Interconnect length
 

Jim Lesurf wrote:

It may be easier to connect a capacitor and measure the time constant
of the edges of a square-wave. Then use RC to work out the effective
output resistance.

Er, how's that wired up? :-) Time constant? Lemme see...

I have a square wave, a scope, and a bunch of caps. If I put a cap across
the o/p, the HF content of the square wave will be attenuated more than the
LF content. I'll get rounded corners. The time it takes the leading edge of
the square wave to reach maximum amplitude is the time constant, yes? If so,
what do I do with it? :-)


--
Wally
www.wally.myby.co.uk

Interconnect length
 

On Mon, 17 Jul 2006 22:24:28 +0100, "Wally" wrote:

Jim Lesurf wrote:

It may be easier to connect a capacitor and measure the time constant
of the edges of a square-wave. Then use RC to work out the effective
output resistance.

Er, how's that wired up? :-) Time constant? Lemme see...

I have a square wave, a scope, and a bunch of caps. If I put a cap across
the o/p, the HF content of the square wave will be attenuated more than the
LF content. I'll get rounded corners. The time it takes the leading edge of
the square wave to reach maximum amplitude is the time constant, yes? If so,
what do I do with it? :-)

Measure the time it takes for the voltage to reach 63% of its final
value. This is the time constant and is equal to R x C. You know C, so
you can find R - the output impedance of the equipment.

d

--
Pearce Consulting
http://www.pearce.uk.com

Interconnect length
 

Don Pearce wrote:

Measure the time it takes for the voltage to reach 63% of its final
value. This is the time constant and is equal to R x C. You know C, so
you can find R - the output impedance of the equipment.

Aha - cheers. Just tried a 100nF across the output of the function generator
running at 10KHz, and got 5ns. So...

5 nanos / 100 nanos = 0.05R o/p impedance.

Have I got that right?


--
Wally
www.wally.myby.co.uk

Interconnect length
 

On Mon, 17 Jul 2006 22:45:34 +0100, "Wally" wrote:

Don Pearce wrote:

Measure the time it takes for the voltage to reach 63% of its final
value. This is the time constant and is equal to R x C. You know C, so
you can find R - the output impedance of the equipment.

Aha - cheers. Just tried a 100nF across the output of the function generator
running at 10KHz, and got 5ns. So...

5 nanos / 100 nanos = 0.05R o/p impedance.

Have I got that right?

You sure about 5nS? It takes quite some scope to measure that.

d

--
Pearce Consulting
http://www.pearce.uk.com

Interconnect length
 

Don Pearce wrote:

You sure about 5nS? It takes quite some scope to measure that.

Lets make it microseconds, then. :-) (Oops.)

Which makes the function generator's o/p Z... 50 ohms. What an amazing
scope-speak surprise! Still, at least I wasn't thinking of 50R when I took
the measurement - says something about my scope tweaking, I guess. :-)


--
Wally
www.wally.myby.co.uk

Interconnect length
 

On Tue, 18 Jul 2006 00:59:40 +0100, "Wally" wrote:

Don Pearce wrote:

You sure about 5nS? It takes quite some scope to measure that.

Lets make it microseconds, then. :-) (Oops.)

Which makes the function generator's o/p Z... 50 ohms. What an amazing
scope-speak surprise! Still, at least I wasn't thinking of 50R when I took
the measurement - says something about my scope tweaking, I guess. :-)

50 ohms sounds dead right. Innit nice when theory and practice match?

d

--
Pearce Consulting
http://www.pearce.uk.com

Interconnect length
 

In article , Wally
wrote:
Jim Lesurf wrote:

It may be easier to connect a capacitor and measure the time constant
of the edges of a square-wave. Then use RC to work out the effective
output resistance.

Er, how's that wired up? :-) Time constant? Lemme see...

