Design Ideas: March 14, 1996
The circuit in Figure 1 uses a phase shifter to provide an output signal that represents the polarity of the slope of an analog signal. You can use it for square-wave shaping of sinusoidal signals or for highpass filtering. V1839 and V1841 can range from 9 to 18V and -9 to -18V, respectively. The unity-gain isolation amplifier IC1 decouples transient spikes from the input source. You can null any dc offset by grounding the two input lines and adjusting resistor R14 until the output signal (pin 10 of IC1) equals 0V.
The noninverting amplifier IC5 provides gains of unity to 51; you can thus amplify weak input signals. You adjust the gain with resistor R15. The phase shifter (IC3) provides a slight time delay and thus shifts the phase of the input signal, thereby providing a "shadow" of the input signal. You can trim the phase shift by adjusting resistor R3. The comparator (IC2) compares the voltage of the original input signal to that of the shadow signal. Because the shadow signal lags the original input signal, the comparator produces a negative output when the slope is positive and a positive output when the slope is negative. Figure 2 shows the analog signals and the comparator output.
The output amplifier IC4 provides signal conditioning for the slope-polarity waveform. R6 provides gain or attenuation of the comparator's output signal, and R9 provides a positive or negative dc offset. These adjustments provide for direct interface to TTL, CMOS, and HCMOS circuits. High-frequency performance depends on the slew rate of the op amps IC2 through IC5. To extend the frequency response of the circuit, you can reduce the value of capacitor C1.
Figure 3 shows measured performance, as captured by a Nicolet Model 440 digital storage oscilloscope. A Fluke Model PM5150 arbitrary-waveform generator creates the input signal, which is a low-frequency sinusoid carrying a higher-frequency sinusoid. Note that when the slope of the input signal is positive, the signal-slope polarity signal is approximately 0V, and when the slope of the input signal is negative, the signal-slope polarity signal is approximately 4.5V. (DI #1838)