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— Projects: ADA, Session —

ADA in use: Description of a Session

In these examples, some complex waveforms are examined and the frequency response of a filter determined. With 8-bit converters, the resolution is not good enough to really measure devices. However, it is sufficient to visualize the test results.

As wave generator, an old analog Roland System 100 VCEMS (Voltage Controlled Electronic Music Synthesizer) was used. It consists, essentially, of two almost identical units with each a voltage controlled oscillator that generates sine, triangle, sawtooth and square waves. Additionally, the duty cycle of the square wave can be controlled. The oscillator output enters a mixer and then feeds through a high pass filter (HPF) followed by a low pass filter (LPF) with controllable Q (quality) up to ringing. The signal leaves via a voltage controlled amplifier (VCA). One unit has a noise generator (NG) while the other features a ring modulator (RM).

Instruments

Above at left part of the Roland System 100 VCEMS (synthesizer and above expander), at right top an oscilloscope Gould Advance OS250B and underneath a Hewlett-Packard Selective Voltmeter 3581C.

Band-Pass Filter with high-Q

The oscillator output of the HP 3581C was fed to the mixer of one VCEMS unit and the output after the filters to the input of the HP 3581C. The Selective Voltmeter swept its oscillator and tracked the analyzing filter. Thus, a frequency plot of the filters could be established.

Bandpass Filter

At left, a photograph of the oscilloscope's display. At right, the data gathered converted to a picture (the caption was added later).

For the right picture: There are 10 kHz/div horizontally and about 2 dB/div vertically. The center frequency of the filter was set to 7.5 kHz and the Q just below the threshold when it began to ring.

Ring Modulator Output

A ring modulator outputs f1+f2 and f1-f2. f1 was set to 238 Hz, f2 to 1 kHz, a rather extreme setting. At left the resulting waveform in the time domain, in the centre in the frequency domain and at right a screenshot from the converted image.

Ring Modulator

In the center, the spectrum generated as seen on the oscilloscope. On the frequency axis (horizontal), the resolution is 1 kHz/div and vertical 18 dB/div. The noise is above -60 dBm.

The spectrum is more clear on the picture at right generated from the data with the ADA than from the photograph of the oscilloscope's screen. On the photograph of the oscilloscope, the noise seems lower, but this is not true, because the trace was adjusted to be on the second lowest grid line.

Frequency Modulation Output

Frequency modulation makes a complex spectrum and rich sounds that can be adjusted by following filters.

Frequency Modulation

At left, the waveform of the frequency modulator output on the oscilloscope, the waveform in the time domain.

In the center, the spectrum generated as seen on the oscilloscope. One VCEMS oscillator was set to 938 Hz. It was modulated by the second oscillator set to 156 Hz. The waveform shown in the frequency domain.

The spectrum is more clear on the picture generated from the data (right) than from the photograph of the oscilloscope's screen (center).

Amplitude Modulation Output

Another example shows the result of an amplitude modulation in the time and frequency domains.

Amplitude Modulation

One VCEMS VCO was set to roughly 1 kHz and the VCA used to amplitude modulate the signal with a 15 Hz triangle wave. Unfortunately, the data from the ADA was not uploaded to the host computer and no picture generated.

The "knots" or bright dots on the oscilloscope are the data points actually measured. They are connected by the trace; the electron beam moving to the next dot.

Summary

An oscilloscope can function nicely as a spectrum analyzer with ADA. The data gathered from the wave analyzer by ADA and output analogue to an oscilloscope works fine. Instead of sending the data to an oscilloscope, an image can be created and displayed on the computer screen.

 
  © 2004 - 2018 by Horo Wernli.