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Old 9th Aug 2018, 2:25 pm   #41
merlinmaxwell
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Default Re: Back terminated cable?

As far as I recall 50 ohms is used for transmitters as this is a balance twixt power loss and dielectric strength. The lower the resistance (as it should be in a matched system, not impedance with a j in it) the lower the voltage, but then you get a higher current and more conductor loss.
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Old 9th Aug 2018, 4:53 pm   #42
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Default Re: Back terminated cable?

Quote:
Originally Posted by kalee20 View Post
A simple, single-ended triode operated with a fixed input level (which is small enough that clipping never occurs) gives its maximum power when Rl = ra. This is the Max Power Theorem at work. But if you allow yourself to always vary the input level up to the point at which clipping occurs - whether it is grid-current clipping, anode current cut-off clipping, or clipping due to the anode voltage 'bottoming' then max power is reached at quite a different value of load.
I think that a lot of misunderstanding has been created in the minds of electronics students when generations of lecturers have explained loudspeaker matching in terms of the maximum power theorem. Maybe back in the day of low impedance triode output valves it could give a reasonable approximation, but ever since the pentode came along, it's been irrelevant and with transistor amplifiers could result in the release of much magic smoke!

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Old 9th Aug 2018, 5:05 pm   #43
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Default Re: Back terminated cable?

Quote:
Originally Posted by Argus25
Still I wonder, would it not be an advantage (putting optimal operation of the output stage aside and power transfer issues) to strive for an RF amplifier output impedance that matched the coaxial line it was connected to, for the purpose of ensuring that reflected waves heading toward the amplifier do not see an impedance bump there and are absorbed rather than re-reflected into the line ? Or is that not considered too important ?
On the contrary, it is often considered important not to have a match. That prevents any reflected power from heating up the output stage; it merely gets reflected back to the load (typically an antenna) so more of the power you originally produced gets radiated.

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What I meant was, after "mismatching" the load, you can restore power output, and maybe get even more, simply by turning up the wick!
Yes, of course.
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Old 9th Aug 2018, 5:10 pm   #44
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Default Re: Back terminated cable?

Thanks Dave, I did not know it was an advantage not to have a match. Now I might change the way I design my RF amplifier output stages. Though often practical considerations of transistor dissipation and waveform linearity have forced the design to a final form with the components I have at hand.
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Old 9th Aug 2018, 7:05 pm   #45
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Default Re: Back terminated cable?

That's all OK so long as what you transmit can withstand "echoed copies" of the original signal. Watch your EVM
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Old 9th Aug 2018, 8:20 pm   #46
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Default Re: Back terminated cable?

Quote:
Originally Posted by Hartley118 View Post
I think that a lot of misunderstanding has been created in the minds of electronics students when generations of lecturers have explained loudspeaker matching in terms of the maximum power theorem. Maybe back in the day of low impedance triode output valves it could give a reasonable approximation, but ever since the pentode came along, it's been irrelevant and with transistor amplifiers could result in the release of much magic smoke!
I'm always very careful to reserve the word "matched" for when source and load impedances at some point are either exactly equal if resistive, or conjugates if complex.

In other circumstances i talk about loading and load impedance transformation, carefully skating around the m-word. It takes a fair while to get recent graduates to see that we don't actually match everything in RF engineering.

It does take some doing to undo some of the impressions people are given.

David
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Old 10th Aug 2018, 1:11 pm   #47
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Question Re: Back terminated cable?

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Originally Posted by kalee20 View Post
50 ohms is common-place for video and most other stuff . . . .
Doesn't most video equipment use 75Ω ?

Al.
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Old 10th Aug 2018, 3:54 pm   #48
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Default Re: Back terminated cable?

Most video gear, if not all, uses 75 ohms and typically there is a low Z output stage and a 75R resistor in series (so in these cases the impedance of the output amplifier always matches the cable) to the typical BNC or RCA video output socket. If the far end of the cable is unterminated with a 75R resistor (often there is a switch on a video monitor to select high Z) the signal level doubles, but it was helpful if a few video monitors were daisy chained then the last one terminated it in 75R.

Most TV related RF test gear is also 75R out, for example the PM5519 pattern generator video and RF outputs are also 75R. If you look into the RF out connector on the generator, with a meter, you will find it measures as a 75R resistor, regardless of the setting of the attenuator. It has a very clever enclosed linear attenuator that maintains a stable input and output impedance as it is rotated.

Also most TV related antenna coax is 75R, and the antennas have baluns, it replaced the early 300 R ribbon balanced line, which incidentally, has got difficult to get now. I needed some for a restoration of a tuner and had to scavenge it from an old set of rabbit ears.

Most commercial RF gear, Comms radios etc, various transmitters appear to have adopted 50R.
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Old 10th Aug 2018, 11:10 pm   #49
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Default Re: Back terminated cable?

