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I just did a little searching for fast probes that match 50 ohm (sampling) inputs. The table on Tekwiki shows only P6150 or P6156 would have the necessary speed. But on ebay the prices are an unpleasant surprise!

If those prices are representative, I can see why a 453 ohm metal film resistor on the end of a piece of good (e.g. PTFE) 50 ohm coax is a common and far cheaper alternative, although the ground hardware still needs to be worked out...

Or the circuit has 50 ohm ports built into it, for direct connections with SMA cables...
thoughts?

An active probe with hi-Z inputs and an amplifier to provide a 50 ohm output isn't cheap.

For the 1S1, I have the P6032 but it's only 850MHz and the plastics degrade quickly and STINK. Then there's the Type 282 Amplifier.

I have a few lying around. Price is not too bad, this one went for 57$ US.

Then you can find some sort of reasonable probe to hook up to that but of course there are few 500MHz+ passive probes since they usually have too high an input capacitance. Tek had the P6010 that was "wide bandwidth" but even it doesn't quite match the risetime of the sampler, and indeed it depends on the length of the cable.

You didn't say what you want to probe, if it's your own circuits I have put SMBs with a SMT resistor as a power-tap probe. Amplitude calibration is up to you, but you will get quite nice results, if you can live with the stolen power from the signal resulting in a lower signal amplitude. That's why I go for a 1K and use the 1S1. SMB cables aren't too expensive. Mating to the GR-874 is up to you.

I just ordered one of these, which I think does basically the same thing:



I am not sure about the voltage output so I also ordered some SMA feed-through attenuators to avoid overloading the input of my 1S1. (However, whether the use of attenuators introduces problems with impedance matching, I don't know.)

If you watch Mr. Carlson's Lab on YouTube, he has a similar circuit available on his Patreon:



The only downside is that you have to provide your own D.C. power.

Yikes, with that layout, I am skeptical. What is the flatness over its bandwidth and what is its true -3dB point? It's basically an antenna: sure, it'll get a signal through, but you can't really rely on it for any kind of absolute measurements.

An SMA attenuator has to be 50 ohms on both its ports, so I don't see how it would be a problem, unless its terrible. Of course each extra connection is itself a potential discontinuity but that's unavoidable. Keep 'em clean, specifically inside the connector where you can see the dielectric. Any junk in there that creates an air gap can get problematic as you get up there.

On Sun, Mar 21, 2021 at 09:15 PM, snapdiode wrote:

of course there are few 500MHz+ passive probes since they usually have too
high an input capacitance.

For the GHz range, I only found the two as listed in my original post... I hadn't considered an active probe, figuring that it had to cost more (and be subject to electrical damage from overload) than a passive one.

Jonathan Pyle 9:19pm #180728

I just ordered one of these, which I think does basically the same thing:

Now that's reasonable... $12 plus $8 shipping. That'd probably work fine with the S-1 head (350 ps), but it would definitely be the slowest part when feeding the 75 ps S-2, or the 25 ps S-4. I think I'll order a couple to play with too. Thanks for the link! Please post risetimes if you have a fast enough pulse to measure it.

It does appear to be less than perfect... you get what you pay for!

Also, the link on the last post of the EEVblog thread takes you to a homemade version that is good to 1.6 GHz (1 dB), -3 dB is 1.7 GHz...

On Sun, Mar 21, 2021 at 07:38 PM, Charles wrote:

I only found the two as listed in my original post.

I wouldn't even consider these "probes", they're part of a system and I bet that these "probes" have to be soldered in-circuit to get best performance. At a previous job I had to solder in some Infiniium probes for 20+ Gbps stuff and it's basically a tiny ceramic PCB with the magic on it and there's two tiny wires to solder in-circuit. It was a differential system for measuring the eye of digital systems. There's simply no way to probe stuff like this the same way you tool around inside a Commodore 64 or Tektronix 547...

Hi Charles,

I suggest Picosecond Pulse Labs (PSPL) 5520C's as alternative for P6156 + 013-0237-00 and P6150 + 013-0251-00/01's. The 013 PN's are the SMA Voltage Pick-Offs for these two Probes.

Greetings,

Egge Siert

Hello: We use several P6156, to use on 7104 aand 7904 TEK with 7A29 Vert.

They are the best but beware used on epay, often abused and missing parts. The best have the original case and all accessories.

We found them over the years at $40-65, but not often.

A 50 Ohm cable SMA is OK. The PCB mount SMA connectors are available from Semtech, etc. to place on your board designs.

