On Mon, 02 Jan 2006 21:56:37 GMT, Owen Duffy wrote:

On Mon, 02 Jan 2006 13:34:54 -0800, dansawyeror
wrote:

Owen,

That idea seems to work. I set it up and then looked and the 'interference', the
change in phase based on changing the pad. Zero pad showed several degrees phase
shift from -40 dbm, 10 dbm showed small shift, and 20, 30, and 40 were all about
equal. I decided on 20 dbm as a practical base.

This doesn't make sense... are you using "dbm" to mean decibels of
attenuation, usually written "dB".

The units "dBm" are usually written to qualify a power level with
respect to one milliwatt.

The attenuator on your sig gen might be marked in dBm, but that
applies to the combination of the oscillator, possibly its level
meter, and the attenuator as a system.

Using the wrong terms for things is often a result of a concept gap!

A 20dB attenuator will reduce the effect of the reflected component to
about the same level as you would expect from a practical directional
coupler, more attenuation is better if you have the power from the sig
gen and the VVM probe chan can operate at the higher input level.

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?