FAQ
For a reflective RF switch, when it is in the "off" state, the non-conducting port presents high impedance (open circuit) or low impedance (short circuit). This prevents the signal input to this port from passing through and instead reflects it directly back to the signal source. For an absorptive RF switch, when it is in the "off" state, the non-conducting port is connected to a matched load (typically a standard 50Ω impedance). The signal input to this port is converted into thermal energy via the load resistor and dissipated (absorbed), thus avoiding signal reflection.
Advantages of absorptive RF switches: They absorb signals when turned off, featuring high isolation and excellent VSWR (Voltage Standing Wave Ratio), which reduces interference.
Application scenarios of absorptive RF switches: Suitable for scenarios requiring high signal purity, such as communication receiving systems and test equipment like spectrum analyzers.
Advantages of reflective RF switches: They have fast switching speed (nanosecond-level), large power handling capacity, and some are compact in size.
Application scenarios of reflective RF switches: Ideal for applications requiring high-speed switching, such as radar transmitters, satellite communications, and high-speed data transmission systems.
- Process variation is the most fundamental reason for the deviation in static gate voltage. During semiconductor wafer manufacturing, although the process is strictly controlled, subtle differences still exist in microscopic parameters such as material doping concentration, epitaxial layer thickness, gate length, and gate width.
- Process variation leads to threshold voltage (Vth) differences of several tens of millivolts even between two adjacent chips on the same wafer. Since Vgs and Idq are determined by Vth (for FETs, Idq ∝ (Vgs - Vth)²), a minor change in Vth can cause significant fluctuations in Idq. The static gate bias range of our company's products is < ±500 mV, and the design should reserve flexibility for adjusting the gate voltage during application.
- As voltage-controlled current devices, the quiescent current of GaN HEMT devices is one of the most critical indicators for determining their operating state. In practical applications, it is necessary to adjust the gate voltage to ensure the device's quiescent current reaches the recommended value.
"Inductance" is merely a parameter that describes an inductor under ideal DC or low-frequency conditions. Actual inductive components exhibit parasitic capacitance (C_p) and parasitic resistance, which vary depending on factors such as package size, ratings, material properties, and winding density, ultimately affecting the performance of the inductor. Therefore, when selecting an inductor, pay attention to the following points:
- Self-Resonant Frequency (SRF): It must be significantly higher (typically at least twice) than your operating frequency.
- Q Factor: Ensure that the Q factor at your operating frequency is sufficiently high to meet insertion loss and selectivity requirements.
- Rated Current: Avoid magnetic saturation, especially when used in output matching for power amplifiers.
- Package and Size: These affect parasitic effects and circuit board layout.
No, selection cannot be based solely on matching capacitance and tolerance values.
- For peripheral matching components used in high-power applications, all specification dimensions must align with the provided component specifications. Taking capacitor components as an example, parameters such as capacitance value, voltage rating, tolerance, Q factor, and package form factor can all impact high-frequency performance.
- The impact of capacitance value is straightforward to understand. Insufficient voltage rating may lead to component breakdown and reliability issues; excessive tolerance can cause deviations in frequency response compared to the recommended application. The Q factor affects insertion loss and bandwidth characteristics, while in high-frequency applications, differences in package form factor can significantly alter frequency response and also influence power handling capability.
- If alternative components need to be selected, the capacitance value should remain the same. The voltage rating (VDC) must be greater than three times the supply voltage. The tolerance should be no wider than that of the recommended component, and the Q factor and package form factor should be comparable to those of the recommended component.
- Mismatched parameters of the loop filter. The loop filter is the core for controlling the dynamic characteristics of the loop. Improper parameter design will lead to loop instability, which may cause the frequency synthesizer to lose lock.
- Abnormal reference clock signal. The reference signal is the benchmark for the frequency synthesizer to lock, and its instability or distortion will directly lead to loss of lock.
- Errors in the configuration word cause incorrect switches inside the circuit to be turned on or off, thereby leading to the loss of lock of the frequency synthesizer.
- Generally, a frequency synthesizer widens its RF output frequency range by integrating multiple voltage-controlled oscillators (VCOs) internally.
- Inside the frequency synthesizer, the RF frequency of the voltage-controlled oscillator (VCO) is divided by the subsequent RF frequency divider to broaden the frequency band range of the frequency synthesizer.
- The wide output frequency range is achieved by selecting, through the subsequent gating circuit, either the RF frequency of the voltage-controlled oscillator or the frequency after frequency division for output.
SPI is a synchronous serial communication protocol, mainly used for short-distance, high-speed communication between chips. Its core features are "synchronization" (relying on a clock signal to synchronize the master and slave devices), "full-duplex" (able to send and receive data simultaneously), and "master-slave architecture" (typically one master device controls multiple slave devices).
The SPI standard defines four core signal lines:
- SCLK (Serial Clock): Clock line, output by the master device, used to synchronize the data transmission rhythm between the master and slave devices.
- MOSI (Master Output Slave Input): Master-to-slave data line, through which the master device sends data to the slave device.
- MISO (Master Input Slave Output): Slave-to-master data line, through which the slave device sends data to the master device.
- SS/CS (Slave Select/Chip Select): Chip select line, controlled by the master device (active low), used to select the slave device to communicate with.
