Question about impedance matching resistors, use one or use several? - Page 1
Forum section: electronics / projects

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 06, 2014, 12:42:56 pm
content: In quite a few circuits designed to drive into a 50ohm line you will see a single 50ohm resistor used as impedance matching, other times the designer might use 4x200ohm resistors in parallel.Are there certain advantages or disadvantages to each of the methods?

author: hlokk
author_rank: Contributor
timestamp: October 06, 2014, 01:18:58 pm
content: Three possible reasons:1) Possibly increased tolerance (i.e. 4x 1% resistors will give an equivalent 0.25% accuracy, assuming a gaussian or even distribution of individual resistor values. Though wont help much if the batch is all on the high or low side).2) Higher total current/power rating.3) Thats what they had surplus of.Advantage of a single 50 ohm is obviously cost and simplicity.

author: bktemp
author_rank: Super Contributor
timestamp: October 06, 2014, 01:32:31 pm
content: Using multiple resistors in parallel reduces its parasitic inductance.

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 06, 2014, 02:31:32 pm
content: Quote from: bktemp on October 06, 2014, 01:32:31 pmWouldn't this be offset by the increase in parasitic capacitance? or is inductance more important?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 06, 2014, 10:58:22 pm
content: Worth noting that the source may not be zero ohms and therefore less than 50 ohms series is required. Depends on the circuit, and at what frequency; correct compensation of a feedback-stabilized amplifier (like you'd find in a function generator) may require an R||C because the output impedance rises at higher frequencies. (I don't think this is usually done, it's just something to think about.)Tim

author: DanielS
author_rank: Frequent Contributor
timestamp: October 06, 2014, 11:18:07 pm
content: Quote from: AlessandroAU on October 06, 2014, 02:31:32 pmThat depends on the application.But in most cases, the main reasons would be power rating and tolerances.

author: G0HZU
author_rank: Super Contributor
timestamp: October 06, 2014, 11:56:30 pm
content: There can be very valid reasons for doing this at microwave frequencies. With small SMD resistors (eg 0603 or 0805) the impedance of a single 50R resistor will deviate away from 50R much quicker than a pair of 100R resistors in parallel as you go up in frequency to several GHz.So the two 100R resistors in parallel will make a much, much better broadband 50R dummy load compared to the single 50R resistor.

author: Dave
author_rank: Super Contributor
timestamp: October 07, 2014, 02:03:51 am
content: Quote from: hlokk on October 06, 2014, 01:18:58 pmAssuming Gaussian distribution (highly unlikely with 4 resistors, but ok), your tolerance would be reduced to 0.5%, not 0.25%. The tolerance is reduced by the square root of the number of resistors used.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 07, 2014, 05:03:55 am
content: Quote from: AlessandroAU on October 06, 2014, 02:31:32 pmFor most resistors the L dominates for low value resistors around 50 ohms. For higher value resistors, the capacitance is more important. According to this document:http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdfthe crossover is around 75 ohm for MELF and 120 ohm for chip resistors. Basically, if you take the parasitic L and C, you can find a 'parasitic impedance' sqrt(L/C). For resistor values less than that, the parasitic reactance looks primarily inductive. Above, the reactance is primarily capacitive.

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 07, 2014, 11:57:55 pm
content: Quote from: ejeffrey on October 07, 2014, 05:03:55 amSo wouldn't it be pointless using 4x200ohm resistors because the reactance is primarily capacitive already?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 08, 2014, 02:07:09 am
content: Quote from: AlessandroAU on October 07, 2014, 11:57:55 pmNot if you cut out the ground plane underneath them, so they look like lossy 200 ohm transmission lines.Four 200 ohm transmission lines in parallel, of course, look like 50 ohms again.Paralleled resistors is probably more common as a throwback to the days of leaded resistors, where impedances were higher due to the thinner lead diameter, higher lead/body elevation above ground, and low dielectric constant of air.Supposedly, an 0603 resistor is fractions of a dB better "upside down". At 10GHz. I'm not sweating it, not for 0805 at anything I'm ever going to work with.Tim

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 08, 2014, 07:21:47 am
content: Quote from: T3sl4co1l on October 08, 2014, 02:07:09 amHow much worse are axial leaded components compared to smd?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 08, 2014, 07:31:37 am
content: 1/4W size axial, values +/-50% from system impedance, maybe noticeable by a gig or a few?Obviously, bigger ones are going to have worse issues. Or vertical positioning, or being high up off the board (common for power resistors). Let alone, inductive types.Tim

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 08, 2014, 07:41:46 am
content: Quote from: T3sl4co1l on October 08, 2014, 07:31:37 amThe reason is ask is because I am working on a Jim Williams Pulse generator, should I use SMD for the matching resistor?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 08, 2014, 07:50:30 am
content: I would go with it, yeah.Protip: if you're doing it on 2-sided 1.6mm thick PCB, a 50 ohm trace (coplanar waveguide) with reasonable clearance is about the same width as a 1206.Tim

author: mazurov
author_rank: Frequent Contributor
timestamp: October 08, 2014, 03:41:26 pm
content: That's how you do it for the JW pulser. 0603 resistors fit perfectly into SMA dimensions. The reason to have multiple resistors spread evenly is to make termination co-axial.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 09, 2014, 01:32:03 pm
content: I'm just putting together a project at the moment with multiple input resistors. I've done it for power dissipation reasons. I'm expecting to see 5v which into a 50R is 1/2 a watt. That's too much for small SMDs so I've put four 50Rs in series/parallel to still give 50R but each individual resistor is only seeing 1/8W

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 09, 2014, 03:33:22 pm
content: What do you think of this avalanche pulse circuit, taken from here;http://cds.linear.com/docs/en/application-note/an94f.pdfIgnoring the changes in the triggering portion of the circuit it uses an inductor in series with the coax and some adjustable damping to reduce overshoot, is this a significant improvement over the more basic jim williams circuit?

author: mazurov
author_rank: Frequent Contributor
timestamp: October 09, 2014, 04:17:58 pm
content: They are similar. Build one, make it work then mod into another. Unless you follow someone else's build notes you won't be able to get nice shaped pulse on the first try (and even if you do it's not guaranteed). Expect to move parts around a lot so lay them out accordingly. I made mine placing everything on or in the air between two sma connectors ( similar to one on the picture 3 posts above ) soldered to the ground plane - one for the output and another to connect random pieces of the coax line.BTW, I've seen another arrangement somewhere where the power is provided via the other end of the coax line. This arrangement reduces clutter around the most sensitive node of the circuit which is the collector pin of the transistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2014, 04:49:44 pm
content: L4 basically compensates for the R+C reservoir, which tends to be peaky (to get a good short fast pulse), and hence you need to let off the TL for a moment (considering the dimensions of the 'inductor', a fraction of an ns or so) to compensate.You could also use a few mm of narrow trace (high impedance looks inductive). The length of which should be proportional to C, and the width, complementary to R (so that the parallel combination exhibits a somewhat more constant impedance as time goes on).If you don't have a GHz scope, you probably won't even begin to be able to see the difference.Tim