|Category||Embedded Solutions => Engineering Tools => RF / Wireless Development Tools => RF Development Tools => 8841394|
|Title||RF Development Tools CFTL|
|Datasheet||Download EVAL-CN0390-EB1Z datasheet
|Product Category||RF Development Tools|
|Manufacturer||Analog Devices Inc.|
|Tool Is For Evaluation Of||ADA4077-1, ADL6010, HMC635, HMC985A|
|Frequency||20 GHz to 37.5 GHz|
|Operating Supply Voltage||- 0.6 V, +/- 5V|
|For Use With||ADA4077-1, ADL6010, HMC635, HMC985A|
|Factory Pack Quantity||1|
Devices Connected/Referenced ADL6010 Circuits from the Lab® reference designs are engineered and tested for quick and easy system integration to help solve today's analog, mixed-signal, and RF design challenges. For more information and/or support, visit HMC985A HMC635 Fast Responding, 45 dB Range, 0.5 GHz to 43.5 GHz Envelope Detector 4 MHz, 7 nV/Hz, Low Offset and Drift, High Precision Amplifier GaAs Voltage Variable Attenuator, 10 GHz to 40 GHz GaAs RF Amplifier, 18 GHz to 40 GHz20 GHz to 37.5 GHz, RF Automatic Gain Control (AGC) Circuit
Circuit Evaluation Boards CN-0390 Circuit Evaluation Board (EVAL-CN0390-EB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials
The automatic gain control (AGC) circuit is useful in multiple applications such as amplitude stabilization of a synthesizer, controlling output power in a transmitter, or optimizing dynamic range in a receiver. The circuit shown in Figure 1
uses the ADL6010 detector, along with the HMC985A voltage variable attenuator (VVA) and the HMC635 RF amplifier, to provide automatic gain control over a wide range of input frequencies (20 GHz to 37.5 GHz) and amplitude. Circuit performance, as measured by the AGC figures of merit described in this circuit note, are very good between 20 GHz and 30 GHz. The overall gain of the circuit falls off above 30 GHz. However, improvements can be made over narrow bands by using matching techniques not explored in this circuit note. The AGC circuit has applications in microwave instrumentation and radar-based measurement systems.
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 determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage)
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2016 Analog Devices, Inc. All rights reserved.Figure 2. EVAL-CN0390-EB1Z AGC Printed Circuit Board (PCB) Photo
Many RF applications require very exact amplitude control with minimal drift over time and temperature. Examples of applications with this requirement are instruments where NBS traceable calibration is required, and where calibration intervals may be long terms such as once or twice per year. Other applications include phased array radar, where the accuracy of the amplitude and phase control limits the beam forming accuracy. The approach used in this circuit, using an op amp in an integrating circuit for the loop controller, provides excellent gain control to compensate for variations in gain of the RF components over input amplitude, RF frequency, and temperature. In operation, the VSET dc bias controls the output amplitude. The most likely application drives this dc bias with to 12-bit DAC, depending on the accuracy required of the loop. This approach allows digital control of the RF output amplitude. Although the DAC is not included as part of this circuit note, there are many options available, such as the AD5621 12-bit nanoDAC from Analog Devices, Inc.
Figure 3 shows that the difference amplifier is used to compare the VSET voltage to a voltage generated by the detector circuit. The detector converts the RF amplifier output amplitude a dc voltage. Because the RF input (X) is injected in the middle of the loop, the effect of any variation X is minimized at the RF output (Y). This effect is true as long as the total loop gain remains high. This effect is explained by the following equations: Y=X×Z
The central idea behind an AGC circuit of this type is to stabilize the amplitude an RF signal that may vary based on frequency, temperature, or time. Typically, this circuit has two inputs. The first input an RF input of a given amplitude whose envelope requires stabilization. The second input a dc control applied to what is called the VSET input, and it is this input that is used to set the output amplitude. This simple loop is shown in Figure 3.
