Distributing Tek 11801 calibration port output to 4 SD-26 heads? - Page 1
Forum section: electronics / general

author: rhb
author_rank: Super Contributor
timestamp: April 02, 2019, 01:56:01 am
content: Subject line says it all. I need to distribute the output of the calibration port to the 8 inputs with minimal delay and degradation of BW to synchronize data acquisition for timing measurements.My current concept is 9 SMA connectors, one input and 8 outputs on a double sided board with a 350 ohm resistor in series with each output SMA in a radial pattern so that each PCB line is the same length.The output of the calibrator is 250 mV. So an 8:1 divider should give me ~30 mV at each channel which is sufficient for timing adjustments for the individual heads.So with a 350 ohm resistor in series, what should the impedance of the traces be? The inputs and output need to see 50 ohms.Is there an alternate solution I should consider?

author: JohnG
author_rank: Frequent Contributor
timestamp: April 02, 2019, 02:35:15 am
content: As a practical matter, I would put the 350 resistors at the center where the input is, and then run 50 ohm lines to the SMAs on the outer ring. The reason is that a 350 ohm transmission line will be very narrow, and it will be difficult to get an accurate impedance value due to variation from the PCB manufacturer. If you want low dispersion, use buried stripline as opposed to microstrip.I would also use an 0402 size resistor, or maybe enough in parallel to get about the same width as the transmission line and still get the correct total value. Finally, if you mount them upside down, so that the element is against the PCB, you will substantially reduce the parasitic inductance of the resistor.Cheers,John

author: tomato
author_rank: Regular Contributor
timestamp: April 02, 2019, 03:04:08 am
content: I think you want 39Ω resistors on all the lines for a 50Ω 1:8 splitter..

author: David Hess
author_rank: Super Contributor
timestamp: April 02, 2019, 10:43:18 am
content: Quote from: tomato on April 02, 2019, 03:04:08 amYep, 39 ohms for a 8-way resistive power splitter:https://www.microwaves101.com/encyclopedias/resistive-power-splittersAll traces are 50 ohms simplifying construction. The resistors meet as a star in the center.

author: rhb
author_rank: Super Contributor
timestamp: April 02, 2019, 01:15:09 pm
content: Thanks for the link. That was very useful.This is going to be even harder than I thought. And I was quite certain it was very difficult to begin with.It's beginning to look like making 1/4-20 hex head bolts and nuts in 1/30th scale. I'm utterly amazed anyone could manage to do that.https://www.craftsmanshipmuseum.com/images/Bolt%20and%20Wrench.jpghttps://www.craftsmanshipmuseum.com/Kieffer3.htm

author: JohnG
author_rank: Frequent Contributor
timestamp: April 02, 2019, 01:17:03 pm
content: For the resistive Y-splitter, the resistors will meet as a star in the middle, but then you have to connect a resistor from the meeting point to the input connector, and that makes it harder to be symmetrical. I think what he is doing is just adding 350 ohms to each line, to get 400 ohms. Connect 8 in parallel at the center to get 50 ohms.Since this is a single-purpose splitter where timing matters the most, this seems like a good approach to me, but if the Y-splitter has an advantage in this case, I'd like to know about it. My microwave knowledge is pretty dusty at this point...John

author: JohnG
author_rank: Frequent Contributor
timestamp: April 02, 2019, 01:40:04 pm
content: Pictures help...

author: rhb
author_rank: Super Contributor
timestamp: April 02, 2019, 04:48:40 pm
content: The error was because I only considered the impedance presented to the calibrator. I had concerns about that, but was tired. It really needs to be 50 ohms in both directions. Happily, David provided a link that gave the needed information.It is still going to be difficult. Mainly because of the issue of cable length matching. But I think I might have hit on a solution in the form of SMA F-F bulkhead connectors. By not fully mating the males and using a locknut I should be able to get a 7 ps adjustable delay. I *think* I can trim cables to within 5 ps by attaching one connector and then using TDR to trim to length. I'll test the delay concept later today. The cable making will have to await my crimpers and connectors. fortunately, I only need to match the cables to a head. I can adjust the head skew which makes life a lot easier. So I can sort cables for best match pairs.The funny part is I made a deliberate decision to limit my T&M kit to 3 GHz for both cost and difficulty reasons. But I bought a couple of Leo Bodnar's pulsers, both square wave and 100 ps pulse. The CSA803 plots he supplied did me in. So now I have 8 channels that go to 20 GHz and am doing battle with picoseconds. But with repairs and shipping, I have $1275 in the setup. Being able to resolve femtoseconds at that price is rather mind boggling. Of course, I now *really need* a 50 GHz SD-32 ;-)

author: tomato
author_rank: Regular Contributor
timestamp: April 02, 2019, 05:20:32 pm
content: Quote from: rhb on April 02, 2019, 04:48:40 pmOooh ... adding delay while introducing a gap in the dielectric? Not an ideal approach.

