# The difference between RF and digital circuit design

For high-speed digital circuits, although the voltage is still concerned, the design method is similar to that of the RF circuit, and impedance impedance matching needs to be considered because the presence of the reflected voltage causes an additional error rate.

RF circuit:

1. Focus on impedance matching or power, which is the two most critical parameters in the design. Other intermediate parameters can be determined by power and impedance.

2. Focus on the frequency response, usually in the frequency domain, because the bandwidth range is important for RF circuit modules;

3. I like to use network analyzers, spectrum analyzers or noise testers. These instruments have low input/output impedance, generally 50 ohms, which often affect the circuit. Therefore, it is necessary to measure under impedance matching conditions. ;

4. Typically, the input/output impedance of the RF module is very low, typically 50 ohms. The lower impedance facilitates the transfer of power to a module or part of the circuit, since for a given power P, P = V2 / Z knows that V2 is proportional to Z, and if the impedance is low, that is, the same power can be transmitted with a lower voltage;

5. The RF module preferentially selects a larger drain current, which is more conducive to power transfer for a given voltage;

6. In the communication system, for the receiver, the RF signal needs to be converted before demodulation. Generally, the ratio of the RF signal power to the noise power at the input of the demodulator is greater than 10 dB; for the transmitter, behind the modulator The modulated carrier needs to be power amplified and transmitted to the antenna, which is large enough for transmission to a farther receiver.

Digital circuits:

1. Pay attention to voltage or current, do not care about impedance matching;

2. Pay attention to the waveform or eye diagram;

3. Like to analyze in the time domain, the response speed is very important for digital circuit performance;

4. The test instrument likes to use an oscilloscope to visually explain the performance of the digital circuit. The probe is a high-impedance sensor. When the probe touches the circuit node, it does not interfere with the circuit.

5. The input/output impedance of the digital circuit is very high, which is beneficial to the voltage swing of the circuit module. For a given current, the higher impedance has a larger voltage swing, so that the switching action can be performed;

6. In a communication system, only voltage is required for digital signal processing or conversion, and power conversion is not required.

The key to causing the above contradictions and differences is that there is voltage reflection and power reflection in the actual circuit, so impedance matching is very important for power transmission or power processing in the circuit! The above are the main differences between RF and digital circuits. Of course, there are other differences, such as the same role. In different systems, the terminology is different, or the commonly used units are different. In addition, for high-speed digital circuits, although the voltage is still concerned, the design method is similar to that of the RF circuit, and impedance impedance matching needs to be considered because the presence of the reflected voltage causes an additional error rate. Circuit layout/wiring, AC grounding, or isolation are all treated like RF circuits.

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