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Old 22nd May 2008, 6:59 pm   #1
Skywave
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Question Telonic rho-tector: advice, please.

In front of me I have a Telonic rho-tector. Its a die-cast metal case with three N-type ports and one BNC. The three N-type ports are labelled RF IN, Z1 and Z2. The BNC is labelled DET (-) OUT. It claims a bandwidth of 500 KHz to 2.5 GHz.

Now I can make an inspired guess as to what I could use this for & sit at the bench with bits of test gear & have a play with it to test my suspicions. However, is there anyone here who has experience of using one of these and what for? VSWR and return loss measurement springs to mind - but how do I use it for these types of measurements?

All responses are welcome; thank you.

Al / Skywave.
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Old 22nd May 2008, 9:49 pm   #2
Ray Cooper
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Default Re: Telonic rho-tector: advice, please.

Gosh, it's years since I played with one of those things... I shall need to dust my memory down a little.

As you speculate, it's used for measuring return loss. Usually it'll be used in conjunction with some form of swept frequency display unit (eg a wobbulator, or something more upmarket). In use, you feed the wobbulator o/p to the RF in socket: the DET out then feeds the Y-axis of your 'scope display, you connect the item to be tested to one of the outputs (Z1 or Z2) and the other output is terminated by a standard load - 50 ohms usually, but I think that you can make measurements based on other references.

What it sounds like you're missing are a calibrated set of mis-match loads - these are loads having specific values of VSWR or return loss. They are used to substitute the load that you are testing, so that the DET output signal can be calibrated. In use, you'd fit one of these mismatch loads and see what deflection you get on the 'scope - maybe mark the screen with a Chinagraph line as a reference - then repeat for other mismatches 'til you've got some idea of what your Y-display actually means.

As you can see, they were a bit fiddly in use, and quite quickly got superseded by rather more user-friendly measuring devices (eg the Wiltron). But if you've got patience, and the full kit, the rhotector can give quite accurate results.
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Old 22nd May 2008, 11:25 pm   #3
Skywave
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Arrow Re: Telonic rho-tector: advice, please.

A Chinagraph pencil! Gosh - it's years since I snapped the end (repeatedly) off one of those! Never could get the hang of sharpening them

But seriously though - thanks. Very informative; gives me just what I want to know.

At work, they've been having one of their bi-centennial clear-outs & I managed to save it before it went into the scrap metal skip (sacriledge)
I'm sure that I saw its wooden box kicking around somwhere - so perhaps a quiet rummage when no-one's looking might unearth the mis-match loads. If that fails, I suppose I could try making my own (N-type plugs & carbon resistors) for use at relatively low frequencies. Having access to a HP network analyser will help with this during one lunch-time
And before you ask - no, they won't let me borrow the analyser at the week-ends, either!

Cheers,

Al / Skywave.
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Old 23rd May 2008, 6:27 am   #4
ppppenguin
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Default Re: Telonic rho-tector: advice, please.

If you are ever up my way you are welcome to use my HP4195A network analyser (only up to 500MHz I'm afraid) to calibrate it. Apart from geography the only snag is that my HP has the 75R directional coupler etc. (I do video rather than RF) I do have a precision 75R>50R minimum loss pad but no precision 50R terminations.
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Old 23rd May 2008, 7:45 am   #5
Skywave
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Thumbs up Re: Telonic rho-tector: advice, please.

Thank you, Jeffrey - very generous. I've made a note of that offer for possible future reference.

Al / Skywave.
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