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Thursday, December 27, 2007

Allpass network shifts signals 90°

Allpass network shifts signals 90°

Unlike lowpass, bandpass, and other magnitude-altering filters, allpass filters can shift the phase of a signal without affecting its amplitude. For a first-order allpass circuit, the transfer function is
As you sweep the variables from zero (dc) to infinity, the sign of H(s) changes from plus to minus, indicating a change in phase from 0 to 180°. You can realize this transfer function in two wideband transconductance amplifiers (WTAs). The circuitry inside the dashed lines in Fig 1 is one allpass network.

A WTA's transfer function is IOUT8VIN/Z, where 8 is simply an internal constant and Z is an external gain-setting component connecting the WTA's Z+ and Z- pins. The transfer function for voltage amplification is VOUT/VINIOUTxZOUT. Most applications require a resistive Z. But the WTA also accepts an inductor, a capacitor, or any other impedance network for Z.

The allpass circuit combines a resistive-Z WTA (IC1) with a capacitive-Z WTA (IC2). At low frequencies, IC1 dominates the circuit's output because the capacitor's high impedance allows only a low IOUT from IC2. Rising frequencies lowers this impedance, causing the current from IC2 to dominate at high frequencies. Moreover, IC2 inverts, and IC1 does not, producing the desired noninverting unity gain at dc and inverting unity gain at high frequencies.

Communications and signal-processing applications use allpass networks widely. An example is a 90°-phase-shift network, which, with appropriate mixers, produces a single-sideband signal. In Fig 1, the two allpass circuits have corner frequencies that differ by a factor of 7.5. The output RC networks determine these corner frequencies. The result is an output-phase difference that remains close to 90° over a wide frequency range. Measurements show 0.2-dB amplitude variations and a phase difference of 90°±7° from 180 to 740 kHz-a 4:1 range. (DI #1696)

Two wideband transconductance amplifiers (WTAs) form an allpass network (within dashed lines). Combining two such networks produces two outputs having a constant 90° phase shift versus frequency between them.

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