CN0518

RF LOW NOISE AMPLIFIER

The CN0518 uses cascaded HMC376 low noise amplifiers for its RF signal path, achieving an overall gain of approximately 25 dB. The HMC376 is a gallium arsenide (GaAs), pseudomorphic high electron mobility transistor (pHEMT), high MMIC, low noise amplifier that operates from 700 MHz to 1000 MHz.

Figure 2 shows the typical S-Parameter performance, with a typical gain of 15 dB and return losses of more than 10 dB throughout the entire frequency range. The HMC376 has a typical noise figure of 0.7 dB, a 1 dB compression point (P1dB) of 21.5 dBm, and a third-order intercept point (OIP3) of 36 dBm.

Impedance Matching Network

The HMC376 RF input and output ports are internally matched for a single-ended 50 Ω impedance. This internally matched topology allows the HMC376 to be readily ported between any printed circuit board material regardless of its dielectric constant, thickness, or composition. The RFOUT also has an integrated DC-blocking capacitor that eliminates the need for an external capacitor. The only requirement needed is an inductor in parallel with the RF input with respect to the ground.

Figure 3 shows the basic configuration and implementation of two cascaded HMC376 amplifiers.

RF SAW FILTER

The first stage amplifier output of the CN0518 is filtered by a surface wave acoustic (SAW) filter, which eliminates unwanted out-of-band amplification. When selecting a filter, a balance must be struck between the band flatness and the out-of-band rejection. SAW filters are also a source of insertion loss, which reduces the overall gain of the signal chain and needs to be considered during selection.

The filter used has a pass band of 902 MHz to 928 MHz, with a return loss of 21 dB and an insertion loss of 1.9 dB at 915 MHz center frequency.

RF DIRECTIONAL COUPLER

The CN0518 includes a multilayer RF directional coupler. This device has an operating frequency of 698 MHz to 2.62 GHz with a coupling factor of 25.5 at 915 MHz. It has an input and output port impedance of 50 Ω and a typical insertion loss of 0.2 dB.

RF SWITCH

The ADG901 is a wideband RF switch that uses a CMOS process to provide high isolation and low insertion loss. It is an absorptive (matched) switch with 50 Ω terminated shunt legs. The switch enables the user to pass DC signals up to 0.5 V without the use of a DC-blocking capacitor.

The ADG901 operates at DC up to 4.5 GHz at -3 dB frequency. At the 915 MHz center frequency, this device has a typical insertion loss of approximately 1.8 dB at "ON state" as shown in Figure 4, and a typical isolation loss of approximately 30 dB at "OFF state" as shown in Figure 5.

Combining the insertion loss from the filter, coupler, and the RF switch results in a total insertion loss of approximately 3.9 dB at the output of the RF switch in normal operating conditions.

RF PERFORMANCE

The resulting S-parameters, phase noise measurements, spurious-free dynamic range (SFDR), noise figure, and stability measure of the CN0518 are shown in the figures below.

At the center frequency of 915 MHz, the CN0518 achieves a gain of 25 dB with input and output return losses of greater than 10 dB. Figure 6 shows the S-Parameters across the operating frequency range.

Figure 7 shows the single sideband phase noise at 915 MHz, with approximately -91 dBc/Hz, -125 dBc/Hz, -129 dBc/Hz, and -142 dBc/Hz at frequency offsets of 10 Hz, 1 kHz, 100 kHz, and 1 MHz offset, respectively.

Figure 8 shows the narrow-band single-tone RF output with an SFDR of 76 dBFS.

Figure 9 shows the corresponding noise figure of approximately 0.8 dB over the entire frequency range.

The system is stable across the entire 915 MHz ISM frequency bandwidth with the Rollet's stability factor (k) being above 1 and the supplementary stability measure (B1) being above 0, as shown in Figure 10. This makes the CN0518 unconditionally stable for any source and load impedance combination.

OVERPOWER PROTECTION

An overpower management feature is implemented in CN0518 wherein the RF path of the board is automatically isolated when the output power reaches the preset threshold. This feature is performed using an ADL5904 RF power detector.

The ADL5904 provides a resistor-programmable detection threshold, which compares the voltage from an internal envelope detector with a user-defined input voltage. When the voltage from the envelope detector exceeds the user-defined threshold voltage at the V_IN- pin, an internal comparator captures and latches the event to a set/reset (SR) flip-flop. Figure 11 shows the overpower protection circuitry for the CN0518.

As shown in Figure 11, the amplified RF input is sampled by a 25.5 dB RF directional coupler. This power is forwarded to the RFIN pin of the ADL5904, which is then sampled by the internal envelope detector. The ADL5904 threshold voltage level on V_IN- pin is set by a resistor divider network, which value can be computed using Equation 1:

where:
V_IN- is the voltage level at the V_IN- pin of the ADL5904.
R₁ is the user-defined resistance value.
R₂ is the user-defined resistance value.
I_BIAS is the input bias current.

