Electronics > Beginners
Impeadence matching vs high impeadence buffering?
David Hess:
--- Quote from: TimFox on February 09, 2020, 03:31:55 pm ---
--- Quote from: rwgast_lowlevellogicdesin on February 09, 2020, 05:16:37 am ---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!
--- End quote ---
I 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.
--- End quote ---
Tektronix also produced the AM502 which 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.
--- Quote ---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.
--- End quote ---
LeCroy 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 pm ---
--- Quote from: imo on February 09, 2020, 06:01:22 pm ---
--- Quote from: David Hess on February 09, 2020, 03:33:14 am ---Do 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%.
--- End quote ---
As 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?
--- End quote ---
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.
--- End quote ---
Exactly, 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.
rwgast_lowlevellogicdesin:
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?
TimFox:
--- Quote from: rwgast_lowlevellogicdesin on February 10, 2020, 12:53:02 am ---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?
--- End quote ---
I 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.
T3sl4co1l:
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 -- that is to 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
ejeffrey:
--- Quote from: T3sl4co1l on February 08, 2020, 11:40:07 pm ---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.
--- End quote ---
One 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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version