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I'm working on restoring an HP 211A square-wave generator and I'm looking to add a shielded cable to the 600 ohm output. I've got some spare 75 ohm RG59 cabling and I was wanting to know if using that would have any detrimental effects on the output such as signal loss due to the impedance mismatching. The length of cable will be 10" - 12", the output will be between 1Hz and 1MHz and mostly used on loads like oscilloscopes and maybe stereo receivers.

To clarify, the RG59 cabling would run from the V10 and V11 plates to the wiper on R71, and then another cable from the non-grounded pin of R71 to the output screw post. I'd likely use 75 ohm cabling to run to any devices on the output though anyways.

Thank you.

600 Ohm Output On The Circuit In Question

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    \$\begingroup\$ What about the 75 Ohm output in the upper right corner? \$\endgroup\$
    – jonk
    Jun 28 '20 at 2:05
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    \$\begingroup\$ Yes, about what you'd expect if you hooked up the 75 Ohm cable to the 600 Ohm output (discounting reflection questions.) If you want to drive 75 Ohms at 55 Vp, you'll need a driver capable of 20 W. Yes? \$\endgroup\$
    – jonk
    Jun 28 '20 at 2:07
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    \$\begingroup\$ I can't say that the circuit won't distort when loaded down like that. Apparently, that's a 2 - 2.5 watt output and you will be loading it down pretty heavy. \$\endgroup\$
    – jonk
    Jun 28 '20 at 2:11
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    \$\begingroup\$ So it's designed to deliver 600 mW into a 600 Ohm load. Pretty light-weight. I don't think those plate loads will do well into 75 Ohm. You may need to adjust what's driving the grids, too, to get a better operating point. \$\endgroup\$
    – jonk
    Jun 28 '20 at 2:35
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    \$\begingroup\$ @P2000 Those 1200 Ohm pots? Those. \$\endgroup\$
    – jonk
    Jun 28 '20 at 8:44
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If I searched right, the unit has a maximum output frequency of 1MHz. A 1MHz wave has 300m wavelength in free space. It will be slightly longer in a cable. So the cable you will use, will be electrically short for sure, and only the capacitive loading of the cable and thus the bandwidth is to be assessed. Such a coaxial cable should have roughly 100pF/m capacitance. 600Ω output resistance and 100pF forms a pole at 2.67MHz, so you are good* up to 1m cable length for the full range of the instrument. If you want a 1MHz frequency with a waveform including harmonics, then you should reduce your cable length, or the output impedance before the cable.

*Only 6% amplitude loss for a 1MHz sinusoid signal

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  • \$\begingroup\$ He said his cable length would be around 10-12" (up to 4m). I don't think it should be a problem but it's longer than the 1m limit you mentioned. \$\endgroup\$
    – Camion
    Jun 29 '20 at 5:51
  • \$\begingroup\$ @Camion. That's true. I've used 1m, because it is a nice unit for cable lengths. I think the poster could divide or multiply by 2.5 easily. I also deliberately used a different unit to make him rethink whether he really needs that 2.5-3m cable length, because it could limit the performance of his setup. Often questioning the assumptions might lead to an idea for a better solution. \$\endgroup\$ Jun 29 '20 at 9:43
  • \$\begingroup\$ I read 10-12" as inches hence up to 30cm. For feet, I would expect 10-12'. \$\endgroup\$
    – badjohn
    Jun 29 '20 at 9:55
  • \$\begingroup\$ @badjohn that's another reason why I stick to 'm'. :) As a European, I sometimes misinterpret these imperial units. \$\endgroup\$ Jun 29 '20 at 9:59
  • \$\begingroup\$ @HorrorVacui I am familiar with feet and inches as I live in the UK but that doesn't mean that I like them. I would like to see them disappear entirely but that seems very unlikely. However, they certainly should be avoided in an international context. \$\endgroup\$
    – badjohn
    Jun 29 '20 at 10:02
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At the frequencies your using there is little effect in using a lower impedance cable. However you will want to terminate it with a 600 ohm load so the source can still drive it at full voltage.

Frequencies at or above 50MHZ are very fussy about impedance matching and standing waves and you are nowhere near that high in frequency.

600 ohms is typical for audio < 0.01HZ up to 1MHZ. That is the range of my 600 ohm analog audio generator. 1MHZ to 10MHZ for ultra-sound is common today, so 10MHZ is nothing special anymore.

You do want to avoid cable with an impedance above 600 ohms, as this could deform the edges of frequencies approaching 1MHZ (for square waves).

