Circuit Note CN-0390 Devices Connected/Referenced Fast Responding, 45 dB Range, 0.5 GHz ADL6010 to 43.5 GHz Envelope Detector Circuits from the Lab reference designs are engineered and 4 MHz, 7 nV/Hz, Low Offset and Drift, tested for quick and easy system integration to help solve todays ADA4077-1 High Precision Amplifier analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0390. GaAs Voltage Variable Attenuator, HMC985A 10 GHz to 40 GHz HMC635 GaAs RF Amplifier, 18 GHz to 40 GHz 20 GHz to 37.5 GHz, RF Automatic Gain Control (AGC) Circuit uses the ADL6010 detector, along with the HMC985A voltage EVALUATION AND DESIGN SUPPORT variable attenuator (VVA) and the HMC635 RF amplifier, to Circuit Evaluation Boards provide automatic gain control over a wide range of input CN-0390 Circuit Evaluation Board (EVAL-CN0390-EB1Z) frequencies (20 GHz to 37.5 GHz) and amplitude. Circuit Design and Integration Files performance, as measured by the AGC figures of merit Schematics, Layout Files, Bill of Materials described in this circuit note, are very good between 20 GHz CIRCUIT FUNCTION AND BENEFITS and 30 GHz. The overall gain of the circuit falls off above 30 GHz. However, improvements can be made over narrow bands by The automatic gain control (AGC) circuit is useful in multiple using matching techniques not explored in this circuit note. applications such as amplitude stabilization of a synthesizer, controlling output power in a transmitter, or optimizing The AGC circuit has applications in microwave instrumentation dynamic range in a receiver. The circuit shown in Figure 1 and radar-based measurement systems. HMC985A VVA HMC635LC4 10dB RF RF OUT RF IN DIRECTIONAL AMPLIFIER COUPLER V CONTROL ADA4077-1 OP AMP ADL6010 DETECTOR VSET Figure 1. Circuit Block Diagram Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Fax: 781.461.3113 2016 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 13968-001CN-0390 Circuit Note Figure 2. EVAL-CN0390-EB1Z AGC Printed Circuit Board (PCB) Photo CIRCUIT DESCRIPTION AGC Application Space RF INPUT (X) (Y) 10dB HMC635LC4 RF OUTPUT DIRECTIONAL RF Many RF applications require very exact amplitude control AMPLIFIER COUPLER VGAINCTRL (Z) with minimal drift over time and temperature. Examples of applications with this requirement are instruments where NBS traceable calibration is required, and where calibration 10dB TAP ADA4077-1 intervals may be long terms such as once or twice per year. OP AMP Other applications include phased array radar, where the VSET DETECTOR accuracy of the amplitude and phase control limits the beam forming accuracy. The approach used in this circuit, using an Figure 3. Simple AGC Loop op amp in an integrating circuit for the loop controller, provides Figure 3 shows that the difference amplifier is used to compare excellent gain control to compensate for variations in gain of the VSET voltage to a voltage generated by the detector circuit. the RF components over input amplitude, RF frequency, and The detector converts the RF amplifier output amplitude to a dc temperature. voltage. Because the RF input (X) is injected in the middle of In operation, the VSET dc bias controls the output amplitude. the loop, the effect of any variation at X is minimized at the RF The most likely application drives this dc bias with an 8-bit to output (Y). This effect is true as long as the total loop gain remains 12-bit DAC, depending on the accuracy required of the loop. high. This effect is explained by the following equations: This approach allows digital control of the RF output amplitude. Y = X Z (1) Although the DAC is not included as part of this circuit note, there are many options available, such as the AD5621 12-bit Gd Y Z VSET (2) nanoDAC from Analog Devices, Inc. 10 Theory of AGC Operation Gd Y Y X VSET (3) The central idea behind an AGC circuit of this type is to stabilize 10 the amplitude of an RF signal that may vary based on frequency, X VSET temperature, or time. Typically, this circuit has two inputs. The Y (4) Gd X first input is an RF input of a given amplitude whose envelope 1 requires stabilization. The second input is a dc control applied to 10 what is called the VSET input, and it is this input that is used to VSET set the output amplitude. This simple loop is shown in Figure 3. Y (5) 1 Gd x 10 where Gd is the detector gain. Rev. 0 Page 2 of 11 13968-002 13968-003