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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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6th Sep 2020, 3:04 am | #21 |
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Re: R.F. probes
The achilles' heel of the scope probe is the need to make a cable as low capacitance and as high impedance as possible. it is then used in highly mismatched and uncertain circumstances. We then want it to give a flat response and clean, fast risetime.
Aye, right! as they say north of the border. Once I get above a few tens of MHz, I prefer to use probes with a real, broadband, well screened 50 Ohm coax, a scope with a switched 50Ohm termination, and a simple series resistor at the pointy end. These do not give megohm impedances at DC, but they can be a lot better than what the routine divide-by-ten 10Megohm probe degrades to at those frequencies. I have some Sealectro probe tip ends with SMC connector back ends which go on a length of thin 50 Ohm coax. Tektronix had a family of complete probes like this. So, once you start climbing up the MHz, you leave attempts at high-Z behind. Often, they were futile anyway. Once I go higher still in frequency I leave scopes behind and switch to the frequency domain and use the resistor-on-coax probe on a spectrum analyser. I'm probably thinking in frequency domain terms anyway at this frequency. I've spent a lot of time in the last several years probing GHz transmitter circuitry and the resistive probe on a 50Ohm spectrum analyser was the only tool left in the box. Pulsed transmitters reduced the net long-term power dissipation, but the instantaneous peak voltages would have instantly destroyed high-Z active probes. The one thing out of this is learning how critically the way you ground the outer of your coax at the pointy end to the circuit under test affects results. Very low inductance is needed. Plan-B is to put resistor networks on the board being developed to act as interfaces with a 50 Ohm connector. No probe as such, just a wandering 50 Ohm cable is needed. With the resistive probe on 50 Ohm cable, slipping a few ferrite beads on the business end of the cable can help control/avoid common mode effects. With circuitry in llittle screening boxes accessed through removed lids, you can rest the grounded shank of the resistive probe against the box wall very close to where the pointy bit is probing. Once you switch from a scope to a spectrum analyser you suddenly get an awful lot more sensitivity available, and you can exploit this to use more lossy probing/sampling arrangements which are the price of getting less loading on the circuit under test. This sensitivity also leaves far behind what diode probes could do. If handling moderate RF powers in the milliwatt and upwards region, thermal power meters enter the scene. These can be very flat, very accurate. They tell you the power level present. They don't do pulses. The tell you not one jot about waveforms and spectra, just how hot the signal makes a 50 Ohm resistor. Frequency wise, this technique works to and beyond optical frequencies, if you can make a terminating absorber at your frequency, you're in! So scopes were common to 100-200MHz with a few at 400MHz, and one at 1GHz. Apart from sampling scopes they were never pushed higher, because people switched to spectrum analysers. Folk working on ultra-fast logic wanted time domain, so multi-GHz sampling scopes were developed. At first these were subsamplers, needing repetitive signals, but nowadays there are digital samplers running at several GS/s that can give you useful one-shot bandwidth and sometimes a few samples up risetimes.. with the resistive probe, I have a useful bit of kit, the probe into transmission line adaptors made for the HP vector voltmeter. A stainless N-type length of line with a side port to allow a probe to hard ground to the outer metal and to just touch the inner line conductor. True 50 Ohm Zo throughout apart from the side port. Stick it on a sig gen, with a good 50 Ohm termination after it, and you now have a fairly accurate voltage on the line to measure your resistive probe calibration. You can look at flatness this way, you can also use an attenuator and analyser after this probing adaptor as the 50 Ohm load, and you can see the effecto of applying and removing your probe as a sanity check. The vector voltmeter, now I've mentioned it, is a handy bit of kit that will act as an RF voltmeter down below mV and up to 2GHz. 1GHz for the classic one with built in probes on cables and moving coil meters. Two probes, and you can get the relative phase measured too.. The probes contain diode sampling bridges driven by an LO in the main instrument. The LO is phase locked to the incoming signal. It has its limitations, but the limitations are different to other instruments, so they let you see things you wouldn't see elsewhere. No one size fits all. Diode probes are cheap to make. Other stuff can be very expensive unless you hit a lucky find. So there you have it. Many methods, plenty of overlap, but each with strengths. The comment on probe ground referencing, and the ferrite beads apply to all. I'll have one of each, please. David
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6th Sep 2020, 9:05 am | #22 | |
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Re: R.F. probes
Quote:
Peter |
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6th Sep 2020, 10:22 am | #23 |
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Re: R.F. probes
If you get stuck, Peter, I've got spectrum analysers, power meters, synth sig gen and things like that if you need to borrow something.