Cables lower than 75 ohm become a "bulk" problem as the center conductor can be thick, making the cable tough to bend. Avoid 50 ohm and 25 ohm cables for this reason, and their higher cost.

For a "cleaner" square wave at 1MHZ you can load the end-point with less than 600 ohms, by going down a 100 ohms at a step, but the signal amplitude will drop. You have to decide if signal amplitude is more important than signal quality.

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  • \$\begingroup\$ "50MHZ are very fussy about impedance matching and standing waves" I take it you are dismissing these effects sub 50MHz because of the typical cable length for lab equipment, or is there a different reason? \$\endgroup\$
    – P2000
    Jun 28 '20 at 6:11
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    \$\begingroup\$ @P2000 Yes the effects begin before 50MHZ and get worse at higher frequencies. There was no need for a graph to plot out increasing errors with frequency and rise/fall times \$\endgroup\$
    – user105652
    Jun 28 '20 at 6:48
  • \$\begingroup\$ @Sparky256 I think what I'll do is try some shielded phono cabling since I have lots. That should be closer to the factory cable impedance while still providing a shield. Shunting a 600 ohm resistor to ground is a good idea and I'll keep those other points in mind. Thank you. \$\endgroup\$ Jun 28 '20 at 10:17
  • \$\begingroup\$ In this frequency range, the signal will not behave as wave, since the wavelength will around 450m for 1MHz. Assuming cable lengths of 2.5m, it is still not an issue. Also I've used many 50Ohm cable which were very flexible and lightweight. Z0 of a coax depends on the ratio of outer and inner diameter, therefore they does not need to have a strong inner conductor. \$\endgroup\$ Jun 28 '20 at 12:23
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RG59 Coax is 75 Ohms and as I recall 30pF/ft. Twisted pair impedance and pF/ft depend on twists /ft. The fewer twists, the higher the impedance and inductance must be considered as well.

It becomes a matter of tradeoffs whether you have rounded edges or ringing or attenuation with properly matched RC impedance divider like an 8:1 to 75 Ohms or unshielded wire prone to magnetic pickup or lightly shielded twisted pair with large twists.

  • degree of squareness at 1MHz
  • length of twisted pair overall
  • the impedance of load
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    \$\begingroup\$ Keep in mind long scope probe grounds are inductive and will cause apparent ringing . \$\endgroup\$ Jun 28 '20 at 11:23
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    \$\begingroup\$ \$Z=\sqrt {\dfrac {L}{C}}\$ where L is ~fixed ~5nH/cm but C ~<1pF/cm depends on insulation gap 1/x of paired wires. Remember flat TV Antenna wire ? that gap is 300 Ohms, old phono Coax will be more than 100 Ohms and more likely 75. Twisted pair 8~12 twist/ft is 100~250 ohms depending on wire gap.. But then you can replace whole box with a single CMOS IC with variable f 18V 10 Hz to 1MHz square wave if you wanted. \$\endgroup\$ Jun 28 '20 at 12:04
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    \$\begingroup\$ nice vintage antique. \$\endgroup\$ Jun 28 '20 at 12:06
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    \$\begingroup\$ OK but AWG 22 or 24 spaced like old flatwire TV cable is 300 Ohms, so just use that. \$\endgroup\$ Jun 28 '20 at 12:32
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    \$\begingroup\$ Yes Grounding one side reduces stray hum crosstalk but increases capacitance 60pF/m . So ground it. \$\endgroup\$ Jun 28 '20 at 13:32
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600 ohm is unrealistic; needs center conductor about the size of an atom, shield with diameter of the universe, from what I heard.

Use the 75 ohm; you are not expecting standing waves, so don't worry.

Regarding achieving 600 ohm COAX is not the same as achieving 600 ohm TWINLEAD. Pole/crossbar mounted telephone cables are twinlead.

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    \$\begingroup\$ not quite the accurate ratio of diameters for 600 Ohms, but 30pF/1ft Coax is OK = 5kohm @ 1MHz.. but @ 8ft =240pF will attenuate 6dB @ 1MHz and turn a square into a triangle wave \$\endgroup\$ Jun 28 '20 at 4:05
  • \$\begingroup\$ "The first long telephone air lines (two wires on the poles separated from each other at some distance) used to have impedance of around 600 ohms." epanorama.net/documents/wiring/twistedpair.html \$\endgroup\$
    – P2000
    Jun 28 '20 at 6:17
  • \$\begingroup\$ A center conductor with a diameter of a human hair and a shield with a diameter of a mile gives a coaxial cable with impedance close to 600 ohms. \$\endgroup\$ Jul 20 '20 at 16:15

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