Gain and Phase Detector

Gain and Phase Detector
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RF engineers are familiar with gain/loss and phase measurements. Those measurements are commonly performed by using quite sophisticated and very expensive measurement equipment such as Power Meters, Spectrum and Network analyzers. Amateurs and hobbyists, usually do not have access to such expensive measurement equipment. Fortunately, there is a solution for them. AD8302 is an integrated circuit from Analog Devices which can be used as a simple Gain and Phase Detector from LF to 2.7GHz. AD8302 itself is a fully integrated system for measuring gain/loss and phase in numerous RF applications. A typical Gain and Phase Detector circuit, using the AD8302, is shown in the next image. The specific design and some other interesting reference applications are covered in detail on AD8302's datasheet.

Gain and Phase Detector Schematic

Brief Circuit Description

The AD8302 measures the magnitude ratio (gain), and phase difference (phase) between two signals using a closely matched pair of demodulating logarithmic amplifiers, a phase detector and some linear subtraction units. Signal from Input A and B are applied to the logarithmic amplifiers' inputs. A subtraction unit takes the difference of their outputs, and a measurement of the magnitude ratio (or gain) between the two input signals is available at the VMAG output. The logarithmic amplifiers drive also a Phase Detector. Phase Detector's output is available at VPHS and carries a signal which is proportional to the phase difference between signals A and B. VMAG output provides an accurate measurement of either gain or loss over a 60dB (from -30db to +30db) range scaled to 30 mV/dB, and the VPHS output provides an accurate phase measurement over a 0°–180° range scaled to 10 mV/degree (for the typical Transfer Characteristics for the Gain and Phase Measurement refer  to page 16 of the AD8302's datasheet). AD8302 may also be used to determine absolute signal level by applying an unknown signal to one input and a calibrated AC reference signal to the other. For more details about the circuit and for many other interesting ideas of how to use the detector, refer to AD8302's datasheet.

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