From Equation 5, it can be seen that the amplitude of X has a minimal effect on the amplitude Y as long as the value Gd/10 1. Two things that can affect this relationship between X and Y are the gain of the detector, Gd, as well as the 10 dB tap on the directional coupler. In the CN-0390 design, however, because an integrator is built around the op amp controller circuit, the dc gain of the loop is only limited by the high open-loop dc gain of the op amp. This gain is high enough to make the AGC near perfectly flat within the range of the control loop. As with any AGC circuit, there are limitations on the operation of the loop. The loop closes for a given range of RF input amplitudes and VSET control voltages. These limitations vary with frequency, as well. In general, the loop is closed and the output amplitude remains flat with RF input variation when the VGAINCTRL (Z) node is between -2.4 V and 0 V, which is the input range of the HMC985A VVA. Note that the transfer functions for both the detector (VOUT vs. RF input amplitude) and the VVA (attenuation vs. voltage control) are very nonlinear (see Figure 5, Figure 6, and Figure 7), and that their combined gain can vary significantly over the input range of the RF input and the VSET input, possibly over frequency and temperature as well. Using the high gain of the integrator in the control loop compensates for these effects. The circuit, as built, is more complex than can be described by the simple model of Figure 3. In the actual circuit, the VVA function is broken into two components. The first component an HMC985A VVA, which provides attenuation from approximately dB at VCTRL V to approximately 40 dB attenuation at VCTRL HMC635 RF amplifier is used for the gain stage, providing dB of gain over the frequency range of interest.
The anchor device in this circuit is the ADL6010 envelope detector. The ADL6010 operates over the frequency range of 500 MHz to 45 GHz. As shown in the functional block diagram in Figure 4, the is a diode-based detector. The response curve for the ADL6010, given in Figure 5, is very nonlinear, which makes straightforward analysis of the feedback loop in this circuit difficult.
Another approach to conceptualizing the response of the AGC loop is to understand that when the loop is closed V > VGAINCTRL > -0 V), the VSET control voltage is equal to the output voltage of the ADL6010 detector. Under this condition, the op amp integrator is balanced, and the integrator capacitor charge is constant, not charging or discharging. If the RF input amplitude changes while VSET is static, the loop responds, charging or discharging the integrating capacitor until equilibrium is reached again. With the loop at equilibrium, the ADL6010 output is equal to the VSET voltage. The VSET voltage can then be referenced to the transfer function of the ADL6010 (see Figure 5) to find the ADL6010 RF input power that corresponds to this voltage. From there, add dB to this RF input power number, because of the 10 dB tap on the directional coupler, to find the output power that corresponds to a given VSET voltage. Using this method, a table of output power vs. VSET voltage can be created. The transfer curves of the VVA and ADL6010 are very nonlinear; therefore, this method may be easier than establishing a mathematical description of these transfer curves.
The HMC985A VVA operates over the frequency range of 10 GHz to 40 GHz and provides attenuation in the range dB to near 40 dB. The HMC985A consists of two pi-pad attenuators in series, the first controlled by VCTRL1, the second VCTRL2. By connecting VCTRL1 and VCTRL2 and driving them together, a combined attenuation of approximately dB is achievable. With the HMC635 RF amplifier in series, the complete voltage controlled gain range is from 15 dB gain dB of attenuation. The is a GaAs amplifier that requires a negative gate voltage (VGG) that must be applied at the same time or before the 5 V VDD supply. Damage can occur to the HMC635 if this VGG requirement is violated. VGG is typically in the range -0.6 V but may vary slightly from device to device to optimize performance of the amplifier. Consult the HMC635 or other GaAs amplifier data sheets on setting the optimal drain current by adjusting VGG. The evaluation board as built uses a diode and resistor to bias VGG to approximately -0.6 V, for ease of use for the user, so that a separate VGG supply is not necessary. It is recommended that the -5 V supply rail be applied first so that the VGG requirement is
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