Impeadence matching vs high impeadence buffering? - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 08, 2020, 09:41:52 pm
content: Ok im not sure if i am using all the right technical terms, so forgive me if my symantics are off. So I understan AC therory and reactence/reluctance, impeadence real and imaginary. But i having hard time grasping something more practical i need to understand for a 3mhz current probe im working on.So in RF and audio/video we use impeadence matching at 50/600/75 ohm, respectively. This is to insure maximum power transfer. Test gear usuall has much higher impeadences in the mega ohm region, and this is done to ensure your gear doesnt load the DUT and effect the measurement.Now why is it if you connect a 50ohm function gen to a 10megaohm scope input using cables well shorter than 1/4 wavelength, without using a 50ohm passthrough on the scope you get power loss and reflection? Wouldnt the scopes input have no effect on the signal, as if the function gen were not even connected? When deigning a device designed to plug in to a scopes BNC shoud the device end have a 10mega ohm termination to match the scopes impeadence? Is just using a current buffer/voltage folllower enoough or is none of this neccisary at 3mhz, and if not what about at 100mhz?

author: Kirill V.
author_rank: Regular Contributor
timestamp: February 08, 2020, 09:58:07 pm
content: I also want to learn a little more about the topic of radio frequency devices.Termination on the source and receiver side as well as the cable is all a single system called the transmission line. Everything is important in it. And the mismatch of cable and termination impedances will not allow the transmission line to function correctly. This is my personal knowledge in this area at the moment. They must be right:)

author: TimFox
author_rank: Super Contributor
timestamp: February 08, 2020, 10:44:12 pm
content: With a 50 ohm source impedance, an arbitrary length of 50 ohm cable, and an open-circuit (high impedance) termination, the following happens for ideal resistors and transmission lines:Imagine the source putting out a narrow pulse (much less than the time delay of the cable). If the source makes 2 V before the internal source impedance, it would deliver 1 V into a 50 ohm termination.At the source output connection, a 1 V pulse starts traveling through the cable. When it reaches the open-circuit termination, there is apositive(non-inverted) reflection which travels back to the source, where it is absorbed in the matched 50 ohm source. At the open-circuit termination, it adds to the original 1 V pulse to produce a 2 V pulse at the oscilloscope input. If the impedance at the oscilloscope were 50 ohms, there would be no reflection, and a 1 V pulse at the input.Replacing the 2 V pulse (before source resistance) with 2 V DC would give 2 V into an open circuit and 1 V into 50 ohms.The sine wave case is left as an exercise for the student.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2020, 10:55:28 pm
content: Quote from: rwgast_lowlevellogicdesin on February 08, 2020, 09:41:52 pmYou don't. Try connecting your oscilloscope to the output of a signal generator set to a 1kHz sine wave output and you'll see a perfect waveform.QuoteIt doesn't. If you have a true RMS multimeter capable of working up to 1kHz, and connect it to the output of the signal generator, the voltage will hardly change between the oscilloscope being connected and open circuit.

author: Kirill V.
author_rank: Regular Contributor
timestamp: February 08, 2020, 11:04:34 pm
content: Wideband oscilloscopes can have both 50 Ohms and 1 MOhm input impedanceConductors for pulse signals may not be transmission lines if their length is small enough

