USB Powered 5.8 GHz RF LNA Receiver with Output Power Protection
“High-speed overpower detectors and switches are included in this circuit to protect downstream sensitive equipment connected to the receiver system. The receiver system also automatically resumes normal operation when the RF power level falls within an acceptable range. The RF input and output are standard SMA connectors, and the entire design is powered by a micro USB connector.
Author: Analog Devices
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Circuit functions and advantages
The International Telecommunication Union (ITU) has allocated the license-exempt 5.8 GHz Industrial, Scientific and Medical (ISM) radio frequency band for global use. Advances in wireless technology and standards, along with minimal regulatory compliance requirements, have made this frequency band popular for short-range wireless communication systems.
Because of the number of channels and bandwidth available, short-range digital communication applications such as WiFi prefer to use the 5.8 GHz band. While the transmission range is shorter than the 2.4 GHz band, it offers 150 MHz of bandwidth and can support up to 23 non-overlapping WiFi channels. Other common use cases include software-defined radios, wireless access points, public safety radios, wireless repeaters, femtocells, Long Term Evolution (LTE)/Worldwide Interoperability for Microwave Access (WiMAX)/4G, transceiver base stations ( BTS) infrastructure.
This small size design provides high gain, reliable overpower monitoring and protection, an added benefit for ISM band applications facing low signal strength or limited coverage.
The circuit shown in Figure 1 is from a high-performance RF receiver system with +23 dB gain, optimized to support the use of a 5.8 GHz center frequency. The input is unfiltered, maintaining a 2 dB noise figure, but the output is equipped with a bandpass filter to attenuate out-of-band interference.
High-speed overpower detectors and switches are included in this circuit to protect downstream sensitive equipment connected to the receiver system. The receiver system also automatically resumes normal operation when the RF power level falls within an acceptable range. The RF input and output are standard SMA connectors, and the entire design is powered by a micro USB connector.
Figure 1. EVAL-CN0534-EBZ Simplified Functional Block Diagram
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RF Low Noise Amplifier (LNA)
The HMC717A is a Gallium Arsenide (GaAs) Pseudomorphic High Electron Mobility Transistor (PHEMT), Monolithic Microwave Integrated Circuit (MMIC) LNA, suitable for operating frequencies from 4.8 GHz to 6.0 Ghz, suitable for a variety of signal communication protocols such as ISM, MC-GSM, W-CDMA and TD-SCDMA) back-end receivers.
As shown in Figure 2, the HMC717A has 14.5 dB gain in its RF operating band. The noise figure is 1.1 dB, and the amplifier is powered from a 5 V supply with a total supply current of 68 mA. To achieve 23 dB total gain, two HMC717A amplifiers are cascaded. The HMC717A offers 1.1 dB noise figure, 27 dBm 3rd-order intermodulation point (IP3), and 15 dB compression point (P1dB) for first-stage LNAs as well as intermediate gain stages.
Figure 2. HMC717A Broadband Gain (S21) and Return Loss (S11) vs. Frequency
LNA impedance matching
As shown in Figure 3, the RFIN (pin 2) and RFOUT (pin 11) pins of the HMC717A are single-ended pins with a nominal 50Ω resistance over the 4.8 GHz to 6.0 GHz frequency range, allowing the HMC717A to connect directly to 50Ω termination system without the use of additional impedance matching circuits.
RFOUT has an integrated DC blocking capacitor, so there is no need for external capacitors in the second stage, allowing multiple HMC717A amplifiers to be cascaded together in a back-to-back fashion without the use of external matching circuits. The only requirement is that the RFIN of the first stage must be AC coupled with a 1.2 pF capacitor.
Figure 3. Basic connections for cascading HMC717A amplifiers
The LNA output is filtered by a bandpass filter. As shown in Figure 4, the filter has a passband range of 5400 MHz to 6400 MHz, a typical return loss of 14.7 dB, and an insertion loss of 1.6 dB at a center frequency of 5.8 GHz.
Figure 4. Typical electrical performance of a bandpass filter: insertion loss (S21) and return loss (S11)
Over power protection
Relatively low power levels can damage sensitive circuits. For example, the absolute maximum power level of the RF input to the AD9363 transceiver is +2.5 dBm. The CN0534 includes an overpower protection that operates an automatic reset circuit when the power level falls within an acceptable range, as shown in Figure 5.
Figure 5. RF Attenuator and Power Detector Protection Block Diagram
RF power detector and automatic reset circuit
The ADL5904 is an RF power detector that operates from DC to 6 GHz. A 470 nF ac-coupling capacitor and an external 82.5Ω shunt resistor are recommended at the input of the ADL5904 to provide wideband input matching. The ADL5904 provides programmable threshold detection based on RF input power levels using an internal RF envelope detector and a user-defined input voltage. When the voltage from the RF envelope detector exceeds a user-defined threshold voltage on the VIN− pin, an internal comparator latches the event onto the flip-flop. The response time for an RF input signal exceeding a user-programmed threshold to output latch is an extremely fast 12 ns. The latched event remains on the flip-flop until a reset pulse is applied to the RST pin.
