Bode Plot

[Chris Tatler]

Hi all.
I'm new to this forum and I was wondering if someone could help me with this bode plot (image attached).
I'm just mucking around with filters on the bread board using op amps etc. I have a reasonably good understanding of the phase difference between the input to the filter versus the output at varying frequencies and what to expect, but I am fairly new to doing this kind of analysis and using the test equipment so it's quite possible that I may have set it up wrong. I can't understand why the phase trace on the scope jumps from -180° to +180° when the signal is around -6db. I can't imagine it doing it for real, and I thought it might be something to do with the way the scope interprets the information. If I set up the same thing in LT Spice, it doesn't do it.
Attached is a snippet of the filter (part of a bigger circuit for a cross over network) and the screen of the scope bode plot.
Thanks   

[Jay_Diddy_B]

Chris,

There are a couple of things happening here:

1) Bode plots are normally log amplitude versus log frequency. Your x-axis is linear.

2) When you measure phase you are limited to +/- 180 degrees. For example -185 degrees is the same as + 175. Some equipment may offer 'extended' phase which shows it as a continuous.

Jay_Diddy_B

[Chris Tatler]

Hi,
Thanks for your help and quick reply. Yes that does make sense now, its wrapping around it would seem. I was comparing what I was seeing on my scope with what I was seeing in LT Spice, and LT Spice doesn't seem to have the limitation of 180° so was confusing me.
Also - I changed my scope to log and it looks a lot better (it defaults to linear so that's why I had it set like that).
Good to know.
Cheers