commit 01f7af4ad712f5389ddb931e3723fb545a08584e
parent 8fc18d46873ce1d9269d9a0d42c61f8b64a15e56
Author: Martin Kloeckner <mjkloeckner@gmail.com>
Date: Sat, 19 Sep 2026 13:30:25 -0300
new blog post: 'owon-hds25s-bode-plot-using-python-and-scpi-commands'
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diff --git a/blog/owon-hds25s-bode-plot-using-python-and-scpi-commands/owon-hds25s-bode-plot-using-python-and-scpi-commands.md b/blog/owon-hds25s-bode-plot-using-python-and-scpi-commands/owon-hds25s-bode-plot-using-python-and-scpi-commands.md
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+%%
+title: "Owon HDS25S Bode Plot Using Python and SCPI Commands"
+date: "19-Sep-2026"
+%%
+
+# Owon HDS25S Bode Plot Using Python and SCPI Commands
+
+As you may know from my previous post, I bought the Owon HDS25S handheld
+oscilloscope, and I think one of its key features is the SCPI interface, which
+allows the device to be controlled over USB by sending text commands. In this
+post I will show how to get a Bode plot (for now, only the magnitude of the
+frequency response) using the Owon HDS25S and the built-in function generator,
+varying the frequency and measuring the output value of the device under test in
+a completely automated way by using a Python script that sends SCPI commands.
+
+## SCPI Commands
+
+SCPI stands for Standard Commands for Programmable Instruments and are used for
+controlling devices, mainly electronic labs equipment, like oscilloscopes,
+multimeters, bench power supplies, functions generators, etc. The control of the
+device is made from a prompt or query, typically over a serial port or USB, but
+modern devices also support SCPI over the network.
+
+SCPI commands can be send interactively, i.e. typing the commands in a serial
+monitor, or in a non interactive way, i.e. through a script which sends the
+commands line by line. This last method of sending the SCPI commands is where
+you get the true power of the commands, because you can automate test runs,
+achieving the same behaviour of the device on every run, or even the same
+behaviour on multiple devices. Unfortunately, only a small set of SCPI commands
+are standardized across different manufacturers. For example, `*IDN?` to query
+the identification of the device. Beyond these common commands and standardized
+SCPI subsystems, manufacturers often implement their own instrument-specific
+commands.
+
+## Owon HDS25S
+
+The Owon HDS25S supports SCPI commands over the USB port, and the HDS200(S)
+Series SCPI standard can be downloaded from Owon's website (also from
+[https://git.kloeckner.com.ar/hds25s/](https://git.kloeckner.com.ar/hds25s/)).
+To make the oscilloscope accept the SCPI commands the USB mode must be set to
+HID on the oscilloscope system settings, in this way when connecting it via USB,
+it reports as a serial device (on Linux based systems as `/dev/ttyUSB*`).
+
+
+
+The cool thing about this oscilloscope is that it accepts SCPI commands for all
+of it three modes: multimeter, function generator and oscilloscope, and also
+that the function generator can be used at the same time as the oscilloscope.
+This last thing is a key point for achieving our goal.
+
+## Bode Plot
+
+
+On a Bode plot, the magnitude and phase of the frequency response of a device is
+plotted as a function of frequency, with the frequency logarithmically spaced
+and the magnitude typically scaled in decibels. To achieve a bode plot, the
+traditional way is to connect a frequency generator on the input of the device
+under test (DUT for now on) and vary the frequency of the function generator,
+measuring and writing down the amplitude of the output of the DUT for every
+frequency.
+
+Given that the HDS25S supports SCPI commands for both the function generator and
+the oscilloscope, the connections for the automated and traditional methods are
+the same. The difference is that, instead of varying the frequency and measuring
+the output manually, SCPI commands are used. Consider the test bench shown
+below.
+
+
+
+To automate the process of varying the frequency and measuring the output, a
+script is used. To obtain the magnitude data, the script must iterate over a
+list of logarithmically spaced frequencies and for every frequency do the
+following:
+
+1. Set the function generator to the current frequency.
+2. Measure the peak value of the output of the DUT.
+3. Save the current frequency and the measured value on a list of results.
+
+For example, consider the following pseudo-code.
+
+```python
+frequencies = [1 10 100 1e3 10e3 100e3]
+magnitude = []
+
+for frequency in frequencies:
+ func_generator.set_freq(frequency)
+ measured_val = scope.measure_peak_val()
+ magnitude.append(measured_val)
+```
+
+## Getting the Data with PyVISA
+
+The script used is written in Python using the
+[PyVISA](https://pyvisa.readthedocs.io/en/latest/) library to send and receive
+SCPI commands to and from the instrument. The PyVISA library abstracts away the
+communication interface used by the instrument (RS-232, USB, Ethernet, etc.),
+making it easy to establish communication with the instrument.
