Re: using an HP 8405A to measure SWR ?

On Sun, 01 Jan 2006 14:01:18 -0700, Wes Stewart <n7ws*@*yahoo.com>
wrote:

>On Sun, 01 Jan 2006 19:15:12 -0000, dplatt@xxxxxxxxxxxx (Dave Platt)
>wrote:
>
>>In article <nfrfr1dfe7osnc9k1c9591uoo6hr074thl@xxxxxxx>,
>>Wes Stewart <n7ws*@*yahoo.com> wrote:
>>
>>>>The measuring setup is a standard signal generator feeding a tee. One branch
>>>>feeds the A input of the 8405A and the other a 20 db directional configured in
>>>>reverse, that is feeding the out port. The assumption is the reflected signal
>>>>will be read correctly on the in port. The in port is then terminated or
>>>>connected to an antenna.
>>>
>>>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.
>>
>>The 8405A manual indicates the use of a power divider, and then a pair
>>of equal-value pads. One side goes to the probe T for the A
>>(reference) probe and thence to the termination, and the other goes to
>>the probe T for the B probe and thence to the device-under-test.
>
>Actually, my manual does not show this. Although I have the full kit
>of a resistive tee, two 50 ohm "N" sampling tees and appropriate
>terminations, I don't believe Dan does.
>>
>>The manual is quite clear that the A and B probes need to be connected
>>to points which are isolated from one another.
>
>I've read this someplace, but again my version of the manual (unless
>I'm really missing something) doesn't say it. Nevertheless, the
>directional coupler provides the isolation between probes.
>
>>You really don't want
>>the oddities of the load connected to the B side to affect the
>>voltage/phase of the reference signal seen by the "A" probe - it'd
>>certainly wreck the measurement.
>
>>Using a power divider, and resistive pads for isolation is one way to
>>do this.
>
>Actually, you do want to measure (include) the effects of the load on
>the source. Although you ideally want the source to be unchangable
>with respect to changes in the load (good source match). That is the
>point of my suggestion to pad the source at the input to the coupler.
>(Just as does paragraph 2.3 of Owen's reference)
>
>But if the load -does- pull the source you damn sure want to know it
>and account for it. The desired ratio is that between the incident
>signal and the reflected signal. Sampling somewhere off in isolated
>space via a tee and additional padding does -not- yield the incident
>signal.
>
>>
>> Using a pad followed by a dual directional coupler (as in
>>the experiment page to which Owen posted a link - thanks!) is another.
>
>My point exactly. With the dual coupler the forward coupled arm -is-
>measuring the incident signal -regardless- of what effects load pull
>have on the source output.

I've got to clarify this a bit if I can...

If you have the full set of parts per figure 11 in AN77-3 and you are
using them as shown, then with equal loads on the two ends, the
circuit is essentially a resistive Wheatstone bridge in balance with
the null detected by the difference between probes A and B.

In this case, the "incident" signal -is- measured by the A probe and
the effects to the source by a changing load are incorporated into the
measurement.

In the case at hand, at least as I imagine it, there is no longer an
nice tidy resistive Wheatstone bridge, but some cabling and a
directional coupler in the mix. In this case, the generator is no
longer the "source", the source is the signal at the input to the
coupler. It is my belief (unless I change my mind later) that a
sample derived from a resistive divider remote from the input to the
directional coupler is not a true measure of the incident signal.

.

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