Question for a "RF" guy! - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 12:48:32 am
content: Hi,I am putting together a small pcb to have a single BNC input an 5 BNC outputs as a calibrator fixture to the Rigol DS4000 series. I guess batronix has already made something like this but took a square approach. The shipping for me with batronix is cost prohibitive and pcb's are cheap at itead so this is what I threw together last night:I do have a ground pour on top and bottom though the top one gets cut up pretty bad...I used some impedance trace calculators to come up with 90 mil width along with a 1.2mm pcb thickness to match the 50 ohm impedance.Is the "ring" a good idea or bad idea??? I could bring out 4 spokes from the center and then adjust the lengths to feed the outer 5 if that would be better (although 2 of the 5 will have to come out of one spoke and then split)...Please give me wisdom!!!Thanks,Alan

author: w2aew
author_rank: Super Contributor
timestamp: October 07, 2014, 01:53:10 am
content: If the intention is to distribute a signal to the five outputs, then your problem isn't with the design, the problem is with the concept. The single input signal is going to see a severe mismatch (10 ohms, instead of 50). That will upset the flatness vs frequency from your source.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 07, 2014, 01:55:59 am
content: The impedance matching will be completely off. At a minimum, you'll need to add resistors.

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 01:57:06 am
content: Point taken guys - will moving to the star with the same track lengths solve the issue?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 07, 2014, 02:23:11 am
content: If you have the correct resistors in line with each port where they join.

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 02:48:06 am
content: Quote from: NiHaoMike on October 07, 2014, 02:23:11 amDo you mean where they endpoint, in the scope in 50ohm mode?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 07, 2014, 02:57:57 am
content: Put all 6 connectors in a circle, with each trace going to the center. Where they connect to the center, put a series resistor.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 07, 2014, 03:27:29 am
content: Terrible idea. Why do you want to "calibrate" with home-made lash ups?http://www.ebay.com/itm/Mini-Circuits-ZN4PD1-50-S-RF-Power-Splitter-500-5000-MHz-/400783353896?pt=LH_DefaultDomain_0&hash=item5d508ca828

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 03:39:58 am
content: What exactly is a terrible idea specifically?I'm trying to do something similar to this - see the calibration kit towards the bottom of this page:http://batronix.com/shop/rigol/DS4000.html

author: qwaarjet
author_rank: Regular Contributor
timestamp: October 07, 2014, 04:00:42 am
content: I think that Rigol splitter is is a case of "get what you pay for" unless those cable are NOT true coax ie they have some weird resistive property, I don't see how it would work across a wide bandwidth because the impedance mismatch would be to big. The basic problem is the signal source is 50ohms (most likely) if you connect a 50ohm cable to a 50ohm cable everything is fine. But is you have a 50ohm cable being split in 2 you are effectively going from a 50ohm to a 25ohm cable which causes reflections that will screw up the signal. This problem only get worse as you add more cables in parallel. To correct the issue you need to add some kind of matching network and since this is just for signals and not power, resistor would be the best way to do this.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 07, 2014, 04:12:34 am
content: The scope's calibration feature is designed specifically for use with the calibration jig and without knowing the exact characteristics of the jig, it's impossible to DIY one that would give correct results.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: October 07, 2014, 04:16:21 am
content: Buy the mini circuits one linked above. It is cheaper than making one.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: October 07, 2014, 04:17:21 am
content: Quote from: NiHaoMike on October 07, 2014, 04:12:34 am+1

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 07, 2014, 04:26:44 am
content: Quote from: alank2 on October 07, 2014, 03:39:58 amCalibration isn't something you do with whatever contraption you think is right. It's a calibration. What do you use to measure the calibrator if you're building it?

author: ejeffrey
author_rank: Super Contributor
timestamp: October 07, 2014, 04:52:18 am
content: What are you trying to calibrate? Is this just a splitter so you can adjust the compensation caps on four probes at once? Or is this for probe time delay calibration? Amplitude calibration? It would help to know your goals for those of us who aren't familiar with that product.Anyway, I want to point out that for your application, impedance matching network on the board may not be a serious problem as long as A) your input signal is source terminated, and B) all of your outputs are terminated at the end. There will be a large signal reflection from the splitter back along the input, but it will be absorbed when it gets back to the source. Likewise, the signals going to the loads will be absorbed, not reflected back at the splitter. Generally you can have one impedance mismatch in a network without too much trouble. It can be in the source, the load, or somewhere in between. It is when you have two impedance mismatches that you get multiple reflections, standing waves, and the frequency dependence that goes with it.That said, the termination at the source and load is never perfect. You will get some ringing. Because there is no impedance matching at the splitter, the ringing will go on longer than necessary. Fortunately, this is less of a problem with low-frequency square waves used for probe calibration. The reason is that reflected signals always arrive later than the initial pulse. If your rise time is much faster than the length of your shortest cable, you can simply ignore all the secondary reflections -- as long as they completely die away by the time the next edge arrives. If your cables are a couple meters long and your rep rate is 10 kHz, you will have 10k reflections. More than enough for any reflections to die down.Anyway, if you want to make a matching network, there are basically two ways to do this. The first is to just add a single matching resistor to the input. The correct value is 37.5 ohm. That will make the load as seen from the source 50 ohm and eliminate the primary reflection. The outputs are still not matched, so any reflected wave from an unterminated load will cause trouble. The other option is to put 30 ohms in series with the input and also with each of the outputs. That will make a symmetric 5-way splitter, and each port will be impedance matched. This will be most effective at damping any reflections from mismatch at the loads.