author: JohnG
author_rank: Frequent Contributor
timestamp: April 03, 2019, 01:10:58 pm
content: Quote from: rhb on April 02, 2019, 04:48:40 pmIf the scope inputs are "50 ohm", then why does the splitter have to be 50 ohms in each direction? I realize that the scope inputs are probably not a perfect 50 ohms, but they have to be close over a wide bandwidth or you would not be able to attach a 50 ohm cable and do a measurement, since many measurements will not have a convenient 50 ohm output impedance. If there was significant reflection from the scope input, this reflection would hit the measurement source and bounce back. Am I missing something here?John

author: rhb
author_rank: Super Contributor
timestamp: April 03, 2019, 02:51:21 pm
content: You *always* get reflections. Right now I'm looking at the reflection of the 11801 calibrator step after it has reflected off the SD-24 input port and then reflected off the calibrator port. The largest reflection has an amplitude of 1.5 mV, 0.6% of the input step. However, one cannot say what the reflection coefficient is without a lot of additional analysis.The signal is I*Tcal*Tcab*Rin*Tcab*Rcal*Tcab*TinwhereI = Dirac impulseTcal = calibrator transmission responseTcab = cable transmission responseRin = input reflection responseRcal = calibrator reflection responseTin = input transmission response* indicates convolution in the time domain or multiplication in the frequency domain.I've lumped the cable reflection terms in with Rcal and Rin. So if Rcal is large the error is large. The diagram you drew, which is the same as what I drew, adds a large Rcab which gets added to the middle of the cable transfer function for an impulse propagating from the input port to the calibrator port.I'm a scientist rather than an engineer, so I generally have to fall back to fundamental analysis when I'm doing things I've not encountered before. If I'd thought about it in the morning rather than just before bed I'd have realized that an N port resistive divider must be symmetric. They are generally referred to as "ring splitters". I'd read about them in one of my RF design books, but without an application for one, it slipped from memory.The SMA F-F experiment sort of worked. I could only get about 1.5 ps from a gap of 1 mm. My best guess is that fringing effects at that close a spacing lowered the velocity factor. Naturally I had a lot of periodic ringing after the step, but it didn't interfere with the ability to add a small delay. It's just not enough of a delay to be worth while. So time to buy a Sherline lathe. To minimize the effort I'll use sliding 50 ohm air dielectric tubes which are permanently connected to the cable and fixed in place with solder after adjustment. The whole setup will be clamped to the sampling heads and all the cables set in position before making the final adjustments. That should preclude shifts due to using cheap cable.

author: JohnG
author_rank: Frequent Contributor
timestamp: April 03, 2019, 06:08:13 pm
content: I understand that you always get reflections, it's just a question of whether or not they are big enough to matter. I don't know for your case. 0.6% is about 44 dB return loss, which is not shabby at 10-20 GHz.What I do know is that it is a challenge to find resistors that look like resistors at 20 GHz, and this will also result in reflections and unwanted behavior. So you pick your poison.If you do a resistive Y-coupler, I would recommend 0201 resistors and make sure the element side is facing down. The following app note will provide some ballpark numbers that may prove useful:https://www.vishay.com/docs/60107/freqresp.pdf. A caution about the app note is that they do not provide any information about the spacing of the ground plane or really any other geometry in the test circuit, or the effects of the pads, etc, all of which can influence the measurement.If you have to lay out a board for a splitter, it's very little work and extra cost to try both designs. The main parts of the layouts would be very similar, especially if you did the Y-coupler first. Might be worth a try and then the questions would be answered definitively.Have you simulated the circuit I drew? It works. I have simulated, built and used a three-output version. My transition times in the circuit were much longer than yours, a few hundred ps, but the electrical length of my cables was several ns, so any obvious reflections would show up. None did. The challenge with an 8-way is finding a 350 ohm resistor that is well behaved at 20 GHz, but it may not matter. If you are just trying to align timing, you look at the first edge and ignore the reflections. YMMV.John

author: rhb
author_rank: Super Contributor
timestamp: April 03, 2019, 08:20:26 pm
content: Very interesting. Thank you. I've not done any simulations. No experience with any electronic CAD programs and a *very* small amount of 2D CAD experience. So I'm facing lots of aggravation figuring out how run the programs. And choosing which ones to use.I could actually solve for the trace lengths between discontinuities much more easily. The 11801 turns out to be better for me because of my seismic background than a 20 GHz VNA. And an 8753B and 85046A were much cheaper.I'm deliberately making the task harder than it needs to be to help me develop good lab technique.I planning to start with some dead bug/Manhattan experiments using leaded parts for the sake of getting a better feel for just how far you can push that with careful construction.

author: JohnG
author_rank: Frequent Contributor
timestamp: April 03, 2019, 08:57:00 pm
content: I highly recommend LTspicehttps://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.htmlas a basic lumped-element circuit simulator (transmission lines are the only distributed element, IIRC). There is a very good Yahoo discussion group that goes with it, and you can resort terminal and text files if you want. Free, but not open source.Note that my simulation is very idealized, so just a place to get started. I'm not modeling any RF effects other than the transmission lines, but it is straightforward to add *lumped* element parasitics as you get comfortable with the tool. I have included the simulation file if you care to run it in LTspice. You can try changing one or more of the load resistor values as a start.The usual cautionary statements about simulation, models, and GIGO apply, of course.John