Bob,
I think that makes sense. I hadn't thought about is as analytically
as that, but I understand the reasoning.
Thanks!
Dave
On 2/12/2013 6:47 PM, Bob Albert wrote:
?
Dave,
Basically there is no frequency dependency of a
probe.? It will divide the input signal according
to the resistors in it and the input impedance of
the load.
Having said that, and for the moment ignoring the
compensation issue, what is often done is to
relate everything to 50 Ohms.
If you are probing in a 50 Ohm circuit, your probe
should present a high impedance relative to 50
Ohms.? Of course for DC that's easy enough.
But at high frequencies, the loading of the
circuit being probed also includes the input
capacitance of the probe.? So the usual (but
incorrect) method is to calculate the input
capacitance of the probe and see at what frequency
it equals 50 Ohms (reactive).? Thus in a resistive
circuit it will be 3 dB down at that frequency.
It will actually change the circuit voltage due to
its loading, but only at frequencies where the
capacitance of the probe is important.? Of course
that could disrupt circuit operation but we don't
care about that do we.
So the probe still divides the input signal by the
same ratio as at DC, but because of its input
capacitance it forces the 50 Ohm circuit to lower
its amplitude.
Frankly I don't see this as a good way to
characterize a probe.? If the circuit is other
than 50 Ohms, the probe will load it different
amounts, less for lower impedances and more for
higher.? So probe bandwidth numbers are of little
use unless you are probing a 50 Ohm circuit.
Bob
--- On Tue, 2/12/13, Dave Daniel
wrote:
From: Dave Daniel
Subject: Re: [TekScopes] Tek Probe Question
To: TekScopes@...
Date: Tuesday, February 12, 2013, 5:32 PM
?
Dave,
Thanks.
That still doesn't answer my question.
Why the differing BW specs for different
'scopes? Is it the 'scope or the probe
or the combination?
Cheers,
Dave
On
2/12/2013 6:21 PM, David C. Partridge
wrote:
?
The probes you want for the 50 ohm
input amplifiers are P6056 (10x) and
P6057 (100x) or P6156(10x or 100x
versions).
Dave
-----Original Message-----
From: TekScopes@...
[mailto:TekScopes@...]
On Behalf Of davidnickdaniel
Sent: 13 February 2013 00:10
To: TekScopes@...
Subject: [TekScopes] Tek Probe
Question
All,
I'd like to buy some used Tek probes
for use with the higher bandwidth
7000 series plug-ins (I'm thinking
7A19, 7A29, 7B10, 7B92A). I am
looking at a Tek P6139A and decided
to download the spec sheet from
Tektronix to see the particulars of
this probe. It is a 500 MHz probe,
but the datasheet calls out:
500 MHz (TDS3054) or TDS500 series
100 MHz (TDS3012)
Now, I understand about passive
probe compensation and all that.
What I don't understand is what the
'scope-dependent bandwidth spec
means. will this type of probe be
useful close to 500 MHz when used
with the >= 600 MHz BW 700
vertical amplifiers (e.g., response
close to 500 MHz), or is there some
probe/'scope dependency of which I
am not familiar? I have several
P6106s which I am currently using.
Thanks.
Cheers,
Dave
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