Advantages of Two Channels in Waveform Generators

Introduction

For many waveform-generator applications, only a single-ended signal referenced to ground is required. In such scenarios, a single-output generator will suffice. However, many other applications are simply better served with a two-channel instrument. Of course, it is possible to synchronize independent single-channel units, but a two-channel instrument provides synchronization that is more precise by virtue of operation from a single internal sampling clock. This document will review some applications that will benefit from the use of a two-channel waveform generator.

Channel Summing

Among the most powerful features of a two-channel waveform generator is the ability to combine the output of one channel with another (Figure 1). As a result, two channels synced to the same clock can produce a composite output. Because the wave shapes produced by each channel are independently programmable, they can be used together to perform several operations, one of which is channel summing. Users may combine signals from the generator’s two channels by employing a simple BNC T-adapter.

Figure 1: In this example, the waveforms M1 and M2 (stored in oscilloscope memory) combine in trace F1 to show the sum of their peak-to-peak values. The measurements appear directly below trace F1

Among the benefits resulting from channel summing is extended dynamic range. By programming different gain values on the two summed channels, the generator can produce large signals with small features. The programming resolution achievable with this technique is greater than the resolution of a single channel. Further, if the DAC’s output limit has been reached on one channel, summing that channel with the second channel effectively increases dynamic range.

For an example, refer to the table:

In this example, the sine wave on Ch1 has a 5-V swing. A 10-MHz filter serves to remove the sharp corners in the waveform resulting from the discrete DAC steps in the programmed waveform. The spike produced on Ch2 is 500 times smaller than the sine wave onto which it is superimposed. Because of the ratio of the programmed gains for the two channels, the resolution of the glitch is as high as that of the sine wave.

Another use of channel summing is adjustable feature size. In the two-channel summed mode, users may create a waveform on Ch1 with an anomaly on Ch2 (Figure 2). By varying the gain on Ch2, one may change the amplitude of the anomaly without changing the amplitude of the Ch1 waveform. In fact, all parameters of both channels are flexible on the fly, including frequency, period, phase, offset, and more. This can be a powerful tool for studying the immunity of various circuits to unwanted phenomena, such as sensitivity to undershoot.

Figure 2: A 1-MHz sine wave with 6-V p-p amplitude is combined with a 2 MHz, 20-ns pulse with 4-V p-p amplitude. The amplitude of the pulse is independently adjustable relative to the sine wave

Bear in mind that channel summing is only one way to create different waveform shapes. An instrument such as Teledyne LeCroy’s WaveStation waveform generators provides arbitrary waveform functionality, such as equation draw or point-by-point sample point adjustment. Having said that, the presence of two channels provides user with greater dynamic range, which is very useful for applications in which a small signal rides on large voltage swings, such as, for example, the output of a switching power supply. It is also worth mentioning that although arbitrary waveform functionality is a boon to many applications involving non-standard signals, a signal created using two channels and channel summing offers greater flexibility. An arbitrary waveform, once stored in the instrument’s memory, is repeated in its exact form unless the user changes the waveform in memory.

Accurate Phase Control

Thanks to direct digital synthesis of dual-channel waveforms on a point-by-point basis, users may precisely adjust the phase relationship between two waveforms. To understand how this is accomplished, think of the oscillation of a sine wave as a vector rotating around a “phase wheel.” The number of discrete phase points in the “wheel” is set by the resolution of a phase accumulator. The output of the phase accumulator feeds a phase-to-amplitude lookup table, which, in turn, feeds a DAC. A phase tuning word length of 14 bits provides 0.022° of phase delay control resolution.

Application Examples

The advantages inherent to a two-channel waveform generator make these instruments ideal for many applications. Consider, for example, the challenge of simulating radar receivers. Typically, the output from a radar receiver consists of two signals: the baseband in-phase signal (or I signal) and the baseband quadrature-phase signal (or Q signal). The waveform generator is used to create such a signal pair for the purpose of testing and/or characterizing the digital signal processing circuitry that follows the receiver itself. With the generator, users may create a variety of test signals ranging from pulsed CW to pulses shaped with amplitude modulation to complex bursts using frequency modulation and sweeps.

Creation of video signals is another application well served by a two-channel waveform generator. Even as high-definition television standards continue to evolve, consumer video technologies require non-standard television signal formats, which many waveform generators cannot produce. A two-channel waveform generator can create the main structure of a given video signal on one channel while placing sync signals or other information on the second channel.

Conclusion

Why consider a two-channel waveform generator? Thanks to both channels being driven by a single internal sampling clock, synchronization between the two channels is much more precise than one can achieve with a pair of single-channel units. The ability to sum the channels vastly extends the instrument’s usefulness. Adding highly accurate phase control of the two channels is yet another benefit.