Cheapest way to get bode plots - Page 1
Forum section: electronics / testgear

author: bsas
author_rank: Regular Contributor
timestamp: March 04, 2018, 12:59:31 am
content: So, if you already have an arbitrary function generator that can sweep and the only thing you want is a bode plot (in my case, audio range, like 5Hz to 20kHz), what would you get?I don’t want to rely on computers and I an OK with used gear.Thanks!!!

author: tautech
author_rank: Super Contributor
timestamp: March 04, 2018, 01:54:27 am
content: Watch Dave's vid.

author: texaspyro
author_rank: Super Contributor
timestamp: March 04, 2018, 02:33:46 am
content: FYI, Bode plot is pronounced boh-dee, not bowed. I once met a relative of Hendrik Wade Bode and he was quick to point out how to pronounce the name.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 04, 2018, 03:11:18 am
content: If you can set the sweep rate low enough, all you need is a DMM and a pencil and paper.It will get you good information in the pass band.Up through the 60s or early 70s a ton of Bode plots were done with a simple signal generator and meter. When more SNR was needed something like an HP 302 wave analyzer would allow tracking the Bode plot down into the weeds.

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: March 04, 2018, 09:19:04 am
content: Hello at USD 139 the PicoScope 2204A (without probe) includes also the AWG I think is the best deal. You will need a power divider and few cables. Maybe for low frequency BNC coaxial cable are also OK. You also need 50Ohm terminations for the oscilloscope.THE SW DOESN'T WORK WITH MSO OSCILLOSCOPE VERSIONYou need the FRA4 PicoScopehttps://bitbucket.org/hexamer/fra4picoscope/wiki/HomeI recently made a quick video, see here:https://youtu.be/fQxuWX8bnBI

author: vk6zgo
author_rank: Super Contributor
timestamp: March 04, 2018, 10:00:17 am
content: Quote from: CatalinaWOW on March 04, 2018, 03:11:18 amDuring the 60s or early 70s that would be called a frequency response test.The term "Bode" plot (at least in Australia), was normally reserved for an idealised version used in circuit analysis texts, where the Amplitude & Phase response was shown as abruptly changing at "break points" rather than gradually as in a practical test.

author: David Hess
author_rank: Super Contributor
timestamp: March 04, 2018, 01:20:47 pm
content: If you just want the magnitude, then a frequency swept source and oscilloscope is the easiest "fast" way. Otherwise an AC voltmeter, signal source, and pencil and paper like CatalinaWOW suggests works.I almost always want phase as well so I use one of my manual function generators and my DSO with automatic phase and amplitude measurement to make the measurements while graphing the points of interest with pencil and paper. If you have a DSO which supports differentiation and FFTs which display both phase and magnitude, then you can do this in real time with a step function input and none of that time wasting swept sine nonsense.

author: rhb
author_rank: Super Contributor
timestamp: March 04, 2018, 09:18:32 pm
content: I realize the OP doesn't want to use a computer, but this is a case where a 24 bit 96 KS/S sound card and Octave/MATLAB really shine. Measuring THD to absurdly low levels is quite possible if you take the time to calibrate the sound card. Amplitude and phase are trivial and .m files to do the work so common you don't even need to know how.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 05, 2018, 03:03:50 am
content: If you want Bode phase response as well as amplitude it is simple to throw together a synchronous filter. If the information is tapped before the remodulation stage it simply becomes a matter of recording two DC voltages at each frequency point. Amplitude is then the square root of the sum of the squares. Phase is arctan of the ratio of the two voltages.

author: rstofer
author_rank: Super Contributor
timestamp: March 05, 2018, 04:45:27 am
content: This is NOT the cheap solution but it is a GOOD solution:Digilent Analog Discovery 2 does magnificent Bode' Plots:https://store.digilentinc.com/analog-discovery-2-100msps-usb-oscilloscope-logic-analyzer-and-variable-power-supply/You can download the software (free) and play with the "Demo" device. The Network Analyzer gadget is the one you want.What I don't understand about using an oscilloscope is how to correlate frequency with amplitude. It's not like there is a frequency scale. At best, it is a guess based on how much time has passed since the scope and signal generator were triggered and somehow computing back to frequency based on sweep rate of the signal generator. I'm not sure how to get phase response. So, you won't see anything related to phase margin. That's kind of a big deal...I guess it could be done with an Arduino and something like MATLAB but the projects I found with a quick search did not include phase response.

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2018, 06:46:34 am
content: Quote from: rstofer on March 05, 2018, 04:45:27 amBut there is, when done correctly. Watch Dave's vid again, especially from ~4.30.The AWG sweep is performed between pair of frequencies and at a defined repetition rate.That repetition rate should fill the horizontal axis of a scope and is adjusted to do so with the timebase.Therefore when the sweep is linear the amplitude can be correlated to any frequency over the duration of the sweep and can be directly read from the scope's graticules or even with cursors.

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2018, 11:24:31 am
content: Quote from: rstofer on March 05, 2018, 04:45:27 amOddly enough some analog oscilloscopes can do this. Some have a universal reciprocal timer/counter which is gated off of the B sweep so you just set the B sweep to like 1/100th of the A sweep and use the intensified zone as a marker where the readings are taken. If the analog oscilloscope lacks the timer/counter, then its b-gate output can be used to arm and gate an external reciprocal timer/counter which reports frequency and phase.

author: Ringmodulator
author_rank: Regular Contributor
timestamp: March 06, 2018, 08:30:39 am
content: Hi,back in the days, I have used a homemade function generator based on the XR2206 IC.I took the Y outut (sawtooth) voltage from my analog scope to modulate the fm input of the xr2206. So the sweep is in sync with the trace.RegardsChris