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On this DI, highschool scholar Tommy Liu modifies a preferred low-cost DIY oscilloscope to boost its enter noise rejection and ADC noise with anti-aliasing filtering and IIR filtering.
Half 1 introduces the oscilloscope design and simulation.
This half (Half 2) exhibits the experimental outcomes of this oscilloscope.
Experimental Outcomes
Three experiments have been performed to judge the efficiency of our precision-enhanced oscilloscope utilizing each analog and digital sign processing strategies.
First, we take a look at the impact of the brand new anti-aliasing filter described in Half 1. For this objective, a 2-kHz sinusoidal sign is amplitude modulated (AM) with a 961-kHz sinusoidal waveform by a Rigol DG1022Z sign generator (Rigol Applied sciences, Inc., 2016) and is used because the analog enter to the oscilloscope.
On this state of affairs, the low-frequency (2 kHz) sinusoidal waveform is our sign, whereas the high-frequency tones brought on by modulation with 961 kHz sinusoidal characterize excessive frequency noises on the sign supply. Within the experiment, a ten% modulation depth is used to make the excessive frequency noise simply identifiable by sight. The time division is about at 20 µs with the ADC sampling frequency of 500 KSPS.
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Outcomes of anti-aliasing filter
The unique DSO138-mini lacks anti-aliasing filter functionality as a consequence of its inadequate -3-dB cut-off frequencies (round 500 kHz to 800 kHz). Because of this, the high-frequency noise tones brought on by modulations move by means of the analog front-end, with out a lot attenuation, and are sampled by the ADC at 500 KSPS. This creates aliasing noise tones on the ADC output and might be clearly seen within the displayed waveform on the DSO128-mini (Determine 1).
Determine 1 The aliasing noise tones on the ADC output on the DSO138-mini.
Our new anti-aliasing filter offers a big decrease -3-dB cut-off frequency of round 100 kHz, and successfully filters away many of the out-of-band excessive frequency noises, on this case, the noise tones brought on by the sign modulation with 961 kHz sinusoidal. Determine 2 is a screenshot with the brand new anti-aliasing filter, indicating a big discount within the aliasing noise.
Determine 2 Discount of the aliasing noise with the brand new anti-aliasing filter.
Detailed evaluation on the captured knowledge with the brand new anti-aliasing filter signifies a ten dB to fifteen dB (3.2x to five.6x) enchancment over the unique DSO138-mini on noise rejection at frequencies larger than the oscilloscope’s sign bandwidth.
In sensible functions, excessive frequency noises with a magnitude of some millivolts RMS will not be unusual. A 5-mV RMS noise at close to 900 kHz is attenuated to 0.73 mV (RMS) with our new anti-aliasing filter versus 2.48 mV (RMS) with the unique DSO138-mini. With an ADC full-scale enter vary of three.3 V, 0.73 mV RMS is of an efficient decision properly above 10 bits (ENOB). With the unique DSO138-mini, the ENOB could be at solely an 8-bit degree.
Outcomes of digital post-processing filter
The second take a look at evaluates the efficiency of the digital post-processing filter. As defined in Half 1, moreover the noises on the analog enter, different noise sources in oscilloscopes, akin to noises on ADC contained in the MCU harm the measurement precision. That is evident in Determine 3, which is a screenshot of the DSO138-mini with its Self-Take a look at mode turned on. In Self-Take a look at mode, an internally generated pulse sign—much less inclined to the noises from the exterior sign supply—is used to check and nice tune the oscilloscope. We will see that there are nonetheless ripple noises on the heart beat waveform.
Determine 3 Ripples on internally generated pulse sign throughout self-test mode on the DSO138-mini.
It isn’t simple to determine the magnitude of those ripples because of the restricted pixel decision of the DSO138-mini’s LCD show (320 x 240). We transferred the captured knowledge to a PC by way of DSO138-mini’s UART-USB hyperlink for exact knowledge evaluation. Determine 4 exhibits the waveform of the captured self-test pulses on a PC. The ripple noises are calculated and proven in Determine 5.
Determine 4 Captured self-test pulse sign waveform on PC for extra precision knowledge evaluation.
Determine 5 Magnitude of noises on self-test pulse with no digital post-processing.
