Time Domain Reflectometry

[colin]

That will be an asymmetric non-50 ohm impedance, and proper TDR needs
a true 50 ohm source; most electrical TDR is done with a step, not a
pulse. But you don't need a lot of voltage - 0.25 volts into 50 ohms
is standard - so you can pad down a 5-volt swing and get a very good
50 ohms.

You only need matched termination for when the pulse returns,
by wich time the transistor will be fully off.

ofc this rules out using a step.

Most ethernet chips have TDR built in for coax connections anyway,
even most PC ethernet ports have this feature,
although its probably quite hard to find any software that can access it.

Another good way is to use a single frequency at anyone time,
and measure the standing wave voltage,
thus avoiding any pulse smearing,
to give absolute distance the frequency is scanned untill it passes through
2 nulls.

all it needs is a 1-2ghz vco and a PIC.

Colin =^.^=

 

[whit3rd]

I'd like design a pulse generator ... I need
to make some TDR work
I saw the circuit by Tomi Engdahl, which uses a 74AC14 (schmitt inverter),

My doubts are about driving the (50 ohm) cable:

what if I connect the chip directly to the cable? It is specced of being
capable of 50mA "short circuit current"

The absolute-max ratings aren't your concern here; the design
uses multiple outputs in parallel to ensure that the output impedance
of the 74AC14 is NOT limiting the current; that output impedance is
not even remotely constant through a transition from high to low,
or from low to high, nor is the low impedance the same as the
high impedance (remember, those impedances result from very
different transistors in the chip).
The only problem with this is that a lot of gates on the chip
are transitioning at the same time, and your power supply
pins will require the best bypass capacitors and wiring that you
can arrange or you'll get power-supply-droop.

Last but not least, some theory: what's the point in having ~picoseconds
rise time, when anyway you drive one hundred meters of 100pF/m cable with
a 50 ohm impedance in serie?
Before the impulse has traveled one meter, it has already been smoothed
out

If your drive is higher impedance than the 50 Ohm cable, the
capacitance
dominates and you get low-pass characteristics. If, on the other
hand, your drive is LOWER impedance than the cable, the series
inductance dominates and you get high-pass characteristics.
If you match the impedance, you get neither.

'Neither' is the right answer here, because it simplifies the
resulting behavior. You were looking for another effect, nicht wahr?

 

[[email protected]]

Tektronix sold a lot of their 1503 long-range TDRs,measuring up to 50,000
ft of cable,IIRC. phone companies,cable companies.It used a 1/2 sine pulse.

IIRC,the TEK 1502 measured up to a couple of thousand feet,using a fast-
rise step pulse.

Ahh - the memories ...
I dropped one out of the back of a van once doing some rental car driving on a
gravel road ;-) Good thing we shelled for the transport ca$$$e - instant ROI!

 

[jasen]

another kiwi!

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most of the world won't get those. the usenet network has strategiically
located filtering software
that strips them out.

You should have access to alt.binaries.schematics.electronic where they are
allowed.

Bye.
Jasen