I have always wished to have something to accurately measure relative amplitude and phase that did not cost a king's ransom to take Bode plots from an actual real working circuit.
I bought myself an ancient HP3575A phase meter a long time ago. These are now getting rather old, but still excellent, and I picked one up from e-bay for a couple of hundred dollars which is typical. The specifications are pretty good 3Hz to 11Mhz, phase to 0.1 degrees resolution, absolute and relative amplitude measurement to 0.1db resolution.
And a full service and repair manual is available free from the internet.
If you are only interested in analog circuitry, amplifiers, filters and such, its probably all you need, along with a suitable function generator of course.
Sadly it will not work with switching power supplies, it needs reasonably clean signals. Trying to resolve millivolt signals below volts of switching spikes and a very noisy ground is just not practical with either an oscilloscope or something like the HP3575A.
So at the moment I am building a very narrow dual channel tracking filter for it. Basically two channels tunable from a very few Hz to 40Khz with only about 3Hz bandwidth. That should block the wideband switching noise and allow only the specific frequency of interest to reach the HP3575A.
Still working on this right now and so far its looking quite encouraging.
Just discovered this thread while surfing the internet looking to see what other people are doing to take Bode plots on real functioning equipment, and not just playing with pie in the sky circuit simulations.
Simulations are wonderful things, a world where noise problems do not exist, there is infinite dynamic range, and components are perfect.
The real world is rather more hostile...