RoadTest: Analog System Lab Kit (STEM Product)
Author: gpolder
Creation date:
Evaluation Type: Development Boards & Tools
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: My old 10 in 1 electronics kit and all other educational kits around.
What were the biggest problems encountered?: Although the manual looks very nice, the theoretical background behind the experiments given is somewhat sparse. Also broken links in Manual references section.
Detailed Review:
This analog system lab kit (ASLK PRO) reminds me of my first encounter with electronics on age of 10.
In the application I mentioned that as a ham radio operator I'm quite interested in HF and ham related projects.
From the experiments described in the manual, I promised to focus on:
So lets give it a try, but first the traditional unboxing pictures:
| {gallery} Unboxing |
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The kit comes in a nice protective box in which:
I really like the overall layout of the board. It's very complete, six operational amplifiers (opamps) are available, two DA converters, three multipliers, and some space for additional components, including a small breadboard. The board is well equipped for all kind of experiments with analog electronics. There is also a LDO regulator and DC/DC converter on the board. As these circuits are for specific experiments, and mainly not will be used in combination with the other components, I don't see the added value of those.
What I also like on this board is that the schematics are clearly drawn on the PCB, both on the front and on the back. Each connection is equipped with several pin headers, which makes it very easy to wire your design using the attached jumper wires.
Quite a number of standard resistors and capacitors are already connected to part of the opamps + and - inputs, which makes it very convenient to connect input and feedback R's and C's.
According to Element14 a RoadTest should be for the hottest NEW Products:
So I was a bit surprised to find a quality check label of more than 18 months ago (March 2016).
Even more surprising was to learn that the manuals are published more than 4 years ago, in May 2013:
| {gallery} May 2013 |
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The board itself is rock solid, the PCB is 2.5mm thick. Small rubber feet are attached to the bottom:
Although the manual states that all components are from Texas Instruments, the inscription on the analog multipliers says they are from Burr Brown. I learned from michaelkellett that Burr Brown was bought up by TI a good while ago and is really just a TI trading name now.
The manual is looking very nice, it has a very extensive description of the board and its components. Some pictures are in full color, the text is in black and red, and each experiment (chapter) first explains the goal, then some brief theory and motivation, followed by an exercise and assignments. Each chapter also has some room for the students to make notes.
I have to admit that sometimes the theoretical background is somewhat sparse and unclear. It also is not always clear how the circuit in reality needs to be built. The symbol for the multiplier for instance is not uniform over all figures, and the use of the x1,x2,y1,y2 and z1,z2 is not properly described. I will give some examples later in this review.
Each chapter has a lot of references to related articles for further reading, as well as training course video's by Prof. K. Radhakrishna Rao, Department of Electrical Engineering, IIT Madras, one of the two authors of the manual.
Unfortunately some video references have a broken link, and also the link to NPTEL (http://nptel.iitm.ac.in) mentioned on the youtube channel of Prof. Rao didn't work. I suppose this is due to the fact the manual is already four years old, and the video lectures of Prof. Rao are even much older, as they were recorded between 1990 and 2003, and published online in 2008. Just to have an idea how this looks like, please find lecture 7 on 'Operational Amplifier In Negative Feedback' below:
When working on one of the experiments I detected some interference on the signals, particularly on low voltage signals.
Below is an example that showed up when experimenting the AGC. Spikes can clearly be seen on the output of one of the multipliers in blue:
After some investigations I detected that the source of this disturbance was the onboard DC/DC converter, which is of a switching type.
After disabling it by placing the jumper to the external input, luckilly the disturbance was gone.
DC/DC converter on.
DC/DC converter off.
Clean signals.
Power is needed in order to do the experiments. This is not completely trivial as the board needs + and - 10V and I'm not in the in possession of a dual power supply. Personally I would be happy to give up the LDO and DC/DC test circuits in favour of a +/- 10V power supply on the board itself, driven by a single 12V or 5V (USB) Power supply. Given the nature of the circuits mentioned that should be not a big deal.
Luckily I'm in the possession of two power supplies instead. A very old home-brew one with an adjustable output from 5-30V and a surplus professional power supply from Amrel. I checked whether the Common eventually was grounded, but this was not the case for both. So by connecting the + from the first to the - of the second I could provide the board with a neat +/- 10V power.
| {gallery} Power to the board |
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Before starting the proposed projects I quickly tried a unity gain circuit and an inverting amplifier, as described in the manual on page 18 and 19 (fig 1.2 and fig 1.5) .
On the pictures below you can see how this looks like in practice:
This is the unity gain circuit, with the scope I'm measuring input voltage on yellow and output on blue:
Next one is the inverting amplifier, input resistor is 1k, feedback resistor is 2k2.
The resulting input and output can be seen here:
The output indeed is negative, but the value is smaller than the theoretic value of 2.2 * 4.97 = 10.934V.
This caused by input resistor losses, and also the value is limited by the supply voltage of 10V.
The first project I proposed is an active filter. Chapter 4 in the manual describes some experiments. The goal of the chapter is:
To understand the working of four types of second order lters, namely, Low Pass, High Pass, Band Pass, and Band Stop lters, and study their frequency characteristics (phase and magnitude).
In stead of explaining the different filter configurations, this chapter gives just one circuit example that can be used for all four filter types in figure 4.1. Table 4.1 shows the transfer functions of the four filter types.