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 08, 2020, 11:40:07 pm
content: It's for power transfer, right. Except when it's not.Noise matching and signal quality are also good motivations.Typically, the ratio between noise voltage and noise current of a port, happens to be close to its small-signal impedance, but it doesn't have to be exact, and it can differ significantly.Unfortunately, I don't have examples or reasons handy for why this can be the case; I'm not well read on low noise design.In general, the small-signal impedancecanbe very different from the large-signal or intended impedance, thanks to device properties and negative feedback, so that is a possible explanation.Where signal quality is paramount, the power transfer need not matter at all. All those digital logic and comm standards using source termination resistors (USB is a common example), are simply loaded by a slightly capacitive CMOS input pin at the end. This load strongly reflects a wave back to the source, where it is absorbed. If the pulse length is longer than the line length, the wave also opposes further current flow, conserving power and allowing smaller transistors to drive the line. If not, the waves superimpose elsewhere on the line, but at the terminal end, signal quality is still maintained -- hence this is usable for strict point-to-point buses.For multi-drop buses, either the pulse length needs to be longer than the line length, or the line must be doubly terminated.This for example limited the clock speed of PCI buses to, whatever they were, 66 or 100MHz was it? The relevant distance being the length from the motherboard system chip, up and down each expansion slot, to the end. Source termination was used, so signal quality at any point along the bus was poor, except for the very ends, but no useful assumptions can be made about that as a card can be plugged in anywhere along the bus.Industrial RS-485 buses (multidrop) use low impedance drivers and a doubly-terminated line. The source impedance doesn't need to match, because signal level is a higher priority than power transfer, and a lower impedance source is able to pull the line closer to VCC/GND. They do need pretty beefy transistors to implement this (while also being rated for short circuits), hence there's usually an external driver chip for the purpose.Audio is impedance matched, when signal quality matters -- the power doesn't matter much at all, it's just some standard line level (or less, since after all, audio isn't constant power; well, much as commercial broadcasters attempt to do otherwise..). Matching the line improves signal quality (with respect to interference and losses), and a well-defined impedance facilitates transformer design (as a transformer is itself another kind of transmission line component).Much audio is not matched; rather, a low impedance driver is used (an op-amp output; some termination resistance may be added, and this is more for preventing the amp from oscillating into an unterminated line, than for signal or power reasons), feeding a high impedance load (10k? 100k?). Simplicity and cost are stronger motivators here, and anyway, interference is most often from low impedance sources (ground loop) where you don't really have any other options besides differential signaling or isolation, both of which cost extra to implement.Another good example of active circuits with odd impedances: the standard audio (loudspeaker) amplifier is very low impedance, fractional ohm, perhaps milliohms; but the speaker is an impedance of, well, notionally 8 ohms or whatever, but it varies quite a lot as well. Speakers can indeed be designed for an impedance-matched system, but they don't need to be, and it seems to be the modern convention to optimize around a CV source rather than a matched one.Needless to say, you won't get much power from your amplifier if you use a 0.01 ohm speaker; but actually, you will, but you need to understand where, and why. The power transfer theorem assumes a linear system. The amplifier is linear for small signals at least; if we deliver an output of say 10mV, and the amplifier's current limit is well in excess of 1 ampere, then we will obviously draw much more power from those 10mV, with a 10mΩ load, than with 4 or 8Ω! The amplifier itself is not a linear device overall, and will either limit extreme currents or voltages, or destroy itself trying to, when the load is very different from nominal.Regarding the oscilloscope and function generator: the generator is source-terminated, so in principle, the signal quality at the end will be fine at any load impedance, including 1 or 10MΩ. (In practice, it's about half as good as doubly-terminated; but again, it's a power tradeoff in practical systems.)Your piece of test equipment can be designed either way. You might even make it switchable. It should have a 50 ohm source impedance regardless, to limit short-circuit current and give good signal quality on any length cable -- but whether the gain is set for a matched load, or open, is your choice.You certainly cannot make it a matched impedance to a probe, because probes simply don't work that way -- they are designed to read a voltage, and their impedance varies from 10M at low frequencies, to ~100s at high frequencies. Between, the impedance is capacitive. This is usually written as 10M || 6pF or something like that, but that's only a mid-frequency approximation, and there are other impedances that show up at high frequency. Hence the impedance doesn't continue to tank, it actually levels out, or perhaps bounces around.You can't match that, because presumably the intent is to have it matched over the whole band of interest -- but that would require a negative capacitor, which is impossible*.*You can synthesize one with an op-amp, but in this case I think you'll just get a very roundabout oscillator. If nothing else, the gain won't be flat, because again, the probe is designed for sensing voltage.Scopes also aren't terribly good examples of signal quality -- some are, or can be; but for the most part, I mean, you have an 8-bit ADC, and about as much display resolution, and even that is a lot to expect from a standard clip-on style probe. They're only designed to measure down to the mV's, and typically have a noise floor of fractional mV (say, 10s of LSBs on the most sensitive range?), so aren't terrifically useful down there to begin with.Incidentally, your current probe itself is an example of -- explicitly and intentionally -- mismatched impedance. The RF equivalent is a directional bridge, which taps off a fraction of the transmitted or reflected wave into respective ports. A typical implementation uses a pair of transformers, effectively one of which senses a fraction of line voltage and the other senses line current. (Actually they serve reciprocal purposes, because they're arranged symmetrically -- the whole network is symmetrical of course.) If you leave off just one, well, you're adding an impedance in series with the line, or in parallel with it, and that's necessarily a mismatch.So, it's simple to see that a PT or CT must be a discontinuity. We take advantage of the fact that, at low frequencies, and for modest gains, we can have a minimal impact on the line impedance (Zpar >> Zo or Zser << Zo), so we don't worry about reflections from them. But as you go up in frequency, it does become more and more important, and you eventually get to a point where you need to implement it as a power tap or directional bridge instead.(At 3MHz, you have absolutely nothing to worry about.)Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 09, 2020, 03:33:14 am
content: Quote from: T3sl4co1l on February 08, 2020, 11:40:07 pmDo not forget efficiency. RF power amplifiers are deliberately *not* impedance matched to their load because if they were, 1/2 of their output power would become heat immediately limiting efficiency to 50%. The same goes for alternators used for power generation. You can get a hell of a lot more power from either by lowing the impedance of their load ... causing the RF power amplifier or alternator to burn up.One of the most useful things I learned about RF power amplifier design was to design backwards. If I know the supply voltage and output power, then I know what the load impedance should be.QuoteAnother reason better than 8-bit digitizers are of limited use with an oscilloscope is that input noise is limited by the high impedance input buffer. Not only is the integrated noise level high, which limits sensitivity, but the spot noise is high and the flicker noise is high because these are compromised in a high impedance buffer which has to support the full bandwidth of the instrument.If you want a low noise oscilloscope, look for a low bandwidth one. 1 MHz can get you to 10 microvolts/division or even better now.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2020, 04:25:42 am
content: Quote from: David Hess on February 09, 2020, 03:33:14 amYup. Discussed basically that, but helps to make the point explicit.And relating to device properties -- RF amps are also deliberately not matched, because the collector/drain output impedance for example is very high, so the available power into a matched impedance is very small -- again, amps are ultimately nonlinear, and those limits on voltage and current constrain how much power you can get into a small-signal match. So we don't bother with matching, and go for maximum power output, or efficiency or the like, instead.Which does mean that waves incident upon a transmitter's output port, are likely to be reflected, and worsen signal quality. But that's usually a pathological case -- we want low SWR for other reasons, anyway -- so we don't have to worry about it.And again, feedback can modify that, so, say, an RF PA with a shunt feedback resistor (often done for stability) will have a lower output impedance, and may end up reasonably well matched.Class D amps need a high Z (for a voltage-fed inverter, or low Z for a current-fed inverter) to keep efficiency high, for the same reason that generators and such also need to.Automotive alternators, with the awful efficiency they usually have, I wonder how close they operate to maximum power point...Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 09, 2020, 04:48:23 am
content: Quote from: T3sl4co1l on February 09, 2020, 04:25:42 amI was referring to the much larger alternators used to generate 50/60 Hz line power.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2020, 05:03:23 am
content: I know. Separate wondering.Tim