+
+To use PyVISA, we need to know the device location beforehand. On Linux-based
+systems, the Owon HDS25S appears as `/dev/ttyUSB*` (when USB mode is set to HID,
+as mentioned). Knowing the device location, the following snippet sends the
+identification query. If everything is good the console should print the device
+response, for example "OWON,HDS25S,24531234,V8.8.2".
+
+```python
+import pyvisa
+
+DEV_PATH="/dev/ttyUSB0"
+
+rm = pyvisa.ResourceManager('@py')
+dev = rm.open_resource(f"ASRL{DEV_PATH}::INSTR")
+print("Identification: ", dev.query("*IDN?"), end="")
+```
+
+If the previous snippet worked it means that commands can be sent to and
+received from the instrument. To configure the oscilloscope, the folloing
+commands sets the channel 1 scale to 2 V per division, DC coupling and probe
+attenuation to X10.
+
+```python
+dev.write(":CH1:SCALe 2.00V")
+dev.write(":CH1:COUPling DC")
+dev.write(":CH1:PROBE 10X")
+```
+
+Similarly for the function generator, the following sets the output to a
+sinusoidal wave with 1 V peak value and 1 kHz of frequency.
+
+```python
+dev.write(":FUNCtion:AMPLitude 1.00")
+dev.write(":FUNCtion SINE")
+dev.write(":FUNCtion FREQuency 1000")
+```
+
+To vary the frequency, a list of logarithmically spaced frequencies is needed to
+iterate over. For this, the [NumPy](https://numpy.org/) library and its
+[logspace](https://numpy.org/doc/stable/reference/generated/numpy.logspace.html#numpy-logspace)
+function are used to create the `freqs` list. An empty list `amps` is also
+created to append the measured amplitudes.
+
+```python
+import numpy as np
+
+freqs = np.logspace(0, 5, num=20)
+amps = []
+```
+
+Given the list of frequencies `freqs`, the following snippet steps through every
+frequency, setting the function generator to that frequency. After a small
+time delay, the peak value of the channel to which the output of the DUT is
+connected is measured and appended to the list of measured amplitudes `amps`.
+The time delay is needed because the measurement function of the scope is not
+instant after setting a new frequency.
+
+```python
+for freq in freqs:
+ scope.write(f":FUNCtion:FREQuency {freq}")
+ tb_str = get_best_timebase(freq)
+ scope.write(f":HORizontal:SCALe {tb_str}")
+ time.sleep(0.5)
+ val = float(scope.query(":MEASurement:CH1:MAX?").strip())
+ amps.append(val)
+```
+
+The function `get_best_timebase(freq)` returns the best horizontal scale for the
+given frequency `freq`. Note that the peak value is measured using the channel 1
+voltage 'max' measurement, equivalent to the voltage 'max' measurement from the
+physical menu. This has the disadvantage that the waveform must fit within the
+visual area to obtain the actual value. If it does not fit, the measurement
+returns, for example, ">10.00", meaning that the peak value is greater than 10
+volts and cannot be measured. A fix for this is to write a function similar to
+`get_best_timebase`, but for the vertical channel scale, setting the best fit
+for the vertical view space.
+
+## Plotting the Data
+
+To get a figure of the Bode plot from the data we can use a python plotting
+library, for example [matplotlib](https://matplotlib.org/stable/). Another
+possibility it to export the data and use another tool. I like
+[Octave](https://octave.org/) which is similar to
+[MATLAB](https://la.mathworks.com/products/matlab.html). To export the data as
+CSV consider the following snippet.
+
+```python
+data = np.column_stack((freqs, amps))
+np.savetxt("data.csv",
+ data,
+ delimiter=",",
+ fmt=["%d", "%.2f"])
+```
+
+In Octave, plotting the data read from the CSV file is easy. In the following
+snippet, after loading the data, the measured values are normalized. This is
+achieved by dividing all the values by the function generator peak value (the
+peak value set in the function generator). Next, the values are converted to
+decibels, given that the values are voltage measurements, the conversion is done
+by taking the base 10 logarithm of every value times 20. Lastly, and optional,
+to smooth the curve a mean between contiguous values can be calculated using
+the `movmean` function, in the following snippet the mean is taken for every 5
+contiguous values.
+
+```octave
+data = csvread("data.csv");
+freq = data(:, 1);
+mag = data(:, 2);
+
+mag_db = 20 * log10(mag);
+mag_db_smooth = movmean(mag_db, 5);
+
+figure();
+semilogx(freq, mag_db_smooth);
+```
+
+In a similar manner as above but tweaking the appearance of the figure, the
+following bode plot is obtained. The orange trace represents the
+frequency response obtained with this method. The red trace, "H", is the
+frequency response of the theoretical transfer function of the DUT. The blue
+trace, "H normalizada", is the frequency response of the theoretical transfer
+function of the DUT but with normalized commercial component values.
+
+
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