Estimates for the bandwidth (-3dB) of a small signal relay? - Page 1
Forum section: electronics / beginners

author: max_torque
author_rank: Super Contributor
timestamp: November 23, 2016, 09:53:52 pm
content: Say i have an op-amp, and the output of that amp has to go through a typical small, signal relay, like this:Given the low impedance output of the opamp, what would be a good estimate of the bandwidth limitation imposed on that output by the relay being in series with it???Obviously the relay contacts have some capacitance (which changes between open & closed states i suspect), and a small amount of resistance when closed?Anyone ever tested such a relay with a vector analyser etc??

author: Zad
author_rank: Super Contributor
timestamp: November 23, 2016, 09:58:01 pm
content: I wouldn't normally use a relay like that for anything higher than HF - 30MHz or so. Couldn't you use diode switching instead?

author: tggzzz
author_rank: Super Contributor
timestamp: November 23, 2016, 10:03:33 pm
content: Don't forget the inductance, which might be dominant in some modes.RF relays exist, which have a well-defined frequency response and impedance.

author: max_torque
author_rank: Super Contributor
timestamp: November 23, 2016, 10:12:23 pm
content: The signal i'm looking to pass has most of it's frequency content significantly below 1Mhz, but contains some fast edges that have content up at a maximum of ~10MHz, and i'm looking for a simple, robust way to switch between that signal and a generated "test" signal (for down stream system calibration checks) so a DPDT relay is an easy option.The data sheets for relays don't generally seem to include the bandwidth, just occasionally some reference to contact capacitance etc. Any specific RF relay is too expensive......

author: dmills
author_rank: Super Contributor
timestamp: November 23, 2016, 10:21:49 pm
content: The IM series do in fact give some RF performance numbers @ 100Mhz and 900MHz, see page 2 o the datasheet. Isolation is not stunningly wonderful and the data is a bit thin compared to what you would expect of a real RF part, but for HF use they seem likely to be fine.73 Dan.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2016, 03:20:48 am
content: It varies. There are many subtle aspects to consider:- Crosstalk between open contacts, and separate sets- Stubs, reflections and impedance mismatch along the length of the contacts- Microvolt DC thermal errors (easily solved with latching relays)- Contact bounce- Arcing at higher voltages- DC resistance; repeatability (how it varies between contact closure events); possibly rectifying contacts (due to oxidation)- DC leakage (mostly over the surfaces of plastic insulators, but including corona discharge at HV)Unless you're doing extremely sensitive or high current applications, we can safely ignore the contact-closed properties. And ditto, but for high voltages instead, we can ignore the contact-open properties. And for low power signals, we can ignore arcing as well.That basically leaves frequency response. For a relay of that size (about 1cm), the contacts and pin lengths are on the order of 1cm as well, and so we expect to see quirks (humps and valleys in the s-parameters when open and closed) in the GHz range.At low frequencies (say, under 1GHz), all those properties are asymptotic, and can be safely approximated as capacitances between open contacts, and inductances (including mutual inductances) between closed contacts.Even a generic, signal type, not-RF relay has enviable properties: on the order of ~2pF per contact (to whichever nearby contacts, or the frame), and ~10nH per closed contact pair. The impedance is broadly around 100 ohms (as most things are!), and the "cutoff" frequency (not a -3dB point!) on the order of 1GHz.The cutoff frequency is simply where behavior becomes more interesting, i.e., the lumped element model breaks down. The response ceases to be asymptotic around here, and you probably won't get useful switching behavior (i.e., on/off ratio).For a relay used at very different impedances (i.e., very far above or below ~100 ohms), one or the other lumped equivalent parameter (L or C) will become significant, at a proportionally lower frequency (say, for a circuit impedance around 10kohm or 1 ohm, there will be a lowpass cutoff around 10MHz).The closer you get to the true characteristic impedance of the device, the more bandwidth you'll be able to use, up to its full frequency range (~1GHz). This is one possible way of measuring its characteristic impedance. (But, because a generic type relay won't have an impedance-controlled design, it won't have a stable characteristic at higher frequencies, so there isn't much value in measuring it precisely. An RF relay will be designed this way, however, and will be able to operate well above its LF 'cutoff' frequency.)Tim

author: tggzzz
author_rank: Super Contributor
timestamp: November 24, 2016, 10:45:19 am
content: Quote from: max_torque on November 23, 2016, 10:12:23 pmThat doesn't sound too bad, but where "non-sinusoidal" signals are concerned it is usually better (and easier) to quote the risetime requirements directly. As any physicist knows, choosing the right coordinates/domain can vastly simply the calculations!Depending on the circuit and signals, it is conceivable that a v=Ldi/dtcalculation might be relevant.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2016, 12:59:09 pm
content: Quote from: tggzzz on November 24, 2016, 10:45:19 amWell, that, or the I = C dV/dt. Or both at once in a network, or even more, a complex transmission line structure. The great thing about the latter is, you can tell at a glance, which of the former it will be!Tim

author: Zad
author_rank: Super Contributor
timestamp: November 25, 2016, 12:49:05 am
content: In this instance, it might be profitable just to mock up a test board and make some measurements. Simulation and mathematical modelling is helpful, but sometimes actually making the thing can be quicker and tell you more.