Making a probe with a series R will not give best response due to series L and shunt C of the resistor.

Bon chance,

Jon

I hope no one is using a through-hole 1/4W resistor for this. The biggest I'd go is 0603, usually a 0402 placed on the PCB at design time.

The through-hole resistor on the end of the coax seems to be OK down to 1 ns risetimes, maybe as far as 0.5 ns, but I wouldn't try and do anything faster than that. On the other hand, it's essentially free.

Yes it should be good enough at those speeds, after all the 1S1 itself is built with leaded components. The ground connection is as responsible as anything to keep inductance down.

Unfortunately I often talk about PCB design, where you can easily add a probe resistor to a trace by putting its pad directly on the trace and use much smaller connectors like H.FL, etc.

This is how I probe fast stuff when it's my own designs.

Sometimes I guess I'm not clear about this when people are talking about flying lead probing on existing circuits.

Charles --

I tested the RF probe from Ukraine that I got on eBay ().

I tested it with my 1S1 and my Leo Bodnar 30 picosecond pulse generator (10 MHz) and found that the measured risetime through the RF probe was around 300 picoseconds, which is the same as when the pulse generator is plugged directly into the 1S1.

However, the RF probe did add a lot of "ringing" to the waveform. See the photo in this album:

/g/TekScopes/album?id=262713

I'm not sure this is a fair test, though, because a 30 picosecond risetime contains frequencies well beyond the stated bandwidth of the probe (1.5 GHz), so maybe that messes things up?

The RF probe comes with a nice telescoping needle probe point, but I desoldered it because I thought it might introduce noise. Removing it did not have any noticeable effect. I soldered an SMA socket to the RF probe input for purposes of attaching the pulse generator without any long leads. (The pulse generator wants 50 ohms so I used a T and a 50 ohm terminator.)

I did another test with a 10MHz signal from my Koolertron function generator (low cost Amazon product) through a 3-foot BNC cable (well within the bandwidth of the probe). The waveform looks the same through the RF probe, just noisier.

I don't have a function generator that can produce frequencies in the hundreds of MHz, and I don't have a spectrum analyzer that can test the frequency response of the RF probe (see the eevblog post above).

For $21.20 including shipping I think it's a pretty handy device. (Shipping to the U.S. took only eight days.)

Jonathan Pyle wrote:

I don't have a function generator that can produce frequencies in the hundreds of MHz,
and I don't have a spectrum analyzer that can test the frequency response of the RF probe.

Good News! You can remedy both deficits by the acquisition of a TinaSA:

The TinySA is a spectrum analyzer that can operate on signals between 100 KHz and 960 MHz (in two ranges), it can generate sine waves up to 350 MHz, and square waves up to 960 MHz, and it fits in the palm of your hand.

Make sure you buy from one of the reliable sources:

When I bought mine, earlier this year, I paid less than $60. I bought through the Zeenko store on AliExpress. It did take more than a month or the item to arrive, as I did not pay or expedited shipping.

-- Jeff Dutky

That dang Jeff Dutky keeps making me spend money on test equipment!

LOL.

Last I'd checked there wasn't any availability. R&L has them now! Get 'em while their hot!
Dave

Dave,

Happy to be of service.

-- Jeff Dutky

That's a lot of ringing (and it persists for a long time, too)!

For the "direct" test, did you also have the tee with the 50 ohm terminator attached? (Is the ringing from the probe itself or from the setup)?

I may still buy one and tinker with it... I have sine-wave sources up to 920 MHz, and of course my 300-ish ps avalanche pulser.

I didn't use the 50 ohm terminator when plugging directly into the 1S1 because I thought that would be too much load, since the 1S1 already provides 50 ohms of load. However, I tried to replicate the setup as best I could by adding in an equivalent number of SMA adapters, including the tee (with an open end where the 50 ohm terminator was). SMA adapters don't seem to change the waveform noticeably. So I don't think the setup itself is causing the ringing, unless the 0.25 inch unshielded line running from the SMA socket to the solder pad of the probe's input is the culprit.

I already wanted a NanoVNA, but now I feel compelled to also own an TinySA. I need more gift-giving holidays in the calendar.

Good point, I forgot your 1S1 is already 50 ohms. What is the period of that ringing? That may help locate something that's the right electrical length. Track on the probe PC board maybe?
I agree, no way should an SMA connector cause that kind of aberration.

I want a TinySA too :)