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Now I do not wish to sound like an 'arrogant tech. know-it-all' in this matter, but to me, I would have thought that anyone using a transmission line in a typical application would also have the elementary knowledge to realise the need for that source Z to match the cable's Zo, therefore rendering the need for the expression 'back-terminated' to be simply redundant. In conclusion, to me, the use of that phrase is simply 'tech. over-kill'.
A simple and practical example where you might not want to match the source to the cable Zo is given below. You might have already made one yourself!

Up at VHF through UHF it's hard to make a high impedance probe for probing low Z circuits without loading the circuit too much. It's also desirable to want a flat frequency response. So a typical scope probe is no good because of the high capacitance up at VHF/UHF. Lets keep it simple and say the probe is for probing typical RF circuit boards where there might be 50R points in the circuit.

If you simply used 50R cable with a 50R spectrum analyser at the far end then you would double terminate and cause loading and loss in the circuit under test. This can sometimes be OK but it generally isn't a good idea.

But if you fitted a 470R series resistor at the feedpoint of the 50R coax cable as a form of probe tip, the circuit under test would see 470R in series with 50R when touched by this 470R resistor + coax. So it won't be loaded as much. As long as the 470R resistor has low parasitic capacitance and inductance and the cable is tightly connected to ground at the test PCB then the above probe should give a flat response up past 2GHz. However, if you just use a small metal film leaded resistor it might only be good to about 1GHz. But the probe should give about a 20dB loss with a flat response and it will load the 50R with just over 500R so this is about ten times. So there will be minimal loss in the circuit as it gets probed.

But the relevant point here is that the source impedance feeding the 50R cable is now about 500 ohms. But it still gives a flat -20dB response as long as the far end is terminated correctly with 50R. 470R isn't the ideal resistance for a flat -20dB response from the probe but it is quite close

Of course, this is really just a form of voltage divider so the -20dB coupling only applies if the 50R test point really is 50R. So it isn't really a -20dB probe for real world 50R circuits where the circuit might be 58R or 39R. But it is still a very useful tool to have on the bench
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Old 10th Aug 2018, 11:22 pm   #50
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Default Re: Back terminated cable?

I suppose the other classic example is the half wave transmission line. As long as reasonable/sensible tline Zo and source/load impedances are used then it doesn't matter what the tline Zo is because (under steady state conditions) the load impedance at the far end appears at the source after having been rotated fully around one lap of the smith chart by the tline/cable. So it is almost as if the cable isn't there even if you change the cable Zo. So the source Z doesn't have to match the cable Zo here at all. The load seen by the source should look the same even if you could somehow magically change the cable Zo on the fly with a rotary control. This obviously assumes steady state conditions.
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Old 11th Aug 2018, 7:11 pm   #51
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Question Re: Back terminated cable?

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Originally Posted by G0HZU_JMR View Post
Up at VHF through UHF it's hard to make a high impedance probe for probing low Z circuits, without loading the circuit too much.
"Low Z circuits"? I'm sure that you really meant "high Z circuits", no?

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If you simply used 50R cable with a 50R spectrum analyser at the far end then you would double terminate and cause loading and loss in the circuit under test.
How does the "double termination" arise in that situation? Surely the only one load that is terminating the line - and which is also 'seen' by the source - is the 50Ω of the analyzer?

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Last edited by Skywave; 11th Aug 2018 at 7:20 pm. Reason: Add 2nd. quote and its response.
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Old 11th Aug 2018, 7:33 pm   #52
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Default Re: Back terminated cable?

Yes, I did mean the probe was meant for probing typical low Z circuits. eg for probing a signal path through a receiver or synthesiser or the low level stages of a transmitter.

I probably didn't explain it very well but if you could imagine a chain of 50R (UHF) circuits in a line, eg amplifier, attenuator, filter, attenuator, amplifier, attenuator, mixer etc the idea is the probe can be used along this circuit path without loading it too much. The idea is to leave the circuit all connected and just sniff along each stage. Because the probe looks like about 500R then it doesn't load the circuit very much. Obviously if you did this with a direct 50R cable to a 50R analyser the circuit would be double terminated at the probe point.

I've used this type of probe here at home and at work for many years and it can work well up to 2 or 3GHz if constructed carefully. For best performance it's best to include a coaxial DC block in the coax at the analyser end. This way you don't have to worry of the circuit has DC on it and a coaxial/SMA DC block will work much better (in terms of maintaining a flat response over the full bandwidth) than trying to add a blocking cap in series with the 470R resistor.
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Old 11th Aug 2018, 8:28 pm   #53
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Ah! I understand now: thanks.
I was thinking in terms of source - with its internal Z - connected to a 50Ω line - connected to a 50Ω spectrum analyzer. But with a chain of interconnected 50Ω modules, 'tapping off' at an intermediate point in that chain to a load (such as an S.A.) - which presents 50Ω to a source - is a different scenario. At that point of measurement, the source will indeed 'see' 50Ω||50Ω.

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Old 11th Aug 2018, 9:22 pm   #54
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Default Re: Back terminated cable?