author: oakkar7
author_rank: Contributor
timestamp: October 07, 2014, 04:53:12 am
content: 1# you need to consider 50 ohm impedance and your desired freq range first. Use thishttp://www.eeweb.com/toolbox/microstrip-impedance2#For routing RF signal, see herehttp://www.eeweb.com/blog/circuit_projects/basic-concepts-of-designing-an-rf-pcb-board3# you will definitely need matching network or balun. As mention above, resistor will be a basic choice. if I were you, I will choose this methodhttp://www.dxing.info/equipment/rolling_your_own_bryant.dx

author: oakkar7
author_rank: Contributor
timestamp: October 07, 2014, 05:35:02 am
content: Quote from: ejeffrey on October 07, 2014, 04:52:18 am+1

author: LukeW
author_rank: Frequent Contributor
timestamp: October 07, 2014, 10:08:23 am
content: For a 50 ohm microstrip line on average FR4 material with a typical thickness of 1.6mm the width should be about 100 thou, so your statement that the width is 90 thou is roughly correct.However, your board has a top layer copper groundplane on it, and that top pour has to be kept well away from the microstrip line or it changes the impedance. You need to keep it well clear of the RF line, or remove the top groundplane entirely, or recalculate the width of the 50 ohm line using a CPWG model. (The 50 ohm line will be significantly thinner.)But your actual design of the transmission lines to make a 5-port splitter doesn't seem to make sense.What you seem to actually want is something like a 5-port cascaded Wilkinson divider suitable for your frequency of interest - or a set of multi-loop Wilkinsons if it needs to be more broadband.If you want something for instrument calibration I agree that it doesn't make sense to build your own, unless you've got calibrated instruments to calibrate what you're building. You can't calibrate anything to a standard better than the calibration of the instruments you're using to calibrate it.Buying something suitable won't be extremely cheap, but buying and calibrating good RF instruments generally isn't.

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 12:14:04 pm
content: Here is the fixture I was trying to duplicate:It is used for vertical calibration and delay calibration.It isn't readily obvious if they put resistors inline in their fixture, but the batronix one appears to not have any and presumably works.

author: bookaboo
author_rank: Frequent Contributor
timestamp: October 07, 2014, 12:39:08 pm
content: Watch out with your footprints whatever you do, be sure to leave enough space for the crimped connectors to be positioned.

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 01:31:41 pm
content: Quote from: bookaboo on October 07, 2014, 12:39:08 pmI appreciate the tip, I'm going to have the center connector mounted on the opposite side of the pcb, but I'll measure the outer ring of 5 to make sure there is room between them.

author: tggzzz
author_rank: Super Contributor
timestamp: October 07, 2014, 02:13:03 pm
content: Quote from: alank2 on October 07, 2014, 12:14:04 pmWhat are the range of delays? What accuracy, total and differential, is required?What frequencies are used for vertical calibration?Does vertical calibration mean that all voltages are the same, or different by a known amount, or...?What is the terminating impedance?I have a suspicion, that isn't grounded in knowledge, that you might be able to get away with virtually any old rubbish provided that the wire lengths are the same and the end-to-end resistances are similar. You might just benefit from having two jigs, one for delay and one for voltage.

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2014, 02:26:09 pm
content: Quote from: alank2 on October 07, 2014, 12:14:04 pmTektronix uses a similar calibration fixture just made up of a BNC-T and x10 attenuator for two channel delay compensation. The impedance mismatch and reflections are not significant as long as the outputs are consistent with each other. The attenuator is removed for vertical calibration which occurs at low frequencies with high impedance inputs.In light of the above, for more than 2 outputs if I did not just use BNC-Ts and matched length cables, I would add series terminations like NiHaoMike and ejeffrey suggest. Anything more will make it useless for vertical calibration.

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 07:52:42 pm
content: Forgive me guys for not spelling out up front very well what I was trying to accomplish.Here is a discussion of the calibration kit/cable here:https://www.eevblog.com/forum/reviews/rigol-ds4014-decided-it-would-be-more-fun-to-be-a-ds4054/15/Also here:https://www.eevblog.com/forum/testgear/rigol-mso4000-and-ds4000-tests-bugs-firmware-questions-etc/msg372708/?topicseen#msg372708Here is a picture of the one Batronix sells:Basically I am trying to think about how cheaply something like this could be put together....

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2014, 09:05:39 pm
content: Like I said, Tektronix used BNC-Ts and 50 ohm patch cables to do the same type of calibration on two channels at a time. Given that this is for delay and vertical calibration, it can be extended to four channels easily and I suspect Tektronix did that on their four channel DSOs.Select five 50 ohm patch cables and 3 BNC-Ts that you trust and you are good go to. For the delay calibration which occurs with 50 ohm terminations, they also used a 50 ohm x10 attenuator just before the BNC-Ts to dampen reflections.

