to clean up an ECL output because the output is low impedance and the voltage swing is too small. On a CML output, the compliance of the output transistor can compensate for the added voltage drop.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: August 25, 2017, 06:09:53 pm
content: Quote from: David Hess on August 25, 2017, 01:45:46 pmWhat do you want to see? I have used ADCMP572 in Rev.A of my pulser and it was OK. I have since cannibalised the prototype but can probably assemble another one if there is anything special about it. It's typical CML. Reasonably fast, 400mVpp into 50R. Not much ringing. It just works. Not much to see really.Leo

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: August 25, 2017, 06:45:44 pm
content: Quote from: David Hess on August 25, 2017, 01:45:46 pmDavid,I have actually found some old screengrabs from ADCMP572 pulser design taken on SD-26 (20GHz) sampling head. I chose 572 because I did not fancy doing split rail PSU for no reason - the output is AC coupled.ADCMP580 does not look that much different apart from breaking out outputs stage rails - why are you specifically interested in 580?CheersLeoP.S. Level is slightly low due to onboard power splitter for trigger pick-off. I have later made a separate pre-trigger output and results were not much different apart from standard CML Vpp.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2017, 08:16:01 pm
content: Quote from: Leo Bodnar on August 25, 2017, 06:45:44 pmThe ADCMP580 has the CML output stage which should produce a cleaner pulse than the PECL output of the ADCMP582.If I am reading your measurement right, the PECL ADCMP582 shows more ripple than would be acceptable for a good calibration although that might be a resonance at 7 GHz do to the circuit layout. At lower frequencies this would not be a problem so it is easily suitable for use to 2 GHz.

author: edavid
author_rank: Super Contributor
timestamp: August 25, 2017, 08:27:45 pm
content: Quote from: Leo Bodnar on August 25, 2017, 06:45:44 pmLeo, this thread is about a different pulser made with the ADCMP580. See the first post.Quote from: David Hess on August 25, 2017, 08:16:01 pmNo, you did not read it right. Leo's V1 pulser used the ADCMP572, which is another CML part (similar to, but slightly cheaper than the ADCMP580).(FYI, his V2 pulser uses the MAX3949, which is a bargain compared to the AD parts.)I don't think anyone has built a pulser with the ADCMP582

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: August 25, 2017, 08:43:27 pm
content: Quote from: edavid on August 25, 2017, 08:27:45 pmOh, my bad, sorry.I thought we have drifted to a more general discussion about Tek calibrators.We probably need to establish what "good calibration" criteria is.You can dampen 580 or 572 to around 100ps risetime and almost all aberrations will be gone.ADCMP582 looks very inviting to experimentation with current sources but maintaining impedance might be a challenge. Again, it all depends on the purpose of the pulser. It does not have to have 50R source impedance if it is injecting the signal directly into the scope jack.Leo