In recent columns, we’ve given a quick review of some basic instrumentation common to most engineering work benches. Here are a few more instruments found in many engineering labs.
Signal Generator is a generic term describing various instruments, all sharing a single rationale, which is that a device under test (DUT) requires a periodic analog or digital signal applied at its input. The input may be swept, stepped or modulated. Controls on the front panel permit the user to vary the signal’s amplitude and frequency in order to evaluate the DUT’s response.
Signal generators were used extensively by the first generation of TV service technicians in the great post World War II consumer electronics boom. Early tube-type CRT TVs were complex energy hogs that required periodic repair. A common procedure was to inject appropriate signals at various stages starting downstream from the tuner, and methodically probing stage, circuit and component inputs and outputs to isolate the fault. Similar procedures were used for radar and other electronic equipment.
Today, the basic signal generator has evolved into a number of related instruments with expanded capabilities, which lend themselves to extensive analytics on the receiving end.
Types of signal generators include but are not limited to:
Function generators, which produce periodic waveforms, individually selected by the user. (Electronics engineers say “waveform” while mathematicians say “function.” They are essentially the same.) These waveform/functions may include Sine wave, Pulse, Ramp, dc, Noise, Sinx/x, Gaussian, Lorentz, Exponential rise, Exponential decay, Haversine and Cardiac among others. Drop-down menus allow the user to vary over a wide range of frequencies/periods, amplitudes and offsets. In instruments having two or more output channels, phase relations can be set. Modern digital storage oscilloscopes usually contain internal function generators. Free-standing bench function generators have enhanced capabilities such as sweep, burst, modulation and others. Function generators contain electronic oscillators or, in newer models, digital signal processing circuitry to synthesize waveforms, with a digital-to-analog converter (DAC) to produce the analog output.
The arbitrary function generator also contains a library of standard waveforms. Additionally, it allows the user to create simple or highly complex waveforms. They may be constructed by altering existing waveforms, drawing traces on the touch screen and then manipulating them, or typing in numerical parameters.
RF signal generators and microwave signal generators are similar, but they operate at higher frequencies. At the lower end, their ranges overlap those of the function generators. Microwave signal generators are capable of generating frequencies as high as 70 GHz over coaxial cable and up to hundreds of gigahertz over waveguide media.
Vector Signal Generator – These instruments generate digitally-modulated radio signals using digital modulation formats such as QAM, QPSK, FSK, BPSK and OFDM. Users may test these communication systems by creating custom waveforms and downloading them into the vector signal generator.
Digital Pattern Generators create digital signals for validation and testing of integrated circuits.
Video Signal Generators output specialized video and TV waveforms including synch and colorburst signals and audio track signals.
A sweep generator is a variation on (and sometimes included in) a function generator. In its most common design it creates a waveform with constant amplitude and frequency that varies linearly. The purpose of a sweep generator is to test the frequency response of active filter circuits.
Sweep generators are widely used to measure the output value over a changing input. They are frequently used to evaluate audio equipment. The principal inputs are:
Glide sweep, in which the input frequency increases or decreases logarithmically with respect to time.
Stepped sweep, also called amplitude sweep, in which the input pauses between sweeps, allowing time for a stable reading to be acquired.
Time sweep, another amplitude sweep, which permits measurement over comparatively long periods of time.
Table sweep, a variation on stepped sweep. The input is a sequence of frequency and amplitude values taken from a pre-established table. It is used only in highly specialized tests.
Developed in the 1930s, the psophometer is an instrument that quantifies the amount of noise in a telephone circuit. Simple ampacity isn’t enough, because different applications tolerate more or less noise and have different frequency limits, depending on whether they are used solely for human speech or for high-fidelity sound. Newer psophometers incorporate weighting networks for this purpose.
Waveform monitor and vectorscope
TV broadcast technicians use the waveform monitor to keep track of the video signal luminance on a real time basis. The instrument can display the
luminance of the entire TV picture or it can zoom in to show one or more lines. It can also display the lack of luminance in the blanking interval during retrace as well as the colorburst between each line in the video signal.
Early waveform monitors were strictly analog, but the newer digital models have enhanced capabilities such as color gamut checking and audio analytics. Additionally, eye pattern and jitter displays measure physical layer parameters.
Modern waveform monitors incorporate vectorscope capabilities. At first, they were separate instruments. The vectorscope displays an X-Y plot of the two signals, the second signal configured to trigger the first signal, revealing relations between the two signals.
The main difference between a vectorscope and an oscilloscope configured to display Lissajous figures is that the vectorscope is manufactured with a specialized graticule, and its inputs are designed to accept TV or video signals, which are demodulated and demultiplexed so they can be analyzed. A waveform monitor displays the overall characteristics of a video signal, while a vectorscope visualizes chrominance. Because chroma is contained in two channels, termed Cb and Cr, this video information lends itself to the X-Y mode.
Volume unit (VU) meter
Measuring sound volume in a broadcast station, sound studio or end-user audio equipment is more complex than simply hooking up an analog ammeter in series with the speaker.
First proposed in the 1930s, the VU meter is a 200 μA dc d’Arsonval ammeter in series with a full-wave copper oxide rectifier. The needle has a relatively large mass, with an intentionally slow response, which integrates the signal so that abrupt peaks and dips are eliminated. This resembles human auditory perception, which is the purpose of the meter. Moreover, brief spikes where the audio exceeds the redline but do not register in the VU meter, are not necessarily as harmful as long duration pulses because the brief temperature rise in the final amplifier stages and speakers does not equate to high caloric heat.
A typical VU meter is set up with 0 VU about two-thirds of the distance between minimum and maximum. Above 0 VU is the red area, which the needle should not be permitted to occupy. 0 VU equals +4 dBu, or 1.28 V RMS. When applied across a 600-Ω load, this translates to 2.5 mW. Modern VU meters are electronic, without the massive needle, but they are configured to duplicate the mechanical meter’s power/time relations.
Specifically, when standards were established in 2009, the meter was mandated to respond to changing audio signals, moving from no signal to 99% of 0 VU when a 1-kHz sine wave was applied for 300 msec. The reading should not depart from the 1 kHz level by more than 0.2 dB from 35 Hz to 10 kHz or more than 0.5 dB from 25 Hz to 16 kHz.
The VU meter and its attenuator should have a 7.5 kΩ input impedance, measured with a sinusoidal signal a 0 dB on the VU meter scale.
Another sound level instrument is the peak meter. It provides a visual indication of the sound level, but rather than the RMS value, it shows the peak. In producing an audio recording, one requirement is to set the recording level to match the maximum capability of the recorder for the loudest sounds that will be encountered in a specific recording sequence.
Modern peak meters consist of horizontal or vertical rows of LEDs. Various colors may be used to indicate normal operation and overloading. In the older analog meters with moving needles, response time could be set by varying the mass of the needle. Today’s digital meters accomplish this by purely electronic means.