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 10:02:22 pm
content: Thanks David. So, let's say I want to try to do this for the minimum $ amount, but instead of going the T route and cables which would be slightly off in length, what if I:Bought 3 50ohm rg58 cables and cut them into 5 short sections around 6" and then a final section to connect to the aux output connector on the back. This is essentially what their pictures 5-in-1 bnc cable looks like except that I'm not sure if they just joined the center conductor, etc. What I was thinking of doing is making a very tiny pcb that I solder the cut off end of the RG58 cables to directly, making sure they are the correct lengths. I get the cables for $13 from mouser and make a tiny pcb 1" x 1" pcb to split them for $5 at dorkbotpdx. Any issues with this idea/plan?

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2014, 10:30:03 pm
content: I expect it will work fine if you take care to keep the signal propagation lengths equal. I have made it a habit to cut my patch cables so they are 2.0 or 5.0 nanoseconds long which comes out to 16.4 or 41 inches if the velocity factor is 0.695 (teflon) so when I last did a delay calibration following the instructions in a Tektronix service manual, I was all set.If I made a dedicated splitter like you describe for delay and amplitude calibration, I would add series resistors where the signal splits to dampen reflections a little bit like ejeffrey suggests. Even better would be to use a resistive-T power divider which just requires one additional resistor. That will allow for an excellent broadband match albeit with twice the loss.http://www.microwaves101.com/microwave-encyclopedia/685-resistive-power-splitters

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 10:38:56 pm
content: Quote from: David Hess on October 07, 2014, 10:30:03 pmWould that cause an issue if the original cable Rigol uses is simply a splitter? We are talking about a resistor from the source to each of the 5? What value would be good to use?

author: TerraHertz
author_rank: Super Contributor
timestamp: October 07, 2014, 11:04:44 pm
content: Quote from: alank2 on October 07, 2014, 12:14:04 pmAnd so why on earth are you even thinking about doing a PCB? That's extreme over-thinking.Surely you have some BNC patch leads? You only need 3 identical leads, from which to cut the 5 end pieces all the exact same length, and one source lead (as long as you need).For delay time equalization and relative amplitude, who cares about reflections? No resistors needed, and so it's just a simple exercise in coaxial cable splicing. Some fine bare tinned wire, a little extra shield braid, some sleeving, some heatshrink tube (or tape), and you're done.If impedance matching mattered for this application, you'd be better off buying a cheap coax splitter, like ebay 191357948051. But it doesn't.

author: alank2
author_rank: Super Contributor
timestamp: October 07, 2014, 11:09:19 pm
content: Quote from: TerraHertz on October 07, 2014, 11:04:44 pmThe prospect of joining those 5 together seems like a mess...

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2014, 01:58:35 am
content: Quote from: alank2 on October 07, 2014, 10:38:56 pmIt should not make any difference in delay or vertical calibration. The delay signal will be a lot cleaner though.The link I gave shows how to calculate the resistors for splitters with a different number of matched outputs.

author: alank2
author_rank: Super Contributor
timestamp: October 08, 2014, 12:28:23 pm
content: Since I don't know when to quit, here is a "joiner" pcb I put together last night. 90 miil traces on top, no ground pour on top, a place to cable tie the rg58 down.I didn't add any resistors to it, but I could do the wye from your link in place of my 5 tracks. I checked out the link you provided David, but it is way above my knowledge. I suspect that the Rigol cable/pcb do not have any resistors, could adding them actually cause it to not work right, or would they only improve it?

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2014, 08:04:08 pm
content: Quote from: alank2 on October 08, 2014, 12:28:23 pmI have done it without the resistors but I used a 50 ohm attenuator on the input to dampen reflections which worked well enough although I was only splitting the signal for two channels.If the resistorsdidcause a problem which I would not expect, then they could always be bridged.The formula given at the link, R=Zx(N-1)/(N+1), yields a value of 33 ohms for the resistors with a 5 way split.

author: alank2
author_rank: Super Contributor
timestamp: October 08, 2014, 09:04:21 pm
content: So David, I would need to add a 33 ohm resistor to each of the 5 legs I have above AND to the source for a total of 6 resistors, right?

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2014, 09:18:01 pm
content: Quote from: alank2 on October 08, 2014, 09:04:21 pmYes.

author: alank2
author_rank: Super Contributor
timestamp: October 08, 2014, 09:50:39 pm
content: I've reworked the pcb to add the resistors, how does this look:

author: TerraHertz
author_rank: Super Contributor
timestamp: October 09, 2014, 09:00:23 am
content: Quote from: alank2 on October 07, 2014, 11:09:19 pmDid you ever try?Does the sample cable you posted a pic of, look like a mess? I don't think so.Here's how it's done. This probably took longer to draw than it would to actually do it.Use solid inner insulation coax. The foam insulation stuff can't stand up to soldering.