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 09, 2020, 05:16:37 am
content: Awesome answers guys, that has cleared up a lot of confusion for me about why some systems are designed with matching impeadence vs High Z connected to random Lower Z.Also I happen to have OLD OLD HP 1200 analog scope (I believe it was HPs first solid state design), it is only 500Khz but I keep it around because it does 100uV a division and its inputs can be either single ended or differential. It is pretty useful for low freq low noise measurement and sees a lot of use actually. I had no idea that its sensitivity/noise was directly related to the bandwidth, is there a more compact modern peace of gear I could use replace it? Obviously an ebay 1mhz scope kit isn't going to be anywhere near as low noise!

author: TimFox
author_rank: Super Contributor
timestamp: February 09, 2020, 03:31:55 pm
content: Quote from: rwgast_lowlevellogicdesin on February 09, 2020, 05:16:37 amI also use a 1200 scope for audio measurements. I think that the noise is close to the physical minimum for the 50 kHz low-pass filter setting. It also has a nice crisp trace. For lower noise, I use a Tektronix 7A22 differential plug-in in a 7600 mainframe. It goes down to 10 uV.div, since the low-pass filter is selectable (1 3 10 sequence) from 100 Hz to 1 MHz.With more modern scopes, you might want to look at external preamps with filter settings. I have two from PAR that have high gain and switchable hpf and lpf.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 09, 2020, 05:57:28 pm
content: Impedance matching is for transmission maximum possible power (or to have less possible loss of power etc) over transmition line.High input impedance usually for low-frequency signal transmission, it gives highest signal voltage at the load (but not the power).

author: iMo
author_rank: Super Contributor
timestamp: February 09, 2020, 06:01:22 pm
content: Quote from: David Hess on February 09, 2020, 03:33:14 amAs a HAM I have a pretty hard time to buy it.Could you somehow elaborate on it?Provided my PA's output impedance is 50+j0, then I have there 40m of 50+j0 cable, at the end a Load. What should be the proper impedance of my Load to get maximum efficiency?

author: TimFox
author_rank: Super Contributor
timestamp: February 09, 2020, 07:19:47 pm
content: Quote from: imo on February 09, 2020, 06:01:22 pmYou are confusing output impedance with load impedance. Matching the load impedance to the cable’s characteristic impedance eliminates reflections and the unnecessary losses in the cable from high standing wave voltages. However, an amplifier designed for high efficiency into the 50 ohms at the amplifier end of the cable probably does not have an actual source impedance of 50 ohms. You can adjust the amplifier output network to obtain maximum power or efficiency when connected to a 50 ohm dummy-load resistor, and then the same power will be applied to the 50 ohm cable driving the matched 50 ohm load. To measure the source impedance of your amplifier output, you could change the load resistance (not so easy at high power) and see how much the output voltage changes.At the other end, a 1/4 wave vertical receiving antenna does have a source impedance close to 50 ohms. It will therefore transfer maximum power to a 50 ohm load. However, the optimum impedance for best snr may be different, even complex. A good noise figure meter works by switching a noise generator on and off, with an accurate 50 ohm source impedance in both states. You get best noise figure of the receiver by adjusting the input matching network (both real and imaginary) for best NF, but the impedance looking into the receiver will, in general, differ from 50 + j0.