I have a square wave, a scope, and a bunch of caps. If I put a cap
across the o/p, the HF content of the square wave will be attenuated
more than the LF content. I'll get rounded corners. The time it takes
the leading edge of the square wave to reach maximum amplitude is the
time constant, yes? If so, what do I do with it? :-)

For obvious reasons the following would be clearer if we had the equipment
or a blackboard in front of us... :-)

If the output impedance is R and you shunt it with a capacitance C then the
result is a time constant of value Tau = RC. This, as you say, 'rounds' the
transitions (edges) of the square-wave.

The rounding is in the form of an exponential. So if the change in voltage
from top to bottom of the square wave is V then the shape of the rounded
edge is

V*(1 - Exp(- t/Tau))

where t is the time *measured from the instant when the transition starts*.

Hence at the time when t = Tau after the transition start, the change will
be
(1 - Exp(-1)) i.e. (1- 1/e) ...or as engineers say, "about two
thirds of the way." ;-

So, set up a square wave and adjust its size and the scope display so that
the peak-top-peak size is, say, three vertical divisions on the scope
graticule. Then alter them time-base and read off using the time scale how
long it takes for the rounded edge to get two thirds of the way from top to
bottom of the square wave. That tells you Tau. Knowing the C you used, you
can then work out R. :-)

In practice, you need to check that the rounding is negligable when you
*haven't* shunted the output with a capacitance, otherwise the C value you
have used won't be the only significant contribution. You also need to
ensure the square wave is low enough in frequency that it still shows clear
'flat bits' at the top and bottom after the rounding. Otherwise the display
isn't showing the full transition amplitude and you may not be measuring
23/rds of the right amount.

The above should be OK for most pre-amps and signal amps. I *don't*
recommend it for power amps in general, though, for various reasons! 8-]

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Interconnect length
 

In article , Rob
wrote:
Jim Lesurf wrote:



FWIW I would not particularly recommend the sparse-braid for such
lengths as it is easy enough these days to get more complete braid,
etc. I just tried it, and then found it worked in my systems as well
as anything else I have tried out since.

Could you recommend a suitable easily available cable (Maplins etc), 5m
phono-phono line level?

No particular cable to be honest. I'd tend to buy from Maplin, choose
cables with capacitances below 100pF/m, and with diameters that fit the
phono plugs I prefer. So far as I can tell, they all work much the same.
Sometimes I buy the 'CT100' types, sometimes types officially called
'audio' cables. The main advantage of the audio ones is that the come in
stereo pairs already bonded together. With the UHF cables I bind them
together with masking tape at circa 10cm intervals to make a stereo pair.
This seems to survive happily for decades. Looks quite pretty and 'hand
made'. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Interconnect length
 

In article , Don Pearce
wrote:
On Tue, 18 Jul 2006 00:59:40 +0100, "Wally" wrote:

Don Pearce wrote:

You sure about 5nS? It takes quite some scope to measure that.

Lets make it microseconds, then. :-) (Oops.)

Which makes the function generator's o/p Z... 50 ohms. What an amazing
scope-speak surprise! Still, at least I wasn't thinking of 50R when I
took the measurement - says something about my scope tweaking, I guess.
:-)

50 ohms sounds dead right. Innit nice when theory and practice match?

In theory this *always* happens. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Interconnect length
 

On Tue, 18 Jul 2006 10:03:54 +0100, Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Tue, 18 Jul 2006 00:59:40 +0100, "Wally" wrote:

Don Pearce wrote:

You sure about 5nS? It takes quite some scope to measure that.

Lets make it microseconds, then. :-) (Oops.)

Which makes the function generator's o/p Z... 50 ohms. What an amazing
scope-speak surprise! Still, at least I wasn't thinking of 50R when I
took the measurement - says something about my scope tweaking, I guess.
:-)

50 ohms sounds dead right. Innit nice when theory and practice match?

In theory this *always* happens. :-)


In theory, theory and practice are the same. In practice, they aren't.

Actually I used to think this, then I found out that I just hadn't got
sufficient theory yet.

d

--
Pearce Consulting
http://www.pearce.uk.com