The output power level of the CN0534’s bandpass filter is sampled by an integrated thin-film coupler with a +13 dB coupling factor and forwarded to the RFIN pin of the ADL5904. The threshold level of the ADL5904 at VIN− is set by a resistor divider to a value of approximately 32 mV, which corresponds to a threshold power of −9 dBm when operating at 5.8 GHz without calibration, as shown in the ADL5904 data shown in the manual. Combined with the loss of the coupler and RF attenuator at 0 dB, the output remains at a level that is safe for sensitive devices.
If higher overpower threshold accuracy is required, a simple calibration procedure can be performed at multiple frequencies to compensate for device-to-device differences within the system. See the ADL5904 data sheet for information on calibration procedures.
During normal operation, the Q output of the ADL5904 holds the LTC6991 programmable low frequency timer in reset. When an overpower event occurs, the LTC1991 is enabled and a 4 ms delay begins. The ADL5904 resets after 4 ms to resample the power level. If the overpower condition persists, the ADL5904 is disconnected again and the attenuator remains at -20 dB. The attenuator control signal is delayed and will remain at -20 dB during the resampling of the power level. If the overpower condition is removed, the attenuator returns to the 0 dB state and resumes normal operation, as shown in Figure 6.
Figure 6. Auto-retry circuit functional block diagram
The HMC802A is a broadband bidirectional 1-bit GaAs IC digital attenuator. The device features a low insertion loss of 1.5 dB at 5.8 GHz in bypass mode and accurate 20 ± 0.6 dB attenuation when enabled. Powered by a 5 V power supply, IP3 is +55 dBm, and the attenuation control signal is CMOS/TTL compatible. While RF switches are typically used in overpower protection applications, at 5.8 GHz, the HMC802A’s 20 dB attenuation is better than the off-isolation state of most RF switches.
As shown in Figure 7, the device has a typical insertion loss of 1.5 dB in bypass mode at a center frequency of 5.8 GHz. Figure 8 shows an isolation factor of -20.5 dB in attenuation mode at a center frequency of 5.8 GHz.
Figure 7. Typical Insertion Loss and Input Return Loss Performance of the HMC802A in Bypass Mode
Figure 8. Typical Insertion Loss and Input Return Loss Performance of the HMC802A in Attenuated Mode
Combining the insertion loss from the bandpass filter, coupler, and from the RF attenuator, the total insertion loss at the output of the RF attenuator is approximately 3 dB under normal operating conditions at 5.8 GHz center frequency, and when in attenuation mode, about 21.5 dB.
Use the setup shown in Figure 9 to test the overpower protection function. The output frequency of the RF signal generator was set to 5.8 GHz, and the input power of the CN0534 was increased from -30 dBm to -20 dBm. The output power of the CN0534 is monitored by the ADL6010 high-speed envelope detector, which provides an accurate measure of the response time from an overpower event to output power decay.
Figure 9. Simplified block diagram of RF overpower response test
Figure 10. Typical Overpower Protection Response Time
Figure 11. Typical Recovery Time After Overload Protection State
Once triggered, the detector latch is reset at 250 Hz, and the output switch is enabled if the output power drops below 2.5 dBm. The switch enable signal is delayed to ensure that it is not asserted while an overpower condition still exists. The results are shown in Figures 10 and 11.
USB power management
Figure 12 shows the EVAL-CN0534-EBZ power tree, which consumes 1.1 W from a 5 V supply via the micro USB interface.
Figure 12. CN0534 System Power Architecture
The LT8335 is a current mode DC/DC converter capable of generating positive or negative output voltages from a single feedback pin. It can be configured as a boost, SEPIC or inverting converter and consumes as low as 6µA of quiescent current. In typical applications, low ripple burst mode maintains high efficiency at low output currents while keeping output ripple below 15 mV. Internally compensated current mode architecture provides stable operation over a wide input and output voltage range. Integrated soft-start and frequency foldback to control Inductor current during startup. To configure the LT8335 to provide a 5.6 V output, the basic connections required are shown in Figure 13.
Figure 13. 5.6V output block diagram of the LT8335
The output voltage is programmed through a resistive divider between the output and the FBX pin. Resistor values are chosen according to Equation 1 to provide a positive output voltage:
Ultralow Noise Linear Regulators
The ADM7150 is an ultralow noise, high PSSR RF linear regulator that uses a 5 V output to maximize the gain of the HMC717A.
The ADM7150 is a low dropout linear regulator that provides 1.0 µV rms typical output noise from 100 Hz to 100 kHz and above 10 kHz with a fixed output voltage selection
Figure 14. Noise Spectral Density vs Frequency with Different Bypass Capacitors (CBYP)
The ADP150 is used to generate 3.3 V for the power detector and auto-retry circuit. As shown in Figure 15, it is a high performance low dropout linear regulator with ultralow noise and ultrahigh PSRR architecture for powering noise sensitive RF applications.
