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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Aug 2018, 11:27 pm   #61
Radio Wrangler
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Default Re: Back terminated cable?

Some of these things have a grounded sleeve which can be adjusted over the series resistor to add C to ground to balance the across-the-resistor to adjust flatness at the top end.

I use a pair of N-type hard lines with fittings to allow HP vector voltmeter probes to stab the line. Add a precision load and you can calibrate your probe quite well.

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Old 13th Aug 2018, 12:01 am   #62
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Thumbs up Re: Back terminated cable?

As the member who asked the question featured in this thread, to me, it seems a good idea briefly to comment on the posts submitted, especially considering the number of those posts.

My original question - "What does the phrase 'back terminated', when applied to a transmission line - actually mean"?
That Q. has been answered. In essence, it refers to a termination at the source end of the transmission line, where that termination (usually) equals the Zo of that line.
David (Radio Wrangler) came up with an excellent alternative definition: source terminated, as opposed to load terminated. To me, that makes things perfectly clear.

There have been many other posts relating to Tx. lines generally. I have read them all and found them informative and useful. My thanks to all members who did so contribute.

Al.
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Old 13th Aug 2018, 6:06 am   #63
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Default Re: Back terminated cable?

Jeremy,
Regarding your post #60, that is really interesting, Whenever I have added resistance in series with a signal feed in RF circuitry (or any signal circuit)it has never created an HF peak that I have detected and has always seemed to create an LPF effect. Mind you I have not tried it in the probe application discussed with 50R cable and probing 50R systems and also I have no test gear that could detect a peak over 1GHz, so the capacitance of the resistor itself has seemed less dominant that the LFP effect of the added resistance and the capacitance it feeds. But now I know this effect exists I will be on the lookout for it and try to observe it, Thanks.
(RW- I like the idea of a small sleeve around the resistor to flatten the response at the high end, I'm going to make a probe)
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Old 13th Aug 2018, 10:59 pm   #64
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Default Re: Back terminated cable?

To get the best from it up towards 2GHz you ideally need to use it with a decent 50R cable that has a good 50R termination at the far end.

You can get some idea of the shunt capacitance needed by multiplying the self capacitance of the 1500R resistor by 1500/50. So if the self capacitance was 0.15pF you would need 4.5pF. But the problem here is that a 4.5pF chip cap won't look like 4.5pF up at 2GHz because of the package inductance and the inductance of the soldered connection. It could easily look like 8pF at 2GHz.

So if you did get some access to some test gear to help you set up the probe you would find that trying to do it with a resistor that has high internal capacitance and a cap that has >0.7nH package/soldering inductance will lead to a lumpy response in the probe.

Best to experiment with several 1500R resistors to find one that is best suited. i.e. the one that gives the least upward slope. Typically it might give an 8dB upward slope by 2GHz without the comp cap. A bad choice of resistor would be much worse. If you really want to make something nice, then try using several 0.7pF and 1pF caps in parallel to make up a low inductance capacitance or try the sleeve method. This would give a flatter response but I'm not sure it's worth it for a cheap/casual/disposable probe like this. The elegance of this probe is the ease with which it can be made from a few parts found around a typical RF bench.

The probe I showed in the earlier image is a good compromise in terms of ease of build and ease of repair and ease of use in terms of how easy it is to splay the resistor and ground spike to suit the dimensions of each PCB artwork you might use it on. I didn't bother with multiple caps as the intended use for this probe is only for casual RF sniffing. The quality of the cable and the termination at the far end will also affect the performance in terms of ripple as it will affect how close to a resistive 50R the entry point of the coax cable looks. You ideally want the cable to look like a resistive 50R black hole with no reflections although any half decent RF cable will do as long as the far end is correctly terminated in 50R. I often use Sucoform 141 or 086 here although this cable type does tend to fray and degrade if it gets used/twisted a lot. So the whole probe has a finite lifetime when built like this. But I tend to misplace them at work before they totally fall apart!
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Old 14th Aug 2018, 6:55 am   #65
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Default Re: Back terminated cable?

Jeremy,

My top Tek generator only goes to 1GHz and I have used it to calibrate my 2465B 400Mhz bandwidth scopes with a recommended 50R Tek cable which is a Tek part.

The 2465B has a 50R termination at its input. Also, as part of the calibration process in line (male to female ) attenuators are used. I didn't have any professional ones and they were pretty expensive (seemed ridiculous) so I made some using 1/8w sized small resistors and some donor housings, a photo of them is on page 26-27 of this article:

http://worldphaco.com/uploads/TEKTRO...THE_DS1225.pdf

But now I wonder if they might have impedance bumps in them, the trouble is I could only test them up to about 500MHz and they seemed OK.

But now I wonder if they should have had compensation capacitors in them. I wonder if the professional versions do ? Or do you think at the frequencies I have been using them < 500MHz, that the response would be unlikely to have any significant peak ?
Thanks, Hugo.
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Old 15th Aug 2018, 9:56 pm   #66
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Default Re: Back terminated cable?

I guess it depends on the layout and the package/quality/parasitics of the resistors. Also the value of the resistors can be significant too. Large value resistors can be problematic if you want lots of bandwidth because the parasitics dominate at high frequencies.... but you presumably only need to go to a few hundred MHz for the Tek scope.

Maybe my little resistive probe deserves it own thread if only to warn anyone (again) that it really does rely on the integrity of the 50R cable and the 50R termination at the other end.

If it was built using BNC connectors and maybe 1 metre of cheapo/ebay RG58 cable and fed to something like a Rigol DSA815 spectrum analyser then it wouldn't matter how much design effort went into the comp cap because the whole probe system would be riddled with mismatch uncertainty. So I'd expect to see disappointment and frustration on the part of the user if they tried this. The Rigol could easily have an input VSWR >1.5:1 and with the cheapo cable inline the VSWR looking into the cable with the Rigol at the far end could easily go above 2:1 by 1.5GHz. But that is just a guess as I've never used the Rigol analyser.

But the problem here is trying to minimise mismatch uncertainty from a 1500R resistor driving a low cost cable and analyser combo that could easily look like a VSWR of 2:1 that does a few laps of the smith chart. So you could expect to see a LOT of confusing ripple (maybe several dB) and the probe would probably be abandoned as a waste of time.

By using an SMA end launcher to make the probe tip it's possible to fit a (low VSWR) coaxial SMA 10dB attenuator right at the probe tip and minimise the mismatch uncertainty caused by cheaper cable and a poor load at the far end. That would help a bit in the case of the Rigol setup. I could post up plots of how mine works on a VNA if that helps but you have to bear in mind that I think an identical probe used with a cable and analyser that has poor VSWR will result in hugely different results because of the difference in mismatch uncertainty in the overall system. With a poor setup, it's really easy to throw away RF performance up in the VHF/UHF region even on something as 'simple' as an RF probe made from a resistor and an SMD cap
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Last edited by G0HZU_JMR; 15th Aug 2018 at 10:06 pm.
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