Using 50 ohm cable or adding an attenuator like Tom suggests would
give better results but I do not think that is the problem in this
case.
Having a -3dB point at 20 MHz is awfully suspicious. Is the 20 MHz
bandwidth limit of the oscilloscope activated? It is controlled by a
front panel button just to the left of the ADD ALT CHOP switch labeled
BW LIMIT. When tested, all of my 22xx series oscilloscopes come very
close to the 100 MHz bandwidth specification and also very close to
the 20 MHz bandwidth specification when the bandwidth limit switch is
pressed.
Note that you can also test bandwidth if you have a pulse generator
with a fast transition time. The rise and fall time of a 100 MHz
oscilloscope should be 3.5 nanoseconds. When the 20 MHz bandwidth
limit is selected, the rise and fall time should be 17.5 nanoseconds.
Rise or Fall Time in nanoseconds = 350 / Frequency in MHz
On Fri, 25 Jan 2013 10:41:33 -0500, Steve <steve65@...>
wrote:
Hi all,
I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least
100MHz" when using the 5mV/division to 5V/division vertical scales.
For no particular reason, I decided to verify that specification. I
connected an HP8657B signal source to the CH1 input with a 36in RG59
coaxial test lead and used a BNC-tee to terminate the scope input with
50 ohms. Then I made some measurements using 0dBm at 1MHz as the
reference. Using this method, the -3dB point is about 20MHz on both CH1
and CH2. It's down about 24dB at 100MHz.
I was expecting the -3dB point to be at least 100MHz. Is my methodology
flawed? Do I have a bad scope?
Thanks.
Steve
David Hess