David
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6th Sep 2020, 10:36 am | #24 |
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Re: R.F. probes
Thanks David. I do appreciate your offer but I don't believe that signals above 110MHz actually exist.
Peter |
6th Sep 2020, 9:50 pm | #25 |
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Re: R.F. probes
David, (Radio Wrangler) wrote:
The one thing out of this is learning how critically the way you ground the outer of your coax at the pointy end to the circuit under test affects results. Very low inductance is needed. ---------------------------- Indeed so. I don't know if anyone here would care to verify - or alternatively to dispute - my calculations, but they did produce the following: 'Scope probe earth lead. Wire diameter: 2 mm. Wire length: 10 cm. (Seems a typical length) Approx. inductance: 0.1 uH At 10 MHz, inductive reactance is approx. 6.3 ohms Really? Al. |
6th Sep 2020, 10:49 pm | #26 |
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Re: R.F. probes
Yes, really. Definitely trouble with risetime rings. with 50 MHz and upwards scopes.
You have to try it to believe it. Ordinary TTL/HC logic will show it up. David
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7th Sep 2020, 1:43 am | #27 | |
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Re: R.F. probes
Quote:
Within the scope of this Forum, I think most people would settle for effective high-impedance RF voltage measurements up to 110MHz (150 megs would be nice), but the need to go the GHz range is generally not required. B
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7th Sep 2020, 6:16 am | #28 |
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Re: R.F. probes
There are diode-based heads for the HP435A and later series of power meters. By using some rather special diodes they can read down to -70dBm. Their low frequency end is set by the values of coupling capacitors and models go to 18 and 40 GHz. They terminate the input signal in 50 Ohms, so they aren't exactly probes, but they show how low a level diodes can be made to work at.
-70dBm is 10E-10W = 100pW. In 50 Ohms this is 70.71 uV RMS so 100uV peak. In other words, at very low power levels, these diodes detect seemingly without the threshold voltage we're used to. They give a DC voltage proportional to RF power. At higher powers they start to rectify and to give a voltage proportional to RF voltage (less a threshold!) So the power meter has to correct for this mode-change region and to know whether the detector head is in square law or not. Some heads have a second diode detector with an attenuator so it operates over an offset dynamic range, so you get heads covering -70 to +20 dBm. So fancy diodes can do some remarkable things. David
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7th Sep 2020, 8:16 pm | #30 |
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Re: R.F. probes
Thanks for that link Jim. I have a feeling that I have seen it previously, but then let it slip my mind. As well as the diodes, it reveals a few features in the design and construction that indicate the detailed development work that went in getting the performance.
B
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7th Sep 2020, 10:09 pm | #31 |
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Re: R.F. probes
I do a certain amount of work with fast digital signals. The scope I use has a bandwidth of 20GHz, and the risetimes of the signals are often in the 100-200ps range. The most reliable connection is a proper 50 ohm feed from the circuit under test, with impedance-controlled PCB tracks and a decent connector (the little U.FL ones popular in mobile phones are really handy) but that's not always possible. My preferred 'quick hack' probe is to simply solder a small (0603 is OK, 0402 if you can manage it) surface mount resistor on to the point of interest, and to the other end of that, connect a bit of microminiature coax with an SMA socket on the other end. The shield of the coax goes to any convenient nearby ground (by nearby, I mean a couple of millimetres away). Resistor value depends on what mismatch you can tolerate and what attenuation you can put up with. A few hundred ohms is usually OK.
This won't give an accurate readout of RF power levels, but can give a representative picture of what's really going on in the analogue domain of a supposedly digital circuit. Chris
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