Keysight DSOX1102G FFT Bode Plot Features Audio Analog Signals

[Chappy425]

My first post.  First off thanks to Dave for all of the great EEVBlog videos especially the teardown videos. 

This post is mainly for the benefit of users interested in audio frequency analog signals.  I would like to share some of my findings with respect to this scope as I was unable to find much on this topic after searching this forum and Youtube videos.  I wound up going with the 1102G 70Mhz model over the entry level Rigol and Siglent models because of it's built in WaveGen option and Bode plot analysis and based on Dave's comments on the poor FFT performance of the cheaper Rigol models.  I took a chance and payed a whole lot more for this model over the Rigol betting that the FFT would be acceptable for my needs.  As it turns out "Bob's your uncle", it is. 

The attached photos depict a guitar overdrive/distortion effects pedal hooked up with a 1Khz signal.  The pedal is set on a low level of overdrive to give assymetrical clipping, followed by a slightly higher setting of overdrive to start the onset of symmetrical clipping.  The FFT analysis clearly shows the harmonics at both even and odd multiples of the fundamental freq.  The bode plot shows both the amplitude and phase response with a sweep over the entire audio range.  With carefull manipulation of the overdrive control I was able to clearly see how the odd order harmonics started to dominate the even order ones as the signal clipping became more symmetrical.   
As well with manipultion of the two cursors, the absolute and relative magnitudes of the frequency components can be read off of the FFT plot.
For anyone interested in analog audio signal processing this scope is a very powerful tool.

Cheers
Steve

[number6photo]

Thanks for posting that, I plan to do similar analysis with my pedals and amps. 

Shifting gear a little bit:  not sure if you have a tube amp, but if so, have you found the 150V max input voltage for the oscilloscope's channels problematic while poking around inside your amp?

[Someone]

have you found the 150V max input voltage for the oscilloscope's channels problematic while poking around inside your amp?

This is only a limitation for direct 1:1 probing, when using the included 10x probes the safety rating is 300 Vrms CAT II (plus derating etc) the same as other brands included probes. Beyond that its getting into specialised probing.

[Chappy425]

I do have several tube amps.  I have also constructed several of my own.  I have a vintage Telequipment vacuum tube oscilloscope which I use for poking around in old gear.  I also have a few old Tek 465 scopes which I believe are rated to 400V on the input. I haven't used those yet.

I am not going to use my Keysight scope, and especially not with the probes that came with it.  One slip of the X10 switch and say goodbye to the input circuitry in the scope.  The B+ rails on tube amps can easily exceed 400VDC. 

One day I plan on making an external voltage divider attenuator for using my newer scopes on old tube gear. I was thinking of a 10X attenuator and two good quality high voltage blocking capacitors in series to block DC from getting to the scope.  I will still use my good quality Tek probes, or I might build my own probe using lead wire with a higher voltage rating (for my own personal safety).  This way I can probe around inside an amp including high level B+ supplies..  For actually measuring the DC levels in the amp I will just stick with the VTVMs that I have.

I hope this helps.

Sincerely
Steve

[tautech]

Sounds like you have never heard of 100x probes.....they are not expensive.