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Impeadence matching vs high impeadence buffering?

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rwgast_lowlevellogicdesin:
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!

TimFox:

--- Quote from: rwgast_lowlevellogicdesin on February 09, 2020, 05:16:37 am ---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!

--- 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.

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.

Vovk_Z:
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).

iMo:

--- 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?

TimFox:

--- 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 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.

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