In stead of building this universal filter circuit I decided to experiment with a more simple, but effective design. A nice explanation and example circuits can be found at http://www.electronics-tutorials.ws/category/filter. As an example I build an Infinite Gain Multiple Feedback Active Bandpass Filter:
This active band pass filter circuit uses the full gain of the operational amplifier, with multiple negative feedback applied via resistor, R2 and capacitor C2. The characteristics of the IGMF filter are as follows:
Using the values:
Results in:
Below are the input (yellow) and output (blue) signals at 1, 3.5, 5, 6.5 and 10kHz.
| {gallery} Filter input/output signal |
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1 kHz |
3.5 kHz (lower -3dB point) |
5 kHz (Fr) |
6.5 kHz (upper -3 dB point) |
10 kHz |
The screenshots clearly show that the signals are accordant to the calculated values. At Fr, the signal is 5 x 2 = 10V. The bandwidth is 3kHz, so the upper and lower frequency are 3.5 and 6.5 kHz. The output signal at these points is -3dB, is 0.7 x 5 x 2 = 7V.
The pictures also clearly show the phase shift. At resonance (5 kHz) it is -180 degrees.
Below is a video of a 10s sweep from 10Hz to 10 kHz. You can see the input and output value at different time scales, as well as an XY plot of the output versus the input. The XY plot clearly shows the -180 degree phase shift at resonance.
The next experiment I did is the Voltage Controlled Oscillator (VCO) from chapter 6. I started this experiment by building an oscillator using a Schmitt trigger and integrator in a feedback loop. The circuit can be found in figure 6.1 or 6.3 from which the multiplier is omitted.
The connections on the board can be seen in the image below:
The values of the components are:
The output frequency can be calculated by: f = (1/4RC).(R2/R1), so f = (1/4.1000.0.1e-6).(2.2/1) = 5.5 kHz.
The scope shows the following signals, in yellow the output of the Schmitt trigger (square wave), in blue the output of the integrator (triangle wave). As one can see, the frequency comes quite close to what has been calculated.
Next step is to build the VCO by adding the multiplier between the output of the Schmitt trigger and R1.
The complete connections of the MPY634 are:
Here a picture how this looks like:
And here a video of the output of the VCO when changing the input voltage:
Now we have a VCO at our disposal, it can easily be converted to a Phase Lock Loop. This experiment is described in chapter 7 of the manual.
The goal of this experiment is to make you aware of the functionality of the Phase Lock Loop commonly referred to as PLL which is primarily used for a frequency synthesizer in high frequency stable clock generators. From a crystal of some kHz range, it is possible to generate waveform of GHz frequency range using a PLL. In this experiment the VCO frequency is locked to an external input signal.
As said earlier the manual is somewhat brief and it is not completely clear how to build the complete circuit.
Luckily on the internet I found a well describe course note from the California Institute of Technology. Looks like this course note is written for this same or a very similar analog design kit (https://labcit.ligo.caltech.edu/~vsanni/ph5/pdf/Ph5.ExperimentProcedure.PhaseLockedLoop.pdf ).
To get an impression on the circuit diagram differences, here is fig 7.2 from the STEM Kit manual:
And here are the circuit drawings from the CalTech course notes:
With the help of this document it was very easy to build the circuit. The result is shown in the picture below:
Another multiplier is used to mix the 4 kHz square wave input signal (yellow alligator clip) with the VCO output signal. The output of the multiplier is filtered using a 1k resistor and a 100nF capacitor and fed into the VCO.
In the video below first the VCO is free running, the frequency changes when the trimmer is adjusted.
Then the output of the PLL multiplier is connected to the X2 input of the VCO. When the trimmer is adjusted in that case, the frequency remains the same, and is locked to the input signal. The output of the VCO is the yellow signal on the scope, the output of the PLL is the blue signal. One can see that the PLL output changes when the trimmer is adjusted.
Finally in my application I promised to do the Automatic Gain Control (AGC) experiment, which is described in chapter eight.
Initially I ran into some problems to get it working, but gecoz helped me out, explaining the circuit.
Several methods are available to implement an AGC. The manual gives two references. First one is to the video lecture of K.R.K. Rao. Electronics for Analog Signal Processing - Part II. AGC/AVC. In this lecture Prof. Rao explains the usage of a rectifier/envelope detector for measuring the amplitude of the input signal:
Another reference in te chapter is an application note from Texas Instruments titled: 'Automatic Level Controller for Speech Signals Using PID Controllers'. Available from http://focus.ti.com/lit/wp/spraaj4/spraaj4.pdf
This application note describes an AGC using a software based PID controller.
The experiment in this chapter although uses another approach, as can be seen in the circuit diagrams of figure 8.1 and 8.3.
Unfortunately the manual doesn't explain the operation of this circuit, it just states:
I would be very happy if someone can explain me this circuit. I don't get the function of U2.
Given figure 8.1 and 8.3 the output of the second multiplier (U2) is the square of the input. For a sine wave input this would be a sine wave with a doubled frequency plus a DC offset, according to the following equation:
When testing this circuit, this is exactly what is shown on my oscilloscope, blue is the input, yellow the output of U2.
Now, for the AGC function, we are interested in the DC amplitude of the output signal only, which can be obtained from the output of U2 by filtering out the component depending on the cos(2a)t, leaving only the DC component proportional to sin(at)*sin(at). This filtering is done by the low pass filter created with the OpAmp. (thanks gecoz)
For the time being I leave it at this, please let me know whether you have suggestions to get the AGC working.
Top Comments
Hi Gerrit,
I'll try to explain the use of U2 in the AGC circuit.
To understand U2, you need to consider it in conjunction with the OpAmp that follows.
As you correctly stated, the action of U2 is to…
Hi Gerrit,
I'm afraid the circuit in fig 8.2 would not work without U2.
You need U2 to transform your signal from a pure sine wave at frequency f to a DC + cosine wave at frequency 2f. The DC component…
Nice road test report.
I spent a lot of time in labs looking at these types of circuits.
DAB