author: David Hess
author_rank: Super Contributor
timestamp: February 10, 2020, 12:52:32 am
content: Quote from: TimFox on February 09, 2020, 03:31:55 pmTektronix also produced theAM502which is a 7A22 with a BNC output for direct connection to any oscilloscope or other test instrument. Essentially it is a 1 MHz 10 uV/div high voltage differential probe which uses a pair of high impedance passive probes.QuoteLeCroy has something like that from when they purchased Preamble but it is either discontinued recently or soon to be discontinued. AM502s are not as common as 7A22s and they cost more on the used market but are certainly available.Quote from: TimFox on February 09, 2020, 07:19:47 pmExactly, the output matching network, which will be an L or T network for narrow-band application and a transformer for wideband applications, sets the output power for a given supply voltage with an impedance transformation. But the impedance at the output of the transistor and the input of the network are deliberately *not* matched to preserve high efficiency at the expense of output power.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 10, 2020, 12:53:02 am
content: after re reading this thread, I have another question. So audio is all over the damn place, but a lot of older pro gear is mostly 600 ohm and everything is matched with transformers... why audio is such a Slow signal it has no transmission effects, and as stated above impeadence matching is not always the best way to transfer maximum power, even at rf an antenna tuner is a great example where you may find the best SWR at a random impeadence. Most pro audio uses either balanced or sheilded cable so I wouldn't think noise is much issue, so what gives with 600ohm?

author: TimFox
author_rank: Super Contributor
timestamp: February 10, 2020, 03:12:39 am
content: Quote from: rwgast_lowlevellogicdesin on February 10, 2020, 12:53:02 amI believe that 600 ohms originates from the characteristic impedance of balanced telephone circuits, which can be adjusted with appropriate inductors inserted periodically to maintain the impedance. It’s not practicable to obtain this value with coax or twisted pairs without such inductors, helical inner conductors, or ferromagnetic materials. In long trunk lines, even slow audio signals can be degraded by reflections.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2020, 05:03:54 am
content: IIRC, telephone is typically over twisted pair with around 120 ohms Zo, but this is the high frequency value, and at low frequencies where ESR is dominant, Zo is apparently higher, and also complex (but the magnitude or real component, of 600 ohms, is best known). I forget exactly how that comes about, or how they determine that it's correct over the entire 500-3000Hz band.Probably, neither is quite the case, hence the aforementioned compensation inductors, and also a number of seminal articles in early BSTJ history which had to do with compensating the frequency, phase and impedance response of long lines. The necessity of which would seem to go along with nonideal characteristics, like all of those changing over a wide band like this.Probably, pro audio adopted 600 ohms as a compatibility standard. It was easy enough to drive with period circuitry (e.g., direct drive from a tube cathode follower).Regarding the antenna tuner -- thatisto match the impedance; the line will see 50+j0 ohms (at perfect match), while on the antenna side it's some oddball value, but whatever that oddball value is, is what the matching impedance at that point is (or more specifically, the impedance and its conjugate, on either side of the point).Tim

author: ejeffrey
author_rank: Super Contributor
timestamp: February 10, 2020, 05:16:43 am
content: Quote from: T3sl4co1l on February 08, 2020, 11:40:07 pmOne simple reason is feedback. Look at an opamp: the noise resistance will typically be a couple hundred ohms to a few kilo-ohms depending on whether they are BJT, JFET, or MOSFET inputs and the optimum noise figure is given when the source resistance matches the noise resistance. But the small signal input impedance is set by feedback to be extraordinarily high.Noise impedance equals signal impedance for thermal noise on resistors, and by extension passive LCR networks, but not always in active circuits.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2020, 05:40:43 am
content: Yeah, feedback can skew the small-signal impedance all over the place, but the noise and current remain more or less the same.Though those can change too, as feedback can have a refrigeration effect on noise sources. I know this... enough to know that it exists; I don't know it well enough to know the practical consequences, or physical limits. So I don't want to make a general statement on it.Heh, also, exceptions to exceptions always being what they are: your classic noise generators, uA741, LM358 and such, end up with a really high Z_n. Mainly because i_n isn't so crazy, but e_n is just an abomination.Noise curves (NF vs. Zin) are plotted in AoE3, and LM358 is actually best with a source like 100kohms or so. What a mess!Tim