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Volume 45 – November 2011
Various software packages enable the stereo sound card found in a personal computer (PC) to provide oscilloscope-like displays, but the low-sample-rate, high-resolution analog-to-digital converters (ADCs) and ac-coupled front end are optimized for 20 kHz or less of usable bandwidth. This limited bandwidth can be extended—for repetitive waveforms—by using a sampling front end ahead of the sound card inputs. Subsampling the input waveform with a high-speed sample-and-hold amplifier (SHA)—followed by a low-pass filter to reconstruct and smooth the waveform—effectively stretches the time axis, allowing the PC to be used as a high-speed sampling oscilloscope. This article describes a front end and probe that provide an appropriate adaptation.
Figure 1 shows a schematic for a plug-in attachment that can be used for sampling with typical PC sound cards. It uses one AD783 high-speed sample-and-hold amplifier per oscilloscope channel. The sampling signal for the SHA is provided by the digital output of a clock-divider circuit; an example of one will be described. The AD783 input is buffered by a FET, so simple ac/dc input coupling can be used. In the two channels shown, 1-MΩ resistors (R1 and R3) provide dc bias when the dc-coupling jumper is open and the input is ac-coupled. The sampled output is low-pass filtered by the two-pole active RC networks shown. The filter need not be an active circuit, but the one shown usefully provides a buffered low impedance to drive the PC sound-card input.
Figure 1. 2-channel analog sampling circuit.
The AD783 SHA provides a usable large-signal bandwidth up to a few megahertz. The effective slew rate at the input is above 100 V/µs. Input/output swing with a ±5-V supply is at least ±3 V. The small-signal 3-dB bandwidth for swings less than 500 mV p-p is close to 50 MHz.
front-end circuit of Figure 1, and a PC’s sound card employing the Visual
Analyser1 software, the screen shot in Figure 2 illustrates a 2-MHz,
single-cycle sine repeated at 1 MHz. The sampling clock provides 250-ns-wide
sample pulses at an
Figure 2. 2-MHz single-cycle sine pulse at 1-MHz repetition rate.
Another screen shot was taken of a Gaussian sine pulse with a 1-MHz repetition rate (Figure 3). The sampling clock rate was again 80.321 kHz, with 250-ns sample pulse width.
Figure 3. 4-MHz Gaussian sine pulse at 1-MHz repetition rate.
of a Sampling Clock Generator
Figure 4. Sampling clock divider circuit.
IC4 is a fixed-frequency metal-can crystal oscillator. Another approach would be to use CMOS inverters (74HC04) and a discrete crystal, X1, to form an oscillator, as shown in Figure 5. This approach, while using more components than the all-in-one metal-can oscillator, permits a small amount of frequency tuning by adjusting Capacitor C1 to pull the crystal frequency.
Figure 5. Discrete crystal oscillator with mechanical tuning.
To avoid the mechanically variable component, use a varactor diode—which has voltage-dependent capacitance—for D1, as shown in Figure 6.
Figure 6. Discrete crystal oscillator with voltage tuning.
of Active Reconstruction Filters
Figure 7. Sallen-Key 39-kHz low-pass filter.
Figure 8 shows another second-order multiple-feedback (MFB) filter with a corner frequency of about 33 kHz, using standard resistance and capacitance values. This filter has a pass-band gain of –1, so—if it is used—select the invert button on the scope software in order for the displayed waveform to be right-side up.
Figure 8. MFB 33-kHz low-pass filter.
another option would be to use the +5 V provided by a spare PC or laptop USB
port. The –5 V could be generated by a
Figure 9. P6040 1×/10× scope probes.
To demonstrate the AD783 sample-and-hold input stage, the probe compensation was first adjusted using a 1-kHz flat-top square wave. The screen shots show the response for various signals with frequencies of 1 MHz and 50 MHz. The two screen shots in Figure 10 show one channel with a 1 MHz, 5-V p-p square wave (a), and a 50-MHz, 5-V p-p square wave (b). In each case, the sample clock was adjusted for a downsampled signal frequency of about 500 Hz, so that any sound-card response differences were eliminated. Thus, the effective time scale is 500 ns/division for the screen shot on the left and 10 ns/division for the screen shot on the right. The sound card input gain was set for the scope software to report a 1.072-V p-p amplitude for the 1-MHz input and a 762.2-mV p-p amplitude for the 50-MHz input. The ratio of 0.7622/1.072 is close to –3 dB. This measurement shows that the combination of the 100-MHz 10× probe and the AD783 has a 50-MHz, 3-dB bandwidth.
Figure 10. Single channel 10× probe 1 MHz (a) and 50 MHz (b) 5-V p-p input square waves.
In Figure 11, the same 1-MHz (a) and 50-MHz signals (b) are applied to both channels. From these two overlaid screen shots of both channels, one can see that there is good gain-, offset-, and delay-matching between the two channels.
Figure 11. Dual-trace 2-channel matching, 10× probes, 1-MHz (a) and 50-MHz (b) 5-V p-p input square waves.
final screen shot (Figure 12) is of a 375-kHz, 5-V p-p square wave (red trace)
and a 1.5-MHz 42 ns wide 5-V p-p pulse (green trace). The horizontal scale is
333 ns/division. The AD783 sampler maintains the full 5-V swing, even for these
Figure 12. Dual-trace 2-channel, 10× probes, 375-kHz, 5-V p-p square wave and 1.5-MHz, 42-ns 5-V p-p pulse.
2Syscomp Electronic Design, Ltd. http://www.syscompdesign.com/Accessories.html.
5The PC-based Soundcard oscilloscope receives its data from the sound card with 44.1-kHz sampling rate and 16-bit resolution. Also available is WaveIO, a Soundcard Interface for LabView software. http://www.zeitnitz.de/Christian/scope_en.
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