Differential signals. These are two complementary signals that have equal and opposite polarity with respect to ground. Fig. 6 shows the single-ended and differential signals. In differential signal, as the signal’s cycle proceeds one path becomes more positive, the other becomes more negative to the same degree. The differential architecture is good at rejecting crosstalk and noise and passing only the valid signal. Single-ended operation is a more common architecture, in which there is only one path plus ground. To design and test the systems that communicate via differential system, you need a signal generator that has this functionality.

Complex waveforms
The above-mentioned are only basic characteristics of the waveform and corresponding specifications of the signal generator. In operational electronic systems, waveforms rarely look like the examples explained above. Certain clock and carrier signals are pure, but most other waveforms will exhibit some unintended distortion. Some waveforms may even include elements of sine, squares, steps and pulses. To test, debug and design such real applications, more sophisticated stimulus signals are required than just simple waveforms. Do check whether the signal generator has these features because you are really going to need them.

Fig. 6: Single-ended and differential digital signals
Fig. 6: Single-ended and differential digital signals

Modulated signals. The most commonly required complex waveform is a modulated signal where the modulation parameters can be controlled. In modulated signals, amplitude, phase and/or frequency variations embed lower-frequency information into a carrier signal of higher frequency. The resulting signals may convey anything from speech to data to video. Such waveforms can be a challenge to reproduce, unless the signal generator is specifically designed for it.

Frequency sweep. If you want to check the frequency response of your device, you need frequency sweep function in your signal generator. It generates a sine wave whose frequency changes over time. This frequency change occurs linearly in Hz/second or logarithmically. Some signal generators even provide sweep sequencing facility, where start, stop, hold frequency and associated time can be defined. Some signal generators also provide a trigger signal synchronously to the sweep to control an oscilloscope that measures the output response of the device.

Digital patterns. A digital pattern consists of multiple synchronised pulse streams that make up ‘words’ of data that may be 8, 12, 16 or more bits wide. The most common signal formats that also come under complex waveforms category are:
1. Non-return-to-zero
2. Delayed non-return-to-zero
3. Return-to-zero
4. Return-to-one

Only specialised signal generators support these waveforms.

Performance specifications
Now after discussing the waveform- related specifications, given below are some performance-related specifications of a signal generator. These specifications have to be checked carefully to select the best signal generator in the range.

Memory depth. Also known as record length, it determines the maximum number of samples that can be stored. Memory depth plays an important role in signal fidelity at many frequencies because it determines how many points of data can be stored to define a waveform. Particularly in the case of complex waveforms, memory depth is critical to reproducing signal details accurately. High-performance mixed-signal generators offer large memory depth and high sample rates. These instruments can store and reproduce complex waveforms such as pseudo-random bitstreams.

Sample rate. The Nyquist Sampling Theorem states that the sampling frequency must be more than twice the highest spectral frequency component of the generated signal to ensure accurate signal reproduction. For example, to generate a 2MHz sinewave signal, it is necessary to produce sample points at a frequency of more than 4 megasamples per second (MSa/s). Although the theorem is usually cited for digital oscilloscopes, it works equally well for signal generators. The stored waveform must have enough points so that the signal can be reproduced reliably. Go for a signal generator with the highest available sample rate specification.

Bandwidth. The analogue bandwidth of the output circuitry should be large enough to support the highest frequency signal that the signal generator can produce. This bandwidth is independent of the sample rate. If there is not enough bandwidth, the signal characteristics may degrade.

Horizontal and vertical resolution. Horizontal resolution is the smallest time increment that can be used to create a waveform. It is actually inversely proportional to the sampling frequency. By this definition, the timing resolution of a signal generator with maximum clock rate of 20 MHz would be 50 nanoseconds. On the other hand, vertical resolution of the DAC defines the amplitude accuracy and distortion of the reproduced waveform. A DAC with inadequate resolution contributes to quantisation errors, causing imperfect waveform generation.

Output channels. Many applications require more than one output channel from the signal generator. Some signal generators can deliver up to four independent channels with full bandwidth. Others offer up to two analogue outputs, supplemented by some high-speed digital outputs for mixed-signal testing. Check in advance your requirement of the number of outputs because this will considerably add to the overall price.

The author is a technical editor at EFY


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