Figure 15. ADP150 PSSR vs. Frequency
In order to achieve a wider operating bandwidth, the HMC8411 can be used instead of the RF LNA. The HMC8411 is a low noise wideband amplifier that operates from 0.01 GHz to 10 GHz. It offers 15.5 dB typical gain, 1.7 dB typical noise figure, and 34 dBm typical output 3rd order intermodulation point power (OIP3), while consuming only 55 mA from a 5 V supply. The HMC8411 also features internally matched 50 Ω inputs and outputs, ideal for surface mount technology (SMT) based high volume microwave radio applications.
The HMC550A can be used in place of an RF switch. It is a low cost single pole single throw (SPST) failsafe switch for applications requiring low insertion loss and low current consumption. These devices control signals in the frequency range from DC to 6 GHz and are ideal for IF and RF applications including RFID, ISM, automotive and battery powered tags and notebook computers.
The ADL6010 can be used as an envelope detector replacement and is a fast response, 45 dB range, 0.5 GHz to 43.5 GHz envelope detector. The ADL6010 is a versatile microwave spectrum wideband envelope detector that offers very low power consumption (8 mW) in a simple 6-pin package. The baseband voltage output by the device is proportional to the instantaneous amplitude of the radio frequency (RF) input signal. Its RF input has a very small slope change in order to envelope the output transfer function from 0.5 GHz to 43.5 GHz.
Circuit Evaluation and Testing
The following sections outline the general setup for evaluating the performance of the CN0534. For complete details, see the CN0534 User Guide.
► EVAL-CN0534-EBZ Reference Design Board
► One RF signal source (R&S® SMA100B)
► One signal source analyzer (Keysight E5052B SSA)
► One network analyzer (Keysight N5242A PNA-X)
► One SMA to SMA cable
► A micro USB to USB cable
► 5 V AC/DC USB Power Adapter
Figure 16. Block Diagram of Phase Noise and SFDR Test Setup
To measure the phase noise and SFDR shown in Figure 16, follow these steps:
1. Set the measurement configuration of the signal source as follows:
► To perform SFDR measurements, set Center Frequency = 5.8 Ghz, Frequency Range = 5.79 GHz to 5.81 GHz, RF Amplitude = 10 dBm.
► To perform phase noise measurements, set Center Frequency = 5.8 Ghz, Offset Frequency Range = 10 Hz to 30 MHz. If the device can handle the amplifier output (approximately 20 dBm at 0 dBm input), please refer to the signal source analyzer data sheet for the maximum input level. If necessary, connect an attenuator to the input of the signal source analyzer.
2. Set the power level of the signal source generator to 0 dBm and the center frequency to 5.8 GHz.
3. Power the EVAL-CN0534-EBZ with a micro USB cable and a 5V power adapter with a power rating greater than 500 mW.
4. Connect the output of the signal generator to the RF input (J2) of the EVAL-CN0534-EBZ.
5. Connect the RF output (J1) of the EVAL-CN0534-EBZ to the signal source analyzer.
6. Perform a measurement run on the signal source analyzer.
Figure 17. Block diagram of S-parameter and noise figure test setup
To measure the S-parameters and noise figure shown in Figure 17, follow these steps:
1. Set the vector network analyzer to the desired measurement conditions, using the following settings:
► To perform S-parameter measurements, set the frequency range = 4.8 GHz to 6.8 GHz.
► To perform phase noise measurements, set the frequency range = 5.3 GHz to 6.8 GHz.
2. Perform a full 2-port calibration of the vector network analyzer using the calibration kit. Note that the RF input (J2) of the EVAL-CN0534-EBZ can be connected directly to the test port, so only one measurement cable is required for the test setup.
3. Power the EVAL-CN0534-EBZ using the 5 V power adapter and microUSB cable.
4. Connect the EVAL-CN0534-EBZ to the test port of the vector network analyzer using the calibrated test setup.
5. Set the measurements to the desired S-parameters.
6. Perform the autoscaling function on the vector network analyzer. The scale can then be adjusted if desired.
The EVAL-CN0534-EBZ amplifies the RF signal input with approximately +23 dB gain and a return loss of −15 dB at a center frequency of 5.8 GHz. Figure 18 and Figure 19 show the gain and return loss of the EVAL-CN0534-EBZ.
Figure 18. EVAL-CN0534-EBZ Gain vs. Frequency
Figure 19. EVAL-CN0534-EBZ Input Return Loss vs. Frequency
The SSB phase noise at 5.8 GHz is shown in Figure 20.
Circuit Evaluation and Testing
Figure 20. SSB Phase Noise vs Offset Frequency for EVAL-CN0534-EBZ at 5.8 GHz
Figure 21 shows a narrowband single-tone RF output with an SFDR of approximately 78 dBFS.
Figure 21. Narrowband single-tone RF output of EVAL-CN0534-EBZ at 5.8 GHz
Figure 22 shows the corresponding noise value versus frequency, which is approximately 2 dB at the 5.8 GHz center frequency.
Figure 22. EVAL-CN0534-EBZ Noise Figure vs Frequency
ESD (Electrostatic Discharge) Sensitive Devices. Live devices and circuit boards can discharge without being aware of it. Although this product has patented or proprietary protection circuitry, the device may be damaged when exposed to high energy ESD. Therefore, proper ESD precautions should be taken to avoid device degradation or loss of functionality.
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