The Waveform Generator Back End Module increases small signal S/N ratio by attenuating, at the test head, an intentionally oversize signal. The result is a small signal waveform to the DUT with an acceptable S/N ratio. Without the Waveform Generator Back End Module, the Waveform Generator must generate a small signal which is highly susceptible to line noise and prone to a poor S/N ratio.

A problem that commonly occurs when waveforms of low amplitude reach the PTH is excessive noise. In many cases, the DUT requires a low amplitude waveform biased at some DC level. Transporting these low amplitude signals from the test system via cables to the PTH often results in a poor signal-to-noise (S/N) ratio. This is because noise is always introduced during the transit through cabling and wiring, and small signals are often partially or completely obscured by this noise. The Waveform Generator Back End Module aids you in using various techniques to increase the S/N ratio. It receives Waveform Generator output, processes the waveforms, then drives the DUT input(s).

A specific technique for increasing the S/N ratio is to increase signal amplitude. Increasing signal amplitude involves generating a waveform of the maximum possible voltage at the Waveform Generator, then using the Waveform Generator Back End Module to attenuate the signal to the proper amplitude for the DUT.

To get the maximum signal out of the Waveform Generator, any DC offset must be eliminated. If the DUT requires a signal biased at a value other than zero volts, that DC offset value can be added by the Waveform Generator Back End. If the application requires independent amplitude control of each tone in a multitone waveform, two Waveform Generator outputs can be added together by the Waveform Generator Back End. When an application requires a differential waveform, it can be generated by the Waveform Generator Back End. Also, the Waveform Generator Back End can filter input waveforms above 50 kHz and AC couple output waveforms.

A PTH will accept up to four Waveform Generator Back End Modules. Figure 13-1 shows a typical application example using the Waveform Generator Back End Module and the Digitizing Voltmeter Front End Module. A relatively large signal is generated by one of the Waveform Generators. The waveform is then attenuated by the Waveform Generator Back End Module. The DUT modifies the waveform. It is then amplified by the Digitizing Voltmeter Front End Module to maintain a large S/N ratio for an accurate reading at the Digitizing Voltmeter.