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Probe into probes. What's up?
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bdunham7:
Here is 1M and 50R superimposed at the same vertical setting.  The slew rate and rise times are similar until the 1M rise slows down.  This is a function of the scope, the Bodnar pulser is fine.  The pulser itself does not need 50R termination to put out a clean square wave, as I have demonstrated with the P6156 or could be demonstrated by using an active-FET probe. 

This is a good discussion to demonstrate the difficulty, or even futility, of trying to use passive Hi-Z probes above 200MHz or so.  The Hi-Z probe will put a complex load on the source, causing the actual signal at the probe tip to be altered--which also can be demonstrated by using a P6156 or active-FET in parallel.  HF probes specific to scopes will attempt to compensate for this to some extent so as to give the appearance of an enhanced system reponse.  The dilemma is then to decide which lie you would rather be told--do you want the probe/scope system to reflect the actual waveform including the alterations caused by the probe load itself or would you like it to attempt to 'compensate' and attempt to show you what would be there if the probe were not in the system?

David Hess:

--- Quote from: bdunham7 on February 01, 2023, 04:59:22 pm ---The dilemma is then to decide which lie you would rather be told--do you want the probe/scope system to reflect the actual waveform including the alterations caused by the probe load itself or would you like it to attempt to 'compensate' and attempt to show you what would be there if the probe were not in the system?
--- End quote ---

Manufacturers take that into account, but Tektronix and HP used different methods.  I forget which was which, but one calibrated their probes to show the signal at the probe tip with the probe loading, and the other calibrated their probes to show what the signal would look like without the probe.
jonpaul:
bdunham7, David H,  very fine notes.

No pulse gen, Leo B 40ps, Tek PG506, PG502 or the many HP high speed pulse gens  will supply the specified output without a 50 Ohm load.


The problem  of high Z probes and FET probes in wideband work is 

a/ Ground return lead inductance (must use a Bayonet probe tip or adapter)

b/ Xc of probe capacitance at the desired frequency.

Those two factors are slight at low F but enormous above 100 MHz.

Here is a  calculation of Xc  impedance of the probe tip C

1/   X10 probe, 15 pF probe tip
Xc = 1/2pi FC
 200 MHz
Xc = 1/(15 exp -9 * 6.28 * 200 exp 6) = 50 Ohms

2/  FET probe, C= 3pF
Xc = 280 Ohms.

Thus the entire raison d'etre, for the X10 or high Z probe is in question due to  capacitance Xc.

For accurate wideband  scope use,  we use  either a Zo probe (P6136) into a 50 Ohm scope,

OR make a 50 Ohm coax RG174/U direct to a 50 Ohm ckt or with 450/4950 series R for Zo probe.

Just the thoughts of an old retired EE, using TEK scopes and probes since 1967!

Have an ABSOLUTELY FANTASTIC day!

Jon

bdunham7:

--- Quote from: jonpaul on February 01, 2023, 05:39:30 pm ---No pulse gen, Leo B 40ps, Tek PG506, PG502 or the many HP high speed pulse gens  will supply the specified output without a 50 Ohm load.

--- End quote ---

But I showed it doing just that in my earlier reply #27....


--- Quote ---2/  FET probe, C= 3pF
Xc = 280 Ohms.

--- End quote ---

3pF is better than 8pF, but a high-BW FET probe will be even lower.  <1pF should give you similar results to  the P6156 even without termination.
BillyO:
While certainly no P6156 I have always made my own Z0 probes.  Cheap and cheerful.  I usually solder them directly to the DUT, but in this case I used a BNC connector.

This one is 1m long, 500R.  It looks like it's suffering from a little bit of reflection.

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