The anti-blocking amplifier, prized for its “high IP1 + low noise” performance, has become the go-to device for rejecting strong interferers in RF front ends. Yet its internal bias inductors—ranging from tens to hundreds of microhenries—can form a low-frequency series resonance with any mismatched trace or matching capacitor. Amplified by the device’s non-linearity, this resonance appears as modulation spurs alongside the desired signal. To prevent this, three design rules must be observed:
- Reproduce the exact component values given in the data sheet.
- Decouple the supply with “small + large parallel capacitors and a π-filter” to suppress ripple and resonance.
- Connect the ground pad through multiple vias directly to the ground plane to shorten return paths and lower Q.
If a narrow-band in-band filter follows, its out-of-band low-frequency region is highly reflective. Reflected energy can return to the amplifier, forming a second amplification loop that regenerates spurs. To break this loop, place a 1 µH shunt inductor to ground after the DC-blocking capacitor. Choose a part with SRF > 3 GHz and low DCR. This inductor shorts reflected energy below 1 GHz, drops loop gain by more than 20 dB, and the spurs disappear.
Log detectors and RMS detectors are core amplitude detection devices in the RF/microwave field, with three key differences:
- Detection Principle: Log detectors utilize the nonlinear volt-ampere characteristics of diodes to convert the logarithm of signal amplitude into a DC voltage. RMS detectors, based on the principle of energy equivalence, calculate the Root Mean Square (RMS) value of the signal and convert it into a DC voltage.
- Output Characteristics: The output of a log detector exhibits a linear-logarithmic relationship, featuring a wide dynamic range and fast response but low accuracy, and it is sensitive to waveforms. The output of an RMS detector shows a linear-energy relationship, with a narrow dynamic range and slow response but high accuracy, and it is insensitive to waveforms.
- Applicable Scenarios: Log detectors are used for RF signal strength monitoring, receiver Automatic Gain Control (AGC), interference signal monitoring, and peak power monitoring. RMS detectors are applied in audio signal amplitude measurement, RF power meters, communication system bit error rate testing, and non-sinusoidal signal measurement.
The main functions of a PI-type attenuator are as follows:
- Precisely control signal amplitude to prevent circuit overload;
- Match the impedance of different modules to reduce signal reflection;
- Adjust the signal dynamic range to optimize system performance.
In a 50Ω system, the core formulas for calculating the resistor values of a PI-type attenuator (PI attenuator) are based on the characteristic impedance Z0=50Ω and the attenuation amount A (unit: dB). First, calculate the voltage attenuation coefficient k=10^(A/20) using the attenuation amount, then calculate the three resistors respectively:
- Series arm resistors:R1=R3=Z0*(k-1)/(K+1)
- Shunt arm resistor: R2=Z0*2k/(k^2+1)
(Note: Z0=50Ω，k=10^(A/20)，A is the required attenuation amount (in dB))
In a demo board, the main function of a de-embedding line is to eliminate the impact of non-Device Under Test (DUT) components—such as test fixtures and connecting lines—on measurement results, thereby obtaining the performance parameters of the DUT itself more accurately.
In high-frequency circuit testing, the DUT on a demo board usually needs to be connected to test instruments (e.g., vector network analyzers) via test fixtures and connecting lines. These test fixtures and connecting lines introduce additional losses, reflections, and phase shifts, which can affect the accuracy of DUT measurements. Through a specific circuit structure and measurement method, the de-embedding line removes the influence of these non-DUT components from the overall measurement results. For example, by measuring the characteristic parameters of the de-embedding line itself, its impact on the signal can then be subtracted from the overall measurement results (which include the fixture and DUT) during data processing. This ensures that the measurement results more truly reflect the DUT’s performance, enabling accurate acquisition of parameters such as the DUT’s impedance, insertion loss, and reflection coefficient.
For the local oscillator (LO) amplification at the mixer end, low phase noise (LPN) amplifiers are preferred over low noise amplifiers (LNAs).
Mixers achieve frequency conversion through the nonlinear mixing of LO and radio frequency (RF) signals. During this process, the phase noise of the LO is directly transferred to the output signal, and its impact on performance is far greater than that of the amplitude noise which LNAs primarily suppress:
If the LO has poor phase noise, the output signal spectrum will exhibit spurious expansion, leading to increased adjacent channel interference (e.g., degraded adjacent channel leakage ratio) or reduced frequency resolution (e.g., difficulty for a spectrum analyzer to distinguish weak signals).
If the LO has slightly higher amplitude noise, as long as it does not exceed the linear dynamic range of the mixer, its impact on key indicators is minimal and can be further suppressed by subsequent intermediate frequency (IF) filtering.
The core requirement for LO amplification is to ensure phase purity, and LPN amplifiers are well-suited to this need. Misusing LNAs will degrade the phase noise of the output, impairing system performance (e.g., channel isolation, frequency resolution).
Our company offers a range of high-performance LPN amplifiers (such as the BR9192TA, BR9191TA, and BR9108TA), which feature an additional phase noise as low as -165 dBc/Hz at a 10 kHz offset. These amplifiers are ideal for LO signal amplification in frequency conversion systems.