The 50 Ohm coax with a resistor and a pin on the end is available as a wide bandwidth passive probe from Tektronix. The price is staggering considering what's in it. But they are awfully useful. (950 Ohms)

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Old 11th Aug 2018, 11:08 pm   #55
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Yes, they are really useful. I've also got a -30dB version that uses a series 1500R resistor but this needs a tiny shunt cap to help compensate for the self capacitance of the 1500R resistor. But this is because I used a basic leaded metal film resistor rather than anything exotic. See the image below.

Quick and dirty homebrew versions like this tend to fall apart because the resistor inevitably snaps away. So I tend to make mine on a SMA end launcher like the image below. It is quick and easy to repair as long as you have lots of 1500R resistors nearby

This is the -30dB version so it has a small compensation cap on it. The inductance of the skinny TCW ground lug limits the frequency response but it typically works up to about 2GHz although probably with about +/- 2dB ripple in the response. But this is plenty good enough for quick and casual signal tracing. It does need that SMA coaxial DC block at the other end of the SMA coax cable though. Eg from Minicircuits.
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Old 12th Aug 2018, 5:57 am   #56
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Default Re: Back terminated cable?

I had a pair of resistor probe-ends made by Sealectro. 5k and 500 Ohm versions. they were short, squat chromed brass with glass bead insulators supporting the spiky bit and a 50 Ohm SMC connector at the other end.

Rather usefully they would plug into HP adaptors intended for their active and vector voltmeter probes.

The 500 Ohm one 'vanished' and people at work dropped the 5k one and the glass bead has broken so it's gone intermittent.

Sealectro are long gone. The chances of ever finding another are roughly zilch.

David
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Old 12th Aug 2018, 8:10 am   #57
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Default Re: Back terminated cable?

Jeremy,
Regarding the remark in post 55 about the capacitance of the resistor. I thought in this situation the reason the HF response would roll off would be due to the capacitance of the circuit the series resistor feeds (in this case the coaxial cable and input it feeds and any capacitance to ground from the proximity of the resistor body to ground) and therefore any tiny self capacitance of the series resistor itself would be helpful, and that to help maintain the HF response it would require a small capacitor in parallel with the resistor, especially for larger values of the resistor, much like what is required in x10 scope probes. In those the ratio of the capacitive divider formed for AC voltages of high frequencies roughly matches that of the resistive divider for DC voltages, to try to get the response flat.
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Old 12th Aug 2018, 9:03 am   #58
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Default Re: Back terminated cable?

Won't that 1500R metal film resistor exhibit some series inductance? - in which case that small capacitor to earth would help to peak up the HF response. I'd be surprised though if it's significant compared with the probe lead capacitance

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Old 12th Aug 2018, 2:42 pm   #59
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It's really just a small fudge capacitor to flatten the response. There are several strays involved and the little cap helps offset the (tiny) internal capacitance of the 1500R resistor for one thing. I chose a resistor with very low internal capacitance despite the large package size but the shunt cap is still needed. I think my little shuntcap is 2 or 3pF but I can't remember now.

The reason I used an SMA end launcher was because it allows me to use several versions of these probes as probe tips and I can swap between them as required with a very good quality SMA RF coax cable (18GHz) into my HP8566B spectrum analyser or into a network analyser. So the far end of the coax is always terminated correctly in 50R with this setup. The probe isn't meant to be used with a 1Meg scope although you could use it if you selected (or fitted) a 50R input termination on the scope.

For more bandwidth I also have versions that either use Sucoform 141 or 086 RF cable and I use a 470R chip resistor for a -20dB version. The ground spike connection can also be much shorter with this setup. It also doesn't need the shunt cap. But this is so fragile I only use it when absolutely necessary. Even with care I often end up replacing the chip resistor more than once in a busy session. But it is cheap and gives good RF performance
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Old 12th Aug 2018, 8:11 pm   #60
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Default Re: Back terminated cable?

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Originally Posted by Argus25 View Post
Jeremy,
Regarding the remark in post 55 about the capacitance of the resistor. I thought in this situation the reason the HF response would roll off would be due to the capacitance of the circuit the series resistor feeds (in this case the coaxial cable and input it feeds and any capacitance to ground from the proximity of the resistor body to ground) and therefore any tiny self capacitance of the series resistor itself would be helpful, and that to help maintain the HF response it would require a small capacitor in parallel with the resistor, especially for larger values of the resistor, much like what is required in x10 scope probes. In those the ratio of the capacitive divider formed for AC voltages of high frequencies roughly matches that of the resistive divider for DC voltages, to try to get the response flat.
Hugo.
The problem is actually the other way around... it's the tiny parallel capacitance of the 1500R resistor that causes the probe to show an increasing response with increasing frequency. The little shunt cap compensates for it to try to flatten the response to -30dB up to about 2GHz. This capacitor is a shunt cap across the coax after the series 1500R resistor.
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