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using an HP 8405A to measure SWR ?
In article , Wes Stewart wrote: 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. The BAMA copy mentions it in paragraph 3-14. Later text indicates that attaching the two probes to a single point is an appropriate way to set phase-zero. Nevertheless, the directional coupler provides the isolation between probes. I see the issue, and I think I was conflating two different sorts of measurement regimes. The splitter/isolator/pad arrangement I was referring to appears on page 3-3 of the 8405A manual available at BAMA. It's what's appropriate for doing an in-line test of a transmission line or other network, where you want to see the effect of the network itself and can measure (via probe B) at the network's output. Page 3-4 shows a somewhat similar hookup, which doesn't include the resistive pads... I presume because the device-under-test (an amplifier) is assumed to have high isolation as part of its design. Neither of these hookups wouldn't work for measuring an antenna, since you can't measure at the antenna's output. Instead, using a directional coupler provides the necessary isolation, and (as you point out) lets you determine the incident and reflected signals accurately. 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. Hmmm. In the general case, I believe you're correct. I suspect that the setup shown in the 8405A manual sets up a specific special case, though. The diagrams and text seem to be defining a case in which: - there is a physical and electrical symmetry in the T arrangement - that is, the power splitter is symmetrical, and the pair of attenuator pads between the splitter and the (A probe tap) and (device under test) are matched. The manual makes a point of this issue. - The pads being used are matched to the system's transmission line impedance, so that any reflected signal coming back from the DUT/coupler sees a proper termination by the source (the pad and signal generator, in this case) and is not re-reflected. In this particular situation, I believe that the incident signals reaching the DUT (the input to the coupler, in this case) and the "A" probe, would be identical... would they not? The proper termination of the reflected wave will mean that it won't re-reflect off of the generator and alter the incident wave. The "A" probe signal (off on its side of the "T") and a signal read out via the incident-wave tap on the directional coupler ought to be the same, once the coupling coefficient is taken into account... no? -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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