Jim Brown wrote:
> On Tue, 14 Jul 2009 18:26:33 -0700 (PDT), Rick Karlquist wrote:
>
>> Also, the waveform on the output of the controller is only a few
>> hundred Hz. The capacitive reactance is much larger than the resistance
>> at these frequencies. Microphones have to go to 20 kHz and the impedance
>> is at least 600 ohms, not a few dozen, so that may not be a great analogy.
>
> You may be right. I haven't looked at the waveform, but a 300 Hz square
> wave will have harmonics well above 3 kHz. As to the capacitance -- 40pF/ft
> is typical for the generic sorts of cable that SteppIR is using.
>
> On the audio side of things, the relatively low reactance of the large
> capacitance of a lot of cable can current-starve the live driver, causing
> distortion peaks above about 10 kHz. Also, a relatively high value of
> capacitance as a load can interact with the feedback network around the
> output stage make it unstable. Deane Jensen published a classic paper on
> that about 40 years ago, and ever since, audio output stages have 50 ohm
> resistors in series with each leg to prevent the instability.
>
> BTW -- pro audio has not used 600 ohm interfaces for about 50 years. Line
> drivers (mics and output stages) have output impedances on the order of 100
> ohms. Line level outputs are designed to drive input circuits on the order
> of 10K ohms, and mics are designed to work into at least 5X their nominal
> output impedance. We describe these as "voltage-matched" systems.
>
> Yes, these are low frequency circuits, but things are not always as simple
> as they look.
>
> 73,
>
> Jim K9YC
Didn't know pro audio was 100 ohms.
Good points, but here is some more evidence: I have 350 feet of cable
feeding my MonstIR. Initially, it wouldn't work because the resistance
was too high. I installed two more runs and tied everything in
parallel. This tripled the capacitance and cut the resistance to 1/3
of the original value. The MonstIR works perfectly now. I have as
much capacitance as a normal run of 1000 feet.
But actually, the cable is a transmission line, and like all
transmission lines, has both capacitance and inductance.
If you terminate a transmission line with less than 30 ohms,
and the transmission line has a characteristic impedance of more
than 30 ohms, the input impedance will be inductive, not capacitive,
for all line lengths less than a quarter wavelength. It is a very safe
bet that any cable used will exceed 30 ohms of characteristic
impedance and I think we can agree that 350 feet is less than a quarter
wavelength for any SteppIR signals, which shouldn't exceed 500 kHz.
Rick N6RK
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