A simple way of measuring the amplitude and phase difference between two high-frequency signals is provided by the AD8302 from Analog Devices (www.analog.com). The two input signals A and B are terminated with 50 Ω and fed to the internal logarithmic demodulators. Taking the difference of the outputs leads to a voltage which represents the amplitude difference in decibels (dB); by multiplying the internal signals an output voltage proportional to the phase difference between A and B is produced.
The circuit produces a voltage at output VMAG (‘magnitude’) between 0 V and +1.8 V. 0 V represents –30 dB while +1.8 V represents +30 dB, each 30 mV step thus representing one decibel. The amplitude of input B is taken as the reference. The phase output also varies between 0 V and +1.8 V, each 10 mV step representing one degree of a phase difference. The outputs can drive up to 15 mA, and so the load impedance must be at least 120 Ω.
The AD8302 can be used, for example, as a level meter by applying a signal with a known amplitude to input B. The input level range runs from –62 dBm to –2 dBm. Error in the device is typically less than 0.5 dB in amplitude and 1 degree in phase. The device operates from a supply voltage between 2.7 V and 5.5 V. If modulation is present on the input signals, the modulation envelope will appear on the outputs.
The IC has a bandwidth of 30 MHz, which can be reduced by fitting a capacitor between pins 8 and 14. Pin 11 is a +1.8 V reference voltage output which can be used when further processing the outputs of the device.