CN0534

RF Low Noise Amplifier (LNA)

The HMC717A is a gallium arsenide (GaAs), pseudomorphic high electron mobility transistor (PHEMT), monolithic microwave IC (MMIC), LNA that is ideal for back-end receivers operating between 4.8 GHz and 6.0 GHz for various signal communication protocols such as ISM, MC-GSM, W-CDMA, and TD-SCDMA.

As shown in Figure 2, the HMC717A has a gain of 14.5 dB through its RF band of operation. The noise figure is 1.1 dB and the amplifier is powered from a single 5 V supply with a total supply current of 68 mA. In order to achieve an overall gain of 23 dB, two HMC717A amplifiers are cascaded. The 1.1 dB noise figure, 27 dBm third-order intercept point (IP3), and 15 dB compression point (P1dB) make the HMC717A suitable for both the first stage LNA and intermediate gain stage.

LNA Impedance Matching

As show in Figure 3, the RFIN (Pin 2) and RFOUT (Pin 11) pins of the HMC717A are single-ended with a nominal impedance of 50Ω over the 4.8 GHz to 6.0 GHz frequency range, allowing the HMC717A to be directly interfaced with a 50Ω terminated system without requiring additional impedance matching circuitry.

The RFOUT has an integrated DC blocking capacitor that elimi-nates the need for an external capacitor at the second stage, enabling multiple HMC717A amplifiers which be cascaded back to back without the need for external matching circuitry. The only requirement is that RFIN of the first stage must be AC-coupled with a 1.2 pF capacitor.

Band-Pass Filter

The LNA output is filtered by a band-pass filter. As shown in Figure 4, the filter has a pass band from 5400 MHz to 6400 MHz with a return loss of 14.7 dB typical and an insertion loss of 1.6 dB at a 5.8 GHz center frequency.

Overpower Protection

Sensitive circuitry can be damaged by relatively low power levels. For example, the RF inputs of the AD9363 transceiver have an absolute maximum power level of +2.5 dBm. The CN0534 includes an overpower protection with automatic reset circuitry for when the power level drops within the acceptable range, as shown in Figure 5.

RF Power Detector and Auto Retry Circuitry

The ADL5904 is an RF power detector that operates from DC to 6 GHz. The input of the ADL5904 is recommended to have a 470 nF ac coupling capacitor along with an external 82.5Ω shunt resistor to provide a wide band input match. The ADL5904 uses an internal RF envelope detector and user defined input voltage to provide a programmable threshold detection function based on the RF input power level. When the voltage from the RF envelope detector exceeds the user defined threshold voltage at the VIN− pin, the internal comparator latches the event to a flip flop. The response time from the RF input signal exceeding the user programmed threshold, to the output latching is a very fast 12 ns. The latched event is held on the flip flop until a reset pulse is applied to the RST pin.

The power level at the output of the band-pass filter on the CN0534 is sampled by an integrated thin film coupler with a coupling factor of +13 dB and forwarded to the RFIN pin of the ADL5904. The ADL5904 threshold level on VIN− is set by a resistor divider with a value set at approximately 32 mV, which is equivalent to a −9 dBm threshold power when operating uncalibrated at 5.8 GHz as indicated in the ADL5904 data sheet. Combined with the losses of the coupler and RF attenuator in the 0 dB state, the output is kept at a safe level for sensitive devices.

If higher overpower threshold precision is required, a simple calibration routine can be performed at multiple frequencies to compensate for device-to-device variations within the system. Refer to the ADL5904 data sheet for information on the calibration routine.

The Q output of the ADL5904 holds an LTC6991programmable low-frequency timer in reset during normal operation. When an overpower event occurs, the LTC1991 is enabled and a 4 ms delay begins. The ADL5904 is reset after 4 ms, effectively re-sampling the power level. If the overpower condition persists, the ADL5904 trips again and the attenuator is held in the -20 dB state. The attenuator control signal is delayed, such that it remains in the -20 dB state continuously as the power level is re-sampled. If the overpower condition has subsided, the attenuator returns to the 0 dB state and normal operation resumes, as show in Figure 6.

RF Attenuator

The HMC802A is a broadband bidirectional 1-bit GaAs IC digital attenuator. This device features low insertion loss of 1.5 dB at 5.8 GHz in bypass mode and an accurate 20±0.6 dB attenuation when enabled. Operating from a single 5 V supply, the IP3 is +55 dBm and the attenuation control signal is CMOS/TTL compatible. While an RF switch would typically be used in an overpower protection application, at 5.8 GHz the HMC802A's 20 dB attenuation is better than the off-state isolation of most RF switches.

Shown in Figure 7, the device performance with typical insertion loss in bypass mode is 1.5 dB at 5.8 GHz center frequency. Figure 8 shows an isolation of -20.5 dB in attenuation mode at 5.8 GHz center frequency.

Combining the insertion loss from the band pass filter, coupler and from the RF attenuator, this results in a total insertion loss of approximately 3 dB at the output of the RF attenuator in normal operating conditions and approximately 21.5 dB when operating in attenuation mode at a center frequency of 5.8 GHz.

Protection Results

The overpower protection functionality was tested using the setup shown in Figure 9. An RF signal generator was set to 5.8 GHz output frequency and the power to the input of the CN0534 was ramped from -30 dBm to -20 dBm. The CN0534 output power was monitored with an ADL6010 high-speed envelope detector, which provides an accurate measurement of the response time from the overpower event to the output power being attenuated.

Once tripped, the detector latch is reset at a frequency of 250 Hz, and the output switch is enabled if the output power has dropped to less than 2.5 dBm. The switch enable signal is delayed ensuring that it is not asserted if the overpower condition is still present. Both results can be seen in Figure 10 and Figure 11.

USB Power Management

Boost Converter

Figure 12 shows the EVAL-CN0534-EBZ power tree, which consumes 1.1 W of power from the 5 V supply available on the micro USB jack.

The LT8335 is a current mode DC/DC converter capable of generating either positive or negative output voltages using a single feedback pin. It can be configured as a boost, SEPIC or inverting converter consuming as low as 6µA of quiescent current. Low ripple Burst Mode operation maintains high efficiency down to very low output currents while keeping the output ripple below 15 mV in a typical application. The internally compensated current mode architecture results in stable operation over a wide range of input and output voltages. Integrated soft-start and frequency foldback functions are included to control inductor current during start-up. Shown in Figure 13 are the basic connections needed to configure the LT8335 for a 5.6 V output.

The output voltage is programmed with a resistor divider from the output to the FBX pin. Choose the resistor values for a positive output voltage according to Equation 1:

Ultralow Noise Linear Regulator

The 5 V output from the ADM7150, an ultralow noise, high PSSR, RF linear regulator, is used to maximize the gain of the HMC717A. The ADM7150 is a low dropout linear regulation with a typical output noise of 1.0 µV rms from 100 Hz to 100 kHz for fixed output voltage options and <1.7 nV/√Hz noise spectral density above 10 kHz, as is shown in Figure 14.

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 featuring an ultralow noise and ultra high PSRR architecture for powering noise sensitive RF applications.