On Tue, 03 Jan 2006 08:25:48 -0700, Wes Stewart
wrote:

....
Owen

Let me pick a nit or two.

More attenuation is not necessarily better. In theory the improvement
in source match is two times the attenuation, so a 10 dB pad improves
the return loss to no less than 20 dB, even with a zero ohm source,
and with a decent source match of RL = 10 dB or so, is as good as you
need. (I know you know this already)

I say this because it's very likely that the return loss of the
attenuator isn't any better than 25-30 dB, regardless of its
attenuation. For example Narda makes a "precision" Type N attenuator:

http://www.nardamicrowave.com/east/P...dPrecision.pdf

Note the VSWR spec, 1.15 at low frequency. That's a 23 dB RL. So
although a 20 dB pad in theory provides a minimum 40 dB RL, the actual
RL can be as little as 23 dB.

Manufacturers have to work really hard and typically use a precision
connector like 3.5mm or 7mm to build a 40 dB RL termination although
Anritsu will sell you a 40 dB RL type N termination for -only- $650
USD.

Also, and this goes back a post or two, where you suggested that if a
pad is used between the generator and the input to the coupler, the
"A" probe (reference/incident) should be between the generator and the
pad.

This is contrary to what I tried to recommend earlier when I said:

"It would also be nice it you had a 6 - 10 dB pad between the
generator and the directional coupler (DC); located right at the DC.
You want the source match to be set right there and the A probe to
sample right there."

Let me offer this thought experiment:

If you had two directional couplers, such that one could be used to
sample the forward signal and the other the reflected, would you place
a pad between them to isolate the generator from the effects of the
load?

No, of course not... it just adds another source of error, and
increases the gap between the measurements being made on both probes.

But Dan does not have two directional couplers. To my mind, if the A
probe is sampling the main transmission line, the sample is of the
resultant of the algebraic sum of the forward and reflected waves
rather than a sample of the forward wave alone (well, nearly alone) as
you would get with a directional coupler. My suggestion of placing the
A probe on the source side of the attenuator is to reduce the
contribution of the reflected wave to the A probe measurement. The
attenuator was proposed mainly for isolation of the forward wave
component for measurement, rather than a source matching issue...
which also exists.

Have I got this wrong?

All comments on practical limits of RL from pads / attenuators noted,
and understood.

Owen



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