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making an 20:1 coax probe
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tggzzz:

--- Quote from: PartialDischarge on July 08, 2022, 06:32:29 pm ---The problem with a 20:1 probe is that loading is high, better find a FET probe from eBay or similar

--- End quote ---

Numbers, not adjectives, please.

Include both resistance and capacitance; the latter is very significant at frequencies over 100MHz or so.

When doing modelling, don't forget the ground lead inductance.

If using a 50ohm terminator at the scope (rather than a pukka 50ohm input such as the Tek 485), don't forget that it will still have the scopes input capacitance in parallel with the 50ohm. That can be significant. Modelling and simulation wil show the relative importance of various parameters.
G0HZU:

--- Quote ---It is not an issue up to 200 MHz.  The oscilloscope response is likely to have more variation than common coaxial cable and the connector will produce.
--- End quote ---

Yes, it probably doesn't matter that much below 200MHz. In case it's useful to anyone I kept plots of the change I saw when going from a cheap BNC crimp to a decent RF connector when using RG316 cable. The plots cover up to 1.5GHz and are shown below. There's a significant difference up at UHF but less so below 200MHz.

At this stage I hadn't compensated the response but you can see how much the choice of connector can change the probe response in terms of mismatch ripple.

The other issue will be the quality of the BNC 50 ohm termination and how it and the input impedance of the scope may contribute to yet more mismatch ripple. it really is up to the individual to decide what is acceptable when the complete probe is tested.
G0HZU:
To try and put some numbers behind this stuff you can crudely estimate the peak to peak mismatch ripple if you know the VSWR looking into the tip end of the coax cable with the 950R resistor removed. Assuming the cable itself is decent this will be defined by the quality of the BNC connector, the 50R BNC termination and the impedance of the scope input.

The input impedance of a typical 200MHz scope is going to be fairly low up at VHF so it can easily degrade the VSWR. The source impedance looking into the cable at the tip end is typically about 1000 ohms because of the series 950R resistor. Ideally, the cable will look like a resistive 50R so the VSWR of the source resistor is about 20:1. In reality, the input to the cable will not be a resistive 50R because the scope and the termination won't be a very accurate 50 ohms.

If the source VSWR is 20:1 and the VSWR looking into the cable is 1.3:1 then there will be about +/- 1dB ripple if the cable is long enough to explore all phase angles. If the cable VSWR is 2:1 then the ripple (uncertainty) increases to about +/- 3dB. I've seen a few probes like this with about +/- 2dB ripple up at UHF because the design hasn't achieved a low VSWR looking into the coaxial cable.
G0HZU:
The other reason for my choice of RG316 cable is that the dielectric material is PTFE so it doesn't melt with soldering. RG174 has a polyethylene dielectric so it melts really easily if soldering the braid or the inner connector.

RG174 is much more flexible though so if you can avoid melting the dielectric it is still a good choice for a 200MHz probe especially if you ever want to be able to fold it all up into a storage pouch.
David Hess:

--- Quote from: G0HZU on July 09, 2022, 12:15:35 am ---To try and put some numbers behind this stuff you can crudely estimate the peak to peak mismatch ripple if you know the VSWR looking into the tip end of the coax cable with the 950R resistor removed. Assuming the cable itself is decent this will be defined by the quality of the BNC connector, the 50R BNC termination and the impedance of the scope input.

The input impedance of a typical 200MHz scope is going to be fairly low up at VHF so it can easily degrade the VSWR. The source impedance looking into the cable at the tip end is typically about 1000 ohms because of the series 950R resistor. Ideally, the cable will look like a resistive 50R so the VSWR of the source resistor is about 20:1. In reality, the input to the cable will not be a resistive 50R because the scope and the termination won't be a very accurate 50 ohms.

If the source VSWR is 20:1 and the VSWR looking into the cable is 1.3:1 then there will be about +/- 1dB ripple if the cable is long enough to explore all phase angles. If the cable VSWR is 2:1 then the ripple (uncertainty) increases to about +/- 3dB. I've seen a few probes like this with about +/- 2dB ripple up at UHF because the design hasn't achieved a low VSWR looking into the coaxial cable.
--- End quote ---

The above may not apply to an oscilloscope at all because of how they are calibrated.  The calibration source for transient response at the input connector is a fast reference edge.  When the transient response calibration is done, the ripple should be removed by the opposite ripple in the response of the oscilloscope, but that does not account for reflections in the cable and who knows how long that is?  At higher frequencies no cable is used, and at lower frequencies it should not matter if the pulse source has a reverse termination to absorb the reflection, and the oscilloscope may not be fast enough to see it anyway.  At higher frequencies the probe itself does part of the transient response calibration for the oscilloscope, which is why Tektronix releases otherwise identical probe revisions for different oscilloscope models, or at least they used to.

Probes themselves are calibrated with a fast edge and 25 ohm source impedance from a 50 ohm termination.  How realistic is that?  Probing digital lines is usually about 60 ohms.  As a practical matter probe bandwidth depends on source impedance.  Probe bandwidth itself is a questionable concept; it is better thought of as the bandwidth below which the probe will perform as specified and faithfully reproduce the signal.

Where it gets really interesting is that different manufacturers are not identical.  In the past Tektronix calibrated their probes so that the oscilloscope shows the signal with the probe loading.  HP calibrated probes to show what the signal should look like without the probe.  Who knows what they do now.

So I would not worry about how accurate the 50 ohm termination is for a 200 MHz oscilloscope because performance depends on too many other things, and 200 MHz is just not that high.  However there are plenty of reasons to want a low-z or active probe even at 200 MHz because the low input capacitance at the probe tip will make many measurements better, and some measurements possible.  Try to get 1 GHz performance out of a 1 GHz oscilloscope and things are very different.


--- Quote from: G0HZU on July 09, 2022, 12:40:37 am ---The other reason for my choice of RG316 cable is that the dielectric material is PTFE so it doesn't melt with soldering. RG174 has a polyethylene dielectric so it melts really easily if soldering the braid or the inner connector.

RG174 is much more flexible though so if you can avoid melting the dielectric it is still a good choice for a 200MHz probe especially if you ever want to be able to fold it all up into a storage pouch.
--- End quote ---

I agree completely.  The various Teflon dielectric cables are easier to work with because the dielectric will not melt, but they are stiffer.  RG-174 is good for flexibility, but can be difficult to solder.  RG-178 is 50 ohm Teflon but even thinner than RG-316 or RG-174 so might have its place in some probing applications.  Double shielded RG-223 and RG-400 are nice because the double shield makes the connector interface much tougher.  In RF applications double shielded may be mandatory to prevent leakage.


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