A high-frequency circuit is just a radio circuit, but it does not involve microwave circuits (microwaves are used to process circuits above 1000MHz, starting from the physical electromagnetic field, which is very different from our common circuits), it is used for transmission, reception, modulation of radio waves , demodulation, amplification, etc.
Digital circuits process digital signals. Digital signals only have high-level and low-level signals <For example, the working range of digital circuits in CMOS technology is 0-3.3v, 0-0.8v is low level, 2.4-3.3v is high level, and other voltages are invalid. All electrical signals are divided into sequences of high and low levels). Suitable for high-speed processing, high-precision processing and computer interface, directly processed by the computer.
Analog circuits do not distinguish between levels, and all continuous signals are processed together (macroscopic physical quantities in nature are continuous) for power supply, amplification, filtering, etc.
Analog circuits and high-frequency circuits are actually very close, but the operating frequency of the circuit is very high, the physical characteristics of many components have changed, and the processing methods and problems are also different.
High-frequency circuit performance including target
High-frequency small-signal amplification has two circuit forms: resonant amplification and broadband amplification. The performance objectives mainly include the following items.
1. Gain
High-frequency circuits, like low-frequency circuits, have indicators of voltage gain and power gain. For a resonant amplifier circuit, it refers to the resonant frequency f0, and for a broadband amplifier circuit, it refers to a range of frequency bubbles.
2. Pass band
Similar to the concept of low-frequency circuits, for a resonant amplifier circuit, the passband refers to the difference between the two corresponding frequencies whose normalized amplitude drops to 0.707 relative to the resonant frequency f0; for a broadband amplifier circuit, it is relative to a segment frequency. Define accordingly.
3. Selectivity
Selectivity is mainly aimed at resonant amplifier circuits, which characterize the ability of the circuit to select useful signals and suppress unwanted signals. It is usually measured by squareness coefficient and suppression ratio, which are based on the vibration characteristic curve of the circuit.
4. Noise figure
When the amplifying circuit is working, the irregular movement of carriers will occur due to various reasons, and noise will be formed inside the circuit, which will affect the signal quality. This effect is usually described by the ratio of signal power Ps to noise power Pn (SNR for short). Noise figure is defined as the ratio of the input signal-to-noise ratio to the output signal-to-noise ratio.
5. Stability
The stability of a high-frequency amplifier circuit refers to the stability of its main performance when the working state or conditions change. For example, changes in ambient temperature or fluctuations in power supply voltage will affect the DC working state of the amplifier circuit; circuit component parameters will also change, resulting in changes in the gain of the amplifier circuit, center frequency shift, and resonance curve distortion. Even self-excited and completely unable to work.
