I picked up some new old stock of LeCroy PP061 resistive probes that I have no documentation for. These were very inexpensive.
I picked up some new old stock of LeCroy PP061 resistive probes that I have no documentation for. These were very inexpensive.
Thanks for posting the VNA tests (a year ago!). Interesting stuff, especially the "bumpy ride" in the amplitude domain. This response tends to be more typical of homemade devices, but considering how old these are, may have been the norm back then.
(The first time you mentioned the probes in the video, you referred to them as PP066: "I also purchased some PP066 probes." Later on you did identify them as PP061 though.)
Re: the PP061, all I can tell you is they were phased out by Feb'99, and replaced by the PP062 probes. Their characteristics don't seem to vary much from the story your VNA told you (500-ohm, 1.5pF, 1 GHz BW rating). See the attached chart...
I noticed earlier in 1999, they still had the PP063 in their passive probe lineup, but it had been dropped by 2000. That one was also a 500-ohm unit, but had an 8 GHz BW, made possible by a <0.5 pF capacitive loading.
[Archaeological digging courtesy of the Wayback Machine.]
I wonder how such setup deals with the capacitance of the scope input? With 1k resistor, even few pF translates to time constant of few ns, which doesn't look good. Am I missing something?
To get flattest HF response it is sometimes necessary to ADD a little parallel capacitance at the input end of the cable to compensate for the (tiny) parasitic capacitance of the 1k resistor itself.
This being said, I know this capacitance simply must have some bad effect on performanceFlattened edges in one-shot transitions? Inverted reflections causing FR ripple? I think somebody reported the latter, blaming it on bad terminating resistor.
Oh, thank you for pointing me at the resistor "not seeing" this capacitance. I completely forgot that RC filter relies on R "knowing" the voltage across C in real time
This being said, I know this capacitance simply must have some bad effect on performanceFlattened edges in one-shot transitions? Inverted reflections causing FR ripple? I think somebody reported the latter, blaming it on bad terminating resistor.
Time for a question from me... In Douglas Smith's implementation, http://www.emcesd.com/1ghzprob.htm why does he include 50R termination (the 4x200R smd resistors) at the probe end of the cable? I realize it's a double parallel termination of the cable (presumably there's 50R termination at the scope end too), but what benefit does it give in this application, given that it seems to work pretty well without? One downside is obviously the increased attenuation for any given tip resistor. Confused.
Time for a question from me... In Douglas Smith's implementation, http://www.emcesd.com/1ghzprob.htm why does he include 50R termination (the 4x200R smd resistors) at the probe end of the cable? I realize it's a double parallel termination of the cable (presumably there's 50R termination at the scope end too), but what benefit does it give in this application, given that it seems to work pretty well without? One downside is obviously the increased attenuation for any given tip resistor. Confused.
Well I think the purpose of a Lo Z probe IS to load the DUT with a matched impedance for HF work.
Why would u need a Lo Z probe otherwise?
No, the purpose of a lo-Z probe is to load the device under test as little as possible: it may have a lower impedance at DC but it has a lower capacitance than a hi-Z probe, so will have a much lower impedance at higher frequencies.
A "Lo-Z" probe is used when it has a higher Z than a conventional probe. A 10pF probe has less than 500 ohms reactance at only 30MHz.
It's also used on naturally low impedance circuits, for example RF amplifiers with 50 ohm system impedances: a 500 ohm load diverts only 10% of the voltage, which is only -0.82dB, hardly noticeable.
A "Lo-Z" probe is used when it has a higher Z than a conventional probe. A 10pF probe has less than 500 ohms reactance at only 30MHz.
It's also used on naturally low impedance circuits, for example RF amplifiers with 50 ohm system impedances: a 500 ohm load diverts only 10% of the voltage, which is only -0.82dB, hardly noticeable.
Tim
A "Lo-Z" probe is used when it has a higher Z than a conventional probe. A 10pF probe has less than 500 ohms reactance at only 30MHz.
It's also used on naturally low impedance circuits, for example RF amplifiers with 50 ohm system impedances: a 500 ohm load diverts only 10% of the voltage, which is only -0.82dB, hardly noticeable.Still in some cases 500 Ohms is too much of a load. In one of my current RF projects I added several 1:50 passive divider test points in the prototype in order to keep the load on the signal low. When measuring LVDS signals a 500 Ohm load is definitely too much so I also have 1:100 Lo-z probes but the attenuation is becoming troublesome. At some point you'll need active FET probes.
A "Lo-Z" probe is used when it has a higher Z than a conventional probe. A 10pF probe has less than 500 ohms reactance at only 30MHz.
It's also used on naturally low impedance circuits, for example RF amplifiers with 50 ohm system impedances: a 500 ohm load diverts only 10% of the voltage, which is only -0.82dB, hardly noticeable.
Tim
20log.9 = -0.91dB , not so?
Still in some cases 500 Ohms is too much of a load. In one of my current RF projects I added several 1:50 passive divider test points in the prototype in order to keep the load on the signal low. When measuring LVDS signals a 500 Ohm load is definitely too much so I also have 1:100 Lo-z probes but the attenuation is becoming troublesome. At some point you'll need active FET probes.