To gather more knowledge about how a typical oscilloscope performs analog-signal processing, the results of the state-of-the-art research (see xy) were further analyzed.
For this purpose, only projects with access to the schematic of the hardware provided enough information.
The three most interesting projects were selected for deeper investigation.
The XMEGA Xminilab developed by Gabotronics [9], for example, designed a front-end which is capable of processing analog signals at a voltage range of -14V to +20V.
This signal processing circuit only consist of 3 parts, or stages, as can be seen in figure 9.
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/sig_proc_osci_hardware/sig_proc_osci_hardware_xminilab.PNG[caption="Figure 1: ",title="Section of the analog front-end of the XMEGA Xminilab by Gabotronics (9)",align="center"]
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The first stage (R2 & R3) is a simple voltage divider and is used to attenuate the input signal by a factor of 5.5. Besides that, this stage is also used as the input impedance of the oscilloscope.
Stage two is a non-inverting amplifier which is used to amplify the signal by a factor of 1.1 or 0.92dB. The last stage adds an offset to the measured signal.
This prevents any negative voltages reaching the input of the ADC. As a second function, this stage defines the input capacitance of the XMEGA Xminilab oscilloscope [9].
This very basic oscilloscope front-end is only capable of processing signals at a maximum bandwidth of 200 kHz which is way below the requirements of the OpenLab oscilloscope as described in chapter 3.2.
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A more advanced and better documented project is the OsciPrime oscilloscope, designed by students of the University of Applied Sciences Northwestern Switzerland [18].
The front-end of this measurement device is capable of processing analog signals with a much higher frequency of up to 8 MHz.
The specifications of this front-end covers more of the requirements of the OpenLab oscilloscope.
So the final design is mainly built on the analyses of the OsciPrime hardware.
The following section of the OsciPrime hardware schematic, seen in figure 10 shows the signal processing path of the front-end.
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/sig_proc_osci_hardware/sig_proc_osci_hardware_osciprime.PNG[caption="Figure 2: ",title="Section of the analog front-end of the OsciPrime oscilloscope (23)",align="center"]
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The more complex hardware consists of a total of 4 analog signal processing stages. The first stage (C1, C2, C3, C73 & R1, R2) will attenuate the signal by a factor of 2.
Additionally, it acts as the input impedance of the oscilloscope as well as the input capacitance.
The next stage consist of a non-inverting amplifier (op-amp A of OP1) and is used, in combination with an analog switch, as an amplifier with variable amplification.
This variable amplification is needed to ensure, that the ADC is able to digitize an optimal signal. This signal should fit the complete input voltage range of the ADC.
The reason for this procedure is to obtain a high resolution of the measured signal [23].
Due to the fact that the OsciPrime oscilloscope uses non-differential ADCs, the voltage of the input signal of those ADCs should not fall below 0V.
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Stage three (op-amp B of OP1) inverts the measured signal and adds an offset. The offset ensures that only positive signals will reach the ADC.
The last stage (R14 & C5) is a basic low-pass filter which improves the general signal quality (23).
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The last interesting project was designed by Stefan Salewski [22] and includes some ideas and thoughts about developing a homemade digital-storage-oscilloscope.
The front-end design is theoretically able to process signals up to 100 MHz but was never tested or simulated before.
The input voltage range is specified from +-0.25V to a maximum of +-18V peak to peak.
Due to the fact that the OpenLab oscilloscope should be designed as a low-cost solution, only some parts of the front-end were further analyzed.
Additionally, the specifications of [22] are way to advanced compared to the OpenLab requirements. The segment of interest of the schematic is shown in figure 11.
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/sig_proc_osci_hardware/sig_proc_osci_hardware_salewski.PNG[caption="Figure 3: ",title="Segment of the analog front-end design of a oscilloscope by Stefan Salewski (22)",align="center"]
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The first stage of the circuit is very similar to the solution of the OsciPrime front-end, and serves again as the input impedance and capacitance of the oscilloscope.
The second stage acts as a high impedance op-amp input and was missing in all previously mentioned front-end solutions.
The book The Art of Electronics [19] emphasizes the usage of so called, source followers, as input stages in oscilloscopes.
The results of the analysis of the previously mentioned projects lead to the final front-end design of the OpenLab oscilloscope, which fulfills all, in chapter 3.2 defined, specifications.
