Newb Hobbyist - Impedence of Signal Generator vs Adaptor and/or cables??

[Robbie010]

Hello everyone!

New forum member here and beginner / hobbyist!

First off, apologies if this is a stupid question or poorly explained.

My main interest is in the repair and modification of audio amplifiers. To help with testing, I've just bought myself a used signal generator that has the old banana type output connections, rather than the standard BNC connection you find on more modern equipment. Unfortunately, the generator does not come with any cables other than a power lead!

Now, I can buy basic banana to alligator cables but was thinking it may be better to buy a banana to BNC adapter and some decent quality BNC to alligator cables.

However, my question is regarding any potential testing issues caused by a mis-matched impedence between the signal output of the generator and that of a banana plug to BNC adapter and/or the cables.

The generator has two outputs, one of 4ohms and one of 600ohms that can be attenuated by x1, x0.1 & x0.01. Many adapters and cables that are available don't seem to stipulate the impedence so how can I be sure of buying the correct type that would allow me to fully utilise both the 4ohm and 600ohm output signals??

Thanks in advance

[Mechatrommer]

why you want BNC cable for audio testing? if you have audio device with BNC input then carry on. but i think banana to alligator is fine for audio work and repair. at audio frequency, cable impedance and what type it is doesnt matter, its all about DC impedance.

[T3sl4co1l]

Doesn't much matter at audio frequencies.

You can solve for the equivalent capacitance and inductance of a cable, based on its impedance, velocity factor and length.
Leq = len * mu_0 * Z/Zo
Ceq = len * e_0 * Zo/Z / vf
where Z is the characteristic impedance of the cable, Zo is that of free space (~377 ohms), and mu_0 and e_0 are the permeability and permittivity of free space (1.257 uH/m, 8.84 pF/m), respectively.

This assumes velocity factor is due to dielectric constant of the insulator, which is correct for almost all cables.  There are some specialty cables that are magnetically loaded, in which case vf contributes to Leq as well.  We'll just ignore those.

Incidentally, Zo == sqrt(mu_0 / e_0) so some simplification of this can be done.  It's about as much memorization either way.

Use unit conversions as needed.  If you input units into Google Calculator, you'll get the right answer out.
https://www.google.com/search?q=6+feet+*+permittivity+of+free+space+*+377+ohms+%2F+50+ohms+%2F+0.67

The last sanity check is to verify that this is the correct calculation to make.  Take the highest frequency you're using (say, 200kHz, a typical output from an audio generator), and find its wavelength, λ = c / F (speed of light divided by frequency).  200kHz is 1500m.  If the wavelength is >20 times the length of the cable (say, >75m), you really don't care about transmission line effects.

You can in turn compare these reactances to your system impedance.  At 4 ohms, the capacitance isn't ever going to matter; but the inductance may.  4 ohms and 10uH rolls off (lowpass) at 64kHz.  At 600 ohms, it's the other way around: 600 ohms and 4nF rolls off at 66kHz.

Incidentally, it's not a coincidence that those two frequencies are close.  I picked L and C such that they would be characteristic of whatever length of 50 ohm cable (uh, ~60m worth, apparently).  It just so happens that 4 ohms is almost as many times below 50 ohms, as the times 600 is above 50.  That is, 600/50 = 12, and 50/4 = 12.5.  This illustrates another handy trick, we can take the 1/4 wave cutoff at 50 ohms and scale it up by the impedance ratio.  And the cutoff will always be inductive if below, and capacitive if above.

For audio testing, you usually have high impedance inputs, so it won't matter if you use the 4 or 600 ohm setting, as long as you get the voltage you need.  If you need accurate measurement, always verify the output voltage from the signal generator.  (You may need to use an oscilloscope to measure this -- common DMMs typically drop off in the kHz range.)

Tim

[capt bullshot]

The generator has two outputs, one of 4ohms and one of 600ohms that can be attenuated by x1, x0.1 & x0.01. Many adapters and cables that are available don't seem to stipulate the impedence so how can I be sure of buying the correct type that would allow me to fully utilise both the 4ohm and 600ohm output signals??

Treat the 4 Ohm output the same way as a speaker output from an audio amp (use wires / litz of appropriate gauge, no need for shielding or whatever.
Treat the 600 Ohm output the same way as a line level output from an CD player (Get an adaptor from banana plugs to your choice of shielded cable connector - any will do)

As perfectly pointed out by our expert, cable / connector impedance doesn't matter at all for your typical requirements at audio frequencies, and is reduced to the effects of DC resistance ratios. Check your cables and adaptors for near zero DC series resistance (for the conductors) and high impedance (isolation between the conductors) - that's all.

The most advanced adaptor you might need in special cases could be an audio transformer for isolation or symmetric (professional audio) / single ended (home audio) conversion.

[spec]

Hi Robbie010,

50 Ohms is not normal for audio work, more for video. 150 Ohms tends to be the standard for audio, but an signal generator with a 50 Ohm (zero Ohms better) will be fine.

If you are dealing with low frequency AF, say up to 440Hz, the situation is less critical, but as you go up in frequency stray capacitances, inductance and skin effect tend to come into play.

The other aspect to consider is the amplitude of the audio signal you will be using. A nice 1VRms voltage will be much less critical than 300uV, which is typical for some transducer outputs (moving coil cartridge of microphone).

The other thing is noise, cable/connector generated, cable generated, and inductively and capacitively induced.

After all that what should you do. I would be inclined to fit a BNC connector (or other suitable screened connector) to your signal generator in place of the terminals, and then make/buy suitable coax leads for your audio work.

It may sound a flaf, but in the long run, you will realise the benefits.