What's the deal with 600 Ohms?

Gentlemen,

Just sorting through my vast collection of obsolete test equipment looking for a milliohm meter (found one!) and came across a *lot* of test equipment with 600 Ohm outputs. It was obviously some sort of standard back in the day but I was just wondering how it was arrived at? Is this a throwback to the valve (toob) gear days or is there some other reason behind it? I'm just thinking Zo=0 would be much more adaptable. ;-)

Just curious....

Reply to
Cursitor Doom
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Den tirsdag den 28. februar 2017 kl. 15.28.37 UTC+1 skrev Cursitor Doom:

afaik it goes back to the first telephone wires, characteristic impedance of two wires one feet apart being roughly 600 Ohm

Reply to
Lasse Langwadt Christensen

It is a throwback to whatever equipment was being used for telephony in the 1920s or something.

Wikipedia claims it has to do with the nominal impedance of relay coils in telephone exchanges, but they only cite one reference and it smells fishy:

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Reply to
bitrex

That was the nominal impedance of telephone lines, including dedicated leased pairs. A twisted pair really has a Zo closer to 100 ohms, but long phone lines are very resistive, so 600 is closer in the audio range.

Lots of audio gear was rated 600 ohms, although it usually wasn't.

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Reply to
John Larkin

I think it also had something to do with being the optimal impedance for a long line used at baseband. The lines were equalized using 88mH coils (pupinisation) and somehow the best results for audio baseband were obtained with a 600 ohm system. It then became a standard impedance much like 50 ohms for most RF systems.

Reply to
Rob

Now you come to mention it, I do remember goofing around making electric fences with surplus Post Office relays half a century ago and they were all AFAICR 600 Ohms. But like you say, I can't see the connection between relay solenoids and transmission lines. :-/

Reply to
Cursitor Doom

I have a few 600-ohm HP voltmeters: an HP 400EL and a couple of HP

3403As. They have dB labelled for 600 ohms, but are really 10M and a few picofarads.

600-ohm open wire line is (or used to be) a common type of feed line used for ham antennas--it has super low loss, and due to its 6-inch spacing, it's pretty well immune to rain, snow, and ice.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Apparently goes back to the telepgraph

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Reply to
bloggs.fredbloggs.fred

Oh, that was before my time.

A long lossy line has an impedance that increases as frequency drops.

600 is just sort of a compromise.
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Reply to
John Larkin

FM Radio and recording studios, mixers, etc., used 600-ohm balanced lines as a standard. Connections were made with transformers, to avoid hum loops, etc., and they expected a final 600-ohm termination. The bridging transformers typically had 10k impedances.

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Reply to
Winfield Hill

Most rhombic antennas I've seen use 600ohm . Possibly because they used telegraph poles and lines as the feed system.

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Brian

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Brian Howie
Reply to
Brian Howie

Almost all old moving-needle meters had ACV logarithmic scales with a 'dBm' designation. It was relative to 0dB at one milliwatt into a 600 ohm load. Though the meters were high-impedance, it meant you could probe inlet and outlet of a driven, loaded amplifier and subtract to find its gain, in decibels.

Anyone who taught electricity or electronics, or lab courses with meters, would eventually hear the question. Until about the late eighties...

Reply to
whit3rd

This was your serious studio low-distortion high-level 20Hx to 20kHz stuff. I worked extensively with it on the 50s to 60s.

Microphones were dynamic 150-ohm or lower with receiving-end step-up transformers.

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Reply to
Winfield Hill

You can have a bit of confusion, 500ohm standard, 600 ohm standard. Get into the audio world, then jump into the Bell world, then stir.

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Reply to
Robert Baer

IIRC the ones I had were 1100 for the slow ones and 1245 for regular speed, and, I think, 75 for the steppers.

1100||1245 comes out close to 600
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Reply to
Jasen Betts

I doubt it.

close to 100 ohms.

300 ohm.
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Reply to
Jasen Betts

Short mismatched cables inside the switchboard didn't matter compared with the long overhead 600-ohm pairs that the standard was originally designed for.

It is relatively easy to check the characteristic impedance of a drum of cable: connect a square wave signal generator with an output of 600 ohms or more to an oscilloscope. Generate a signal somewhere between 100kc/s and 1Mc/s and view the voltage waveform, it should be nice and square. Connect one end of the cable to the signal generator, the reflections from the other end will be visible as distortion of the square wave (ripples, overshoots etc.).

Now connect a non-inductive resistance box to the far end of the cable and adjust the resistors for minimum distortion of the square wave. Check by disconnecting and reconnecting the cable at the signal generator, the amplitude of the square wave will change but, if the terminating resistance equals the characteristic impedance of the cable, its shape should not.

Most ordinary equipment wire will turn be found to have a characteristic impedance around 100 to 120 ohms.

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Reply to
Adrian Tuddenham

Given that the dielectric constant of paper is not greatly different to that of air, or PVC, that's basically impossible.

the wires are too short for impedance to matter much at 3.3kHz but the hybrid in the telephone is terminated with 600 ohms.

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Reply to
Jasen Betts

With an actual 600 ohm load 0dBm is almost equal to -2 dBV(RMS) good enough for government work; the inflection point happens at about 630 ohms.

Reply to
bitrex

600 ohms was a standard old-timey balanced transmission line impedance, for RF.
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Tim Wescott

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