The CN0518 was set at -25 dBm input threshold power, which is equivalent to 0 dBm RF output to protect any sensitive downstream equipment connected to the receiver system. Table 1 can be found in the ADL5904 data sheet and shows the -25 dBm threshold power corresponds to a V_IN- voltage level of 17 mV when operating at 900 MHz.

The threshold level on V_IN- is set by a resistor divider. The value of R₁ is primarily chosen to minimize loading on the 3.3 V rail, and R₂ is chosen for an output impedance that is insensitive to leakage and bias currents. Setting R₁ to 10 kΩ and using the power threshold value of 17 mV, the R₂ value is calculated using Equation 1:

V_IN- = 17 mV
I_BIAS = 20 μA
R₁ = 10 kΩ

Solving Equation 1 using the closest standard resistor value sets the V_IN- voltage level at 17 mV. This user-defined V_IN- voltage is then compared to the output voltage of the internal envelope detector, which is driven by the RF input level. When the power exceeds the threshold value, an overpowering event occurs which opens or isolates the RF path. The series combination of R₁ and R₂ results in a negligible divider current of 328 μA.

The ADL5904 introduces an error level of up to +2.5 dBm on its RF threshold power and can vary from device-to-device. If an accurate threshold power is required, a simple calibration routine must be performed to compensate for the device-to-device variation. Refer to the ADL5904 data sheet for more information on the calibration routine.

Automatic Reset Feature

The CN0518 also includes an automatic reset circuitry that activates when the power level returns within the acceptable range. This feature is done through the LTC6991, a programmable low frequency timer.

As shown in Figure 12, the Q output of the ADL5904 holds the LTC6991 in reset mode during normal operation. When an overpowering event occurs, the LTC6991 is enabled and a 4 ms delay begins. The ADL5904 resets after 4 ms, effectively resampling the power level. If the overpowering condition persists, the ADL5904 trips again and the control signal of the RF switch enters the low state. This shift in signal isolates the RF input and output of the ADG901 switch. Once the overpower event subsides, the ADL5904 starts to resample the power level and then returns to its normal operation.

Overpower Protection Test

Figure 13 presents the setup used to test the overpower protection functionality of the CN0518. In this test, the RF signal generator is set to 915 MHz center frequency and the power to the input is ramped up from -25 dBm to -15 dBm. The CN0518 output power is monitored using a high speed oscilloscope, which shows the response time from the overpowering event to the output power being attenuated.

Figure 14 shows the overpowering protection response time. Based on the figure, the output of the CN0518 has a response time of approximately 10 ns from its normal operation to the RF output power being attenuated.

Figure 15 shows the recovery time from when the overpowering condition subsides, and the power level returns back to its acceptable range. The data demonstrate a 5 ns delay from the attenuated RF output to its normal operation.

USB POWER MANAGEMENT

The CN0518 sources its power through a micro-USB adapter, which typically provides a 5 V, 1 A supply via the micro-USB port. The nominal 4.75 V to 5.25 V input from the USB port is boosted to 5.6 V, then post-regulated to 5.0 V, 3.3 V, and 2.5 V. The circuit requires an overall current of approximately 260 mA from the USB supply during normal operation.

Figure 16 shows the complete CN0518 power tree.

SEPIC Converter

The LT8330 is a current-mode DC/DC converter capable of generating either positive or negative output voltages. It can be configured as boost, SEPIC, or inverting converter consuming as low as 6 μA of quiescent current. Figure 17 shows the basic configuration needed to set the LT8330 for a 5.6 V output.

The output voltage is programmed with resistor divider from the output to the FBX pin of the LT8330. Equation 2 shows how to calculate a desired output voltage with the user-defined resistor values:

where:
VOUT is the desired output voltage in Volts.
R1, R2 are the user-defined resistor values.

For the required output voltage of 5.6 V, the equation results with R1 = 590 kΩ and R2 = 232 kΩ.

Ultralow Noise Linear Regulator

The 5 V output from the ADM7150 is used to power the two HMC376 LNAs. The ADM7150 is a low dropout linear regulator with a typical noise of 1 μV rms from 100 Hz to 100 kHz for fixed output voltage options and <1.7 nV/√Hz noise spectral density above 10 kHz.

The ADP150 is used to generate 3.3 V for powering the RF detector and auto retry circuit. It is a high performance low dropout linear regulator featuring an ultralow noise and ultra high PSSR architecture for powering noise sensitive RF applications.

An ADM7170-2.5 LDO regulator is used to generate the supply voltage required by the ADG901 RF switch. This device has an input voltage range of 2.3 V to 6.5 V and a fixed output voltage of 2.5 V. The ADM7170-2.5 only requires capacitors for proper operation. Particularly, the ADM7170-2.5 is compatible to use small decoupling capacitors of 4.7 μF on its input and output pins.