DS1052E Displays to 150 mHz

[obbm]

I have a Rigol DS1052E oscilloscope, and a Munston Military signal generator TS497B/URR.  The S/G goes to 400 mHz.  The DS1052E is advertised to be a 50 mHz scope.  I haven't dared to do the 100 mHz hack on the scope, it is strictly stock.

Last night I probed the output of the S/G with the Rigol scope.  The scope has a freq readout.  The scope displayed a clean sine wave to 150  mHz, with the freq readout very closely matching the freq setting on the S/G. 

Can anyone explain the sine wave being displayed to 150mHz and the freq readout matching? I will appreciate any discussion on this subject.

Thank you for your attention and reply,

obbm

[PedroDaGr8]

I have a Rigol DS1052E oscilloscope, and a Munston Military signal generator TS497B/URR.  The S/G goes to 400 mHz.  The DS1052E is advertised to be a 50 mHz scope.  I haven't dared to do the 100 mHz hack on the scope, it is strictly stock.

Last night I probed the output of the S/G with the Rigol scope.  The scope has a freq readout.  The scope displayed a clean sine wave to 150  mHz, with the freq readout very closely matching the freq setting on the S/G. 

Can anyone explain the sine wave being displayed to 150mHz and the freq readout matching? I will appreciate any discussion on this subject.

Thank you for your attention and reply,

obbm

Oscilloscopes bandwidths are quoted at the -3db point. Meaning, the frequency at which the signal is attenuated by 3db.You will see frequencies well beyond that point. The highest frequency is often determined by the roll off of the low pass filter. The highest theoretical frequency you can see is determined by the Nyquist limit. Most scopes try to roll off long before that to prevent aliased signals.

Sent from my LGLS990 using Tapatalk

[rsjsouza]

As a consequence of what PedroDaGr8 mentioned, the bandwidth will only show as an amplitude reduction on perfect sinusoidal waveforms, but it will severely distort non-sinusoidal waveforms (square, triangle, sawtooth) due to the higher frequency components.

[obbm]

PedroDaGr8, thanks very much for your reply.  I am an electronics tinkerer for 65 years with no training, and am not a ham, I am an SWL.  Your reply is beyond my expertise to understand.  I realize what you have posted is certainly correct, but I don't fully understand it.

While I was typing this, rsjsouza posted a reply, and thank you.  I think what he said is that because the S/G produces a sine wave, the scope can display higher frequencies than 50 mHz.

Thanks for helping,

obbm

[nitro2k01]

MHz = Megahertz.
mHz = Millihertz.

Not the same thing, and still such a common typo!

[Alex Eisenhut]

MHz = Megahertz.
mHz = Millihertz.

Not the same thing, and still such a common typo!

Thanks! I was wondering if someone was doing seismology or something.

[Creep]

Since the author still seems to be at a loss, I'll try to explain this in a nutshell.
Every scope's front end has a filter that attenuates any signal with a freqency higher than the scope's bandwidth (the scope bandwidth is determined by the freqency at which the front end attenuates the signal to 70.7% (the 3dB level) of the accual amplitude). No filter can leave a 50MHz signal untouched and compleatly block a signal with a freqency say 50.1MHz. An ilustration how a filter attenuates a signal can be seen here:
http://upload.wikimedia.org/wikipedia/commons/6/66/Butterworth_filter_bode_plot.png. How steep the roll-off of the filter is is determined by the scope.
What all this means is that you will still see your signal, just with a lot lower amplitude.
Of course, not all signals are pure sine waves, however a little mathematical invention called the Fourier series suggest that every signal consists of a lot of (infinitely many) sine waves with different freqencys and amplitudes which people usually call components of the signal. Since each component has a different freqency, the filter is going to attenuate each component differently and the result will be a distored waveform (for example if we run a triangle wave through a low-pass filter, we might end up with a sine wave).
Tried to keep it as simple as  possible, hope it helped.

[obbm]

Creep, your nutshell explanation is more suited to my ability.  I did notice that the amplitude went down as the freq went higher.  Thanks for your help in understanding this.

Nitro, you cleared up a matter that I didn't know existed.  I will attempt to use the capital "M" in the future, as I probably will never use millihertz.  I still think of cycles when it involves freq. Thanks,

obbm