Simple Phase Meter Operates to 10MHz (HA5024, HFA3102) ® Application Note Introduction You need phase measurements to set up and verify electronic devices in amplifiers and in audio, control, ultrasound, and echo systems. Phase measurements can be problematic, because not many simple, inexpensive phase meters are available. Moreover, using an oscilloscope is time consuming and imprecise. The phase meter described here uses a standard voltmeter as an output device. It measures the phase difference between two signals with better than 1% accuracy and it operates to 10MHz. It’s inexpensive to build, and it’s simple to calibrate. Measuring phase involves converting two periodic signals to square waves, then measuring the phase difference between the two square waves. If the amplitude of both square waves is identical and constant, the average of the time difference between the square waves is proportional to the phase shift. Usually the value you wish to measure is the phase shift between the input of a circuit under test (called the “reference”) and the output of the circuit (called the “signal”). But, these signals may have different amplitudes. The amplitude differences affect the slew rate of the analog signals. The differing slew rates may result in a phase error if the squaring mechanism is not perfect. In Figure 1, IC1 (an HA5024) is configured as a switchedgain amplifier that has four gain selections. IC1 can, thus, make the reference and signal amplitudes nearly equal, thereby minimizing slew-rate errors. This design switches amplifiers rather than resistor-feedback networks, so you can optimize each amplifier for bandwidth, overshoot, and propagation delay. The propagation delay of IC1 introduces a phase-measurement error that has the same magnitude as the error arising from the propagation delay of lC2. Because the two op amps have identical schematics and undergo the same IC processing, their propagation delays match closely, thus, canceling any propagation-delay errors. 1 October 27, 2004 AN9637.1 After undergoing amplification by IC1 and IC2, the reference and signal go to IC3 (an HFA3102), a matched set of longtailed pairs that function as matched, high-speed comparators. The bases of the reference transistors in the comparators connect to ground, so the input signals must use a ground reference. If the bases are not grounded, it’s easy to reference symmetrical inputs to ground by coupling them through a capacitor. Clamp diodes D1 through D4 protect the input bases. R18 and R19 bias the current sources at -2.2V, and R16 and R17 set the comparator currents at 10mA, so the transistors operate at their maximum fT. The value of R23 ensures that the collector voltage drop is greater than 5V (to ensure that D6 turns on). The outputsignal swing is constant at VOS = V+5 -VD5 -VD6. Because the input signals have approximately the same amplitude, the comparators are matched, the output-voltage swing is constant, and the HFA3102 has a 10-GHz fT. The only variable is phase. When the inputs are in phase (phase shift = 0o), the average collector voltage is 0V. When the inputs are 90o out of phase, the average collector voltage is VOS/4. When the inputs are 180o out of phase, the average collector voltage is VOS/2. The relationship is linear, so it indicates phase shift as a function of the average collector voltage. You need to effect level shifting and an offset null to compensate for initial errors. The divider comprising R24, R25, and R26, reduces the average collector voltage from the matched comparators by a factor of two. R26 is a gain, or span, adjustment, and C1 functions as an integrator that yields the average value of the voltage developed across the divider network. IC4 (an HA5170) has low input current, so it won’t discharge C1. IC4 buffers C1 and presents a low-impedance output. R29 provides an offset adjustment you use to set the zero-phase voltage. To calibrate the phase meter, first set the signal to 0o phase shift, adjust R29 for 0V, then set the phase shift to 180o and adjust R26 for 1.8V. The output scale factor is, thus, 10mV/degree. You may have to iterate the adjustments to obtain 1% accuracy. The PC board is critical in this design: Use a ground plane, keep trace lengths and component leads short, and use good components. 1-888-INTERSIL or 321-724-7143 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 1996, 2004. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Application Note 9637 5V REFERENCE INPUT 0.01µF X1 X2 X5 18 6 + IC1A 19 17 R1 2K 20 5V X10 5V S1 REFERENCE GAIN 0.01µF 3 R8 2K R2 1K R25 51K -5V 0.01µF R24 51K 5V R26 2K R3 681 11 D5 1N4148 10 R27 20K R28 20K R29 5K D6 1N5817 R14 200 +5V 14 1 D2 1N4148 D1 R6 510 13 6 4 IC3A 11 R15 200 R16 147 R18 3.4K 2 IC3C R10 2K 3 R19 4.64K 7 IC3B 9 8 IC3D 10 R17 147 5V 3 + IC1D 2 4 R6 43 R30 13k 0.01µF 8 R5 120 0.01µF -5V 5V R23 750 6 -5V R9 2K R4 681 + IC1C 9 7 R22 499 PHASE OUTPUT 7 OFFSET 13 15 + IC1B 12 14 + IC4 C1 2 50µF 4 3 + IC2 2 - R11 2K R7 383 R12 2K 0.01µF 7 4 SIGNAL IN -5V 0.01µF 1 6 0.01µF R20 200 R21 200 D3 1N4148 D4 1N4148 -5V R13 1K SIGNAL IN R31 1K NOTES: 1. IC1 = HA5024 2. IC2 = HA5020 3. IC3 = HFA3102 4. IC4 = HA5170 FIGURE 1. FOUR ICs AND A HANDFUL OF COMPONENTS PROVIDE 1% ACCURATE PHASE MEASUREMENTS TO 10MHz IN THIS LOW-COST CIRCUIT. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 2 AN9637.1