CN0555

RF AMPLIFIER STAGE

The CN0555 uses two ADL5523 low noise amplifiers for its RF signal path. The ADL5523 is a high performance gallium arsenide (GaAs), pseudomorphic high electron mobility transistor (pHEMT) RF low noise amplifier that provides a high gain and low noise figure. Figure 2 shows its typical S-parameter performance, with a typical gain of 21.5 dB and return losses of more than 10 dB over the entire frequency range.

The ADL5523 has a typical noise figure of 0.8 dB, a 1 dB compression point (P1dB) of 21 dBm, and a third-order intercept point (OIP3) of 34 dBm. Two ADL5523 amplifiers are cascaded to achieve an overall gain of 40 dB.

Impedance Matching Network

The ADL5523 requires an external matching network whose impedance is tuned for the desired frequency band to achieve optimal performance. The input matching network includes inductors placed in series with the RFIN pin and a shunt capacitor. Similarly, for the output side, the matching network uses inductors and capacitors on the bias line. Figure 3 presents the complete impedance matching network and the implementation of cascading two ADL5523 amplifiers.

Proper placement of these components is also critical to the input/output impedance matching; therefore, the CN0555 follows the recommended layout and component sizes for the 500 MHz tuning band in the ADL5523 data sheet.

SAW FILTER

The LNA output of the CN0555 is filtered by a surface acoustic wave (SAW) filter, which helps eliminate 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 SAW filter used in CN0555 has a typical insertion loss of 2 dB and a terminating impedance of 50 Ω.

RF DIRECTIONAL COUPLER

The CN0555 includes a low profile, subminiature high performance 3 dB, 90° hybrid coupler. The device has an operating frequency of 400 MHz to 900 MHz, with an input and output impedance of 50 Ω and a typical insertion loss of 0.3 dB at 433.92 MHz.

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 switch with 50 Ω terminated inputs and outputs. 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 from DC to 4.5 GHz, with an insertion loss of 3 dB at 4.5 GHz. At the 433.92 MHz center frequency, this device has a typical insertion loss of approximately 0.4 dB at "ON state", as shown in Figure 4; and a typical isolation loss of approximately 70 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 2.7 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 CN0555 are shown below.

At the center frequency of 433.92 MHz, the CN0555 achieves a gain of 40.5 dB with input and output return losses of greater than 10 dB. Figure 6 shows the S-parameter values across the operating range.

The single sideband phase noise at 433.92 MHz is shown in Figure 7 with approximately -98 dBc/Hz, -131 dBc/Hz, and -149 dBc/Hz at 10 Hz, 1 MHz, and 10 MHz offset, respectively.

Figure 8 shows the narrow-band single-tone RF output with an SFDR of 58.38 dBFS.

Figure 9 shows the corresponding noise figure over a frequency range of approximately 0.8 dB at the center frequency of 433.92 MHz.

The system is stable across the entire 433.92 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 CN0555 unconditionally stable for any combination of source and load impedance.

OVERPOWER PROTECTION

An overpower management feature is implemented in CN0555 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 CN0555.

As shown in Figure 11, the amplified RF input is sampled by a 3 dB, 90° hybrid 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 VIN- pin of the ADL5904.
R₁₀ is the user defined resistance value.
R₁₁ is the user defined resistance value.
I_BIAS is the input bias current.

The CN0555 was set at 0 dBm input threshold power to protect any sensitive downstream equipment connected to the receiver system. As shown in Table 1, the 0 dBm threshold power corresponds to a VIN- voltage level of 241 mV when operating at 900 MHz.

The threshold level on V_IN− is set by a resistor divider. The absolute values of R₁₀ and R₁₁ are chosen to minimize loading on the 3.3 V rail, while providing an output impedance that is insensitive to leakage and bias currents. Setting R₁₀ to 13.7 kΩ results in an R₁₁ value of 1.02 kΩ, which produces a negligible divider current of 224 μA and an output impedance of 991 Ω.

V_IN- = 241 mV
I_BIAS = 20 μA
R₁₀ = 13.7 kΩ

R₁₁ = 991 Ω or 1.02 kΩ (using standard resistor value)

Solving Equation 1 using the closest standard resistor value sets the V_IN- voltage level at 241.9 mV. When the power exceeds the threshold value, an overpowering event occurs which isolates the RF path.

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 CN0555 also includes an automatic reset circuitry that activates when the power level returns within the acceptable range. This feature is performed by the LTC6991, a programmable low frequency timer.

As shown in Figure 12, the Q output of 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 displays the setup used to test the overpower protection functionality of the CN0555. In this test, the RF signal generator is set to 433.92 MHz center frequency and the power to the input is ramped up from -50 dBm to -40 dBm. The CN0555 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 CN0555 has a response time of approximately 9 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 7 ns delay from the attenuated RF output to its normal operation.

USB POWER MANAGEMENT

The CN0555 sources its power through a micro USB adapter, which typically provides a 5 V, 1 A supply via the micro USB port. This circuit requires a current of approximately 113.61 mA during normal operation. This power requirement is provided using two supply voltages. The first supply gives a power of 3.3 V for the ADL5523 low noise amplifier, ADL5904 RF detector, and LTC6991 low frequency timer. The second power supply provides 2.5 V for the ADG901 RF switch. Figure 16 shows the complete CN0555 power tree.

The LT3042 is a high performance low dropout (LDO) linear regulator featuring ultralow noise and ultrahigh power supply rejection ratio (PSRR) architecture for powering noise-sensitive RF applications. Designed as a precision current reference followed by a high performance voltage buffer, the LT3042 can be easily paralleled to further reduce noise, increase output current, and distribute the heat on the printed circuit board. Figure 17 shows the basic configuration needed to set the LT3042 for a 3.3 V output.

The LT3042 incorporates a precision 100 μA current source at the SET pin, which also ties to the inverting input of the amplifier. Figure 17 illustrates that connecting a resistor from the SET pin to GND generates a reference voltage. This reference voltage is simply the product of the SET pin current, 100 μA, and the SET pin resistor, as shown in Equation 2.

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 input and output 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.