# o'scope b/w question

Discussion in 'Hobby Electronics' started by S Roby, Nov 12, 2004.

1. ### S RobyGuest

Is this correct

"For a scope to have a BW of 20 MHZ, it needs at least a 0.05 micro sec /Cm
(0.1 mico sec /Cm for 10 Mhz one). Your scope gets nowhere near that
figure."

I sold a BWD 15M scope & the guy is wanting money back saying its only 2M

2. ### S RobyGuest

Is my math correct (its been a while)

20MHz across the width of the screen =
= 10devisions x 2uS = 20uS
now convert the measurement into freq
1/20uS =20Mhz

3. ### Phil AllisonGuest

"S Roby"

** Scope bandwidth does not depend on time base speed.

.................. Phil

4. ### BushyGuest

Rather than how big the screen or the divisions across it are, I thing you
might find that the manufacturer was quoting the bandwidth of the vertical
amplifier.

If the amplifier (and probes/leads) will not carry the signal to the
vertical deflection plates in the tube, it will be the limiting factor in
what the cro tube will display.

The way to test the cro's bandwidth is to feed it with a signal from an
oscillator (or any other convenient frequency source) at the frequencies
concerned and see if it will display it. As the frequency response drops off
the vertical deflection will drop off. The bandwidth is probably defined as
the 3 decibel point in vertical response. There is generally a fairly fast
drop-off in response above this at which the frequency will not display at
all, rather than having it fill up one division, or the full screen. Some of
the cro's I remember from years ago had a response graph in their manuals so
you could make meaningful readings at a range of frequencies.

It appears that your purchaser is looking for something that is not in line
with the original manufacturer's specifications. However, from your post it
appears you are also confused or under the same impression. As there is
confusion, then maybe you can come to an agreement that suits both of you,
did you charge a substantial price for this old BWD cro?

Probes are sold with a bandwidth as well, and need frequency compensation
adjusted to suit the temperature and humidity, their age and the amount of
capacitance that is coming from you hand when you hold them, and the way you
hold your mouth. Did you include probes or leads and are they included in
this figure, or is it the cro itself that is the "bone of contention"?

Hope this helps,
Peter

5. ### Phil AllisonGuest

"Bushy"

** CRO probe bandwidth is a great mystery to many - what the hell
limits the bandwidth of a meter or so of co-ax to 30 or 40 MHz ??? The

Because the load presented to the probe cable by the CRO is typically 1
Mohm shunted by a few pF the co-ax line is effectively unterminated and so
will have a deep impedance null when its length becomes a quarter wavelength
of the input frequency. For a 1.5 metre length of RG 58 or similar this
happens at 33 MHz, ie 300 x 0.66 (velocity factor) divided by 1.5 x 4.

The standing wave problem can be eliminated by use of a 50 ohm BNC
terminator - but then the load is 50 ohms at all frequencies. When testing
a CRO's bandwidth with an RF generator the use of such a terminator is
essential.

BTW Frequency compensation trimming is used only in divider probes - ie
10:1 or 100:1 probes - to match the ratio of the capacitance of the
probe's cable to the stray capacitance of the resistors/s in the probe's

.............. Phil

6. ### RichardGuest

Lots of info on Oscilloscopes and probes here:

Richard.

7. ### Clifford HeathGuest

No. 1/20us = 50KHz.

8. ### Franc ZabkarGuest

I have a 10MHz Leader LBO-514 dual trace scope. The minimum time base
setting is 0.5 usec/cm. Selecting the x5 magnification gives a min of
0.1 us/cm which agrees with the 10MHz label. Of course this says
nothing about the frequency response of the vertical amp(s), but would
it make sense to have a vertical amp that is linear to 100MHz, say, if
your time base is only good for 10MHz?

- Franc Zabkar

9. ### Franc ZabkarGuest

I think the buyer's idea is to fit one full cycle of the maximum
frequency in one cm of screen. If the minimum timebase setting is
2us/cm, then the max usable frequency is 500kHz. If your scope has x10
horizontal magnification, then the max frequency would be 5MHz. This
is what the buyer probably considers to be the "bandwidth", and that's
how I would interpret oscilloscope frequency specs. Having said that,
a more correct description of the 5MHz spec would probably be the
"maximum usable displayable frequency". The term "bandwidth" is
probably more correctly applied to the frequency response of the
vertical amp(s). A vertical frequency response of 15MHz would make
5MHz pulses look reasonably square.

I guess you and the buyer both have valid interpretations, but if it
were me, I'd expect a refund. Having said that, if we were to use the
buyer's interpretation, then his expected minimum timebase setting
would have been 0.067 us/cm, which is clearly impractical. This should
have rung warning bells for him.

- Franc Zabkar

10. ### Phil AllisonGuest

"Franc Zabkar"

** From S Roby's posts I get the idea that his old BWD has a sweep that goes
to 0.5 uS/cm - hence the erroneous claim that it has only 2MHz bandwidth.

There is nothing essential about seeing one cycle per division at
frequencies like 10 or 20 MHz since a general purpose service CRO like the
539B ( made in the early 1970s ) would not need to measure such frequencies
with any accuracy. As long as the instrument could cover the video signal
bandwidth of a TV set comfortably and had good triggering it was a viable
unit.

........... Phil

11. ### budgieGuest

Analog CRO manufacturers almost invariably quote instrument bandwidth based on
the -3dB point on the vertical deflection system. Nothing to do with timebase,
nor should it be. If your buyer doesn't understand the product or how it is
specified then that is HIS problem.

12. ### RobGuest

I was taught the same thing at school - BW is reached when the vertical
deflection drops by 3dB.

r.

13. ### David L. JonesGuest

The bandwidth of a CRO is the -3dB "bandwidth" of the vertical
amplifiers, always has been, and is universally accepted as such. It
has nothing to do with what timebase ranges are available.
If the guy didn't know that then that's his problem. If he needed a
certain timebase range then he should have checked it had it before