Contemplating the voltage division setting (1 V, -20 dB on Enter) and attenuation setting (x1), the ripple on the self-test pulse has a peak-peak magnitude of 8 mV. This error is about 10 LSB and the calculated RMS worth is about 3 mV, yielding an efficient decision of 8.3 bits. Digital post-processing can be utilized to suppress a few of these noises.
Determine 6 is the waveform after first-order infinite impulse response (IIR) digital filtering (α = 0.25) is carried out on the PC, and Determine 7 exhibits the noises on the self-test pulse.
After IIR filtering, the noise RMS worth reduces to about 0.75 mV, or by an element of 4. This brings again the efficient decision from 8.3 bits to 10.4 bits. We discover that the rise and fall transition edges of the heart beat look a bit much less sharp than the sign earlier than post-processing.
That is because of the low-pass nature of the IIR filter. With α=0.25, the passband (-3 dB) is at round 23 kHz, protecting an enter bandwidth as much as audio frequencies (20 kHz). For monitoring sooner indicators, akin to quick transition edges of a pulse sign, we will loosen up α to a better worth permitting for extra enter bandwidth.
Determine 6 Self-test pulse with first-order IIR digital filter the place α = 0.25.
Determine 7 Noises on self-test pulse with first-order IIR filter the place RMS noise reduces to ~0.75 mV.
The consequences of each filters
Lastly, we take a look at the general impact of each the brand new anti-aliasing filter and the digital put up processing by inputting a sinusoidal enter of two kHz from a sign generator to our new oscilloscope. We will see from Determine 8 that even with the brand new anti-aliasing filter, there are nonetheless some noises on the waveform, because of the ADC noises contained in the MCU. The RMS worth of the noises is about 2.8 mV and the efficient decision is proscribed to beneath 9 bits.
Determine 8 Noises on a 2 kHz sinusoidal enter waveform regardless of having the brand new anti-aliasing filter.
As proven in Determine 9, with the first-order IIR filter in impact, the waveform cleans up. The RMS noise reduces to 0.7 mV and, once more, this brings up the efficient decision from beneath 9 bits to above 10 bits. Different enter frequencies, as much as 20 kHz (audio), have additionally been examined and an general efficient decision of 10 bits or extra was noticed with the brand new anti-aliasing filter and the digital post-processing algorithm.
Determine 9 A 2 kHz sinusoidal enter waveform after digital post-processing the place the RMS noise reduces to 0.7 mV.
Low-cost oscilloscope
Many conventional low-cost DIY kind digital oscilloscopes have two main technical drawbacks, particularly insufficient anti-aliasing functionality and enormous ADC noises. Because of this, these oscilloscopes can solely attain an efficient decision of 8 bits or much less, despite the fact that most of them are based mostly on an MCU, outfitted with built-in 12-bit ADCs.
These issues restrict DIY oscilloscopes from extra demanding skilled highschool tasks. To deal with these points, a well-designed first-order analog low-pass filter on the analog front-end of the oscilloscope, plus a programmable first-order IIR digital post-processing filter, are carried out on a preferred low-cost DIY platform (DSO138-mini).
Experimental outcomes verified that the brand new oscilloscope may preserve an general efficient decision of 10 bits or above with the presence of excessive frequency noises at its analog enter, as much as an enter bandwidth of 20 kHz and real-time sampling of 1 MSPS. The implementations are cheap—the BOM value of the brand new anti-aliasing filter is simply the price of a ceramic capacitor (far lower than a greenback), and the digital post-processing program is totally carried out within the PC software program.
Costing lower than fifty {dollars}, this precision digital oscilloscope can be utilized in lots of excessive faculties. This contains excessive faculties with out the funds for expensive industrial fashions and, thus, allow college students to carry out a variety of duties: from the first-time electrical sign seize and commentary to the extra demanding precision measurement and sign evaluation for complicated electrical and digital tasks.
Tommy Liu is at present a junior at Monta Vista Excessive College (MVHS) with a ardour for electronics. A devoted hobbyist since center faculty, Tommy has designed and constructed numerous tasks starting from FM radios to easy oscilloscopes and sign mills for college use. He goals to pursue Electrical Engineering in school and aspires to turn out to be knowledgeable engineer, persevering with his exploration within the area of electronics.
Associated Content material
- Constructing a low-cost, precision digital oscilloscope—Half 1
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- Construct your personal oscilloscope probes for energy measurements (half 2)
- Primary oscilloscope operation
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The put up Constructing a low-cost, precision digital oscilloscope – Half 2 appeared first on EDN.
