Jan 02, 2021
Parametric frequency converter, what does parametric frequency converter mean
A mixer is a circuit whose output signal frequency is equal to the sum or difference of the two input signal frequencies or other combinations of the two. The mixer usually consists of non-linear components and frequency selection loops. After the input frequency fc and the local oscillator frequency f1 from the local oscillator are acted on by the mixer, the output frequency becomes fi. Their relationship can be expressed by fi=│±pf1±qfc│, where p and q are any positive integers. If mixing and local oscillation are completed by the same device, it is called a frequency converter.
In addition to the change in the center frequency of the output signal of the mixer, the other parameters, such as the relative relationship between the envelope waveform and the contained spectral components, remain unchanged. The frequency of the output signal is higher than the frequency of the input signal is called up-mixing (changing) frequency, and vice versa, it is called down-mixing (changing) frequency. Transistor frequency converter is a circuit with both oscillation and mixing functions. The transistor T, the inductance coils L4, L3 and the capacitor C3 constitute a mutual inductance coupled oscillator. The signal voltage introduced by the input circuit (L1,) and the local oscillator voltage added to the two ends of the resistor Re are mixed in the transistor, and the difference frequency is taken out by the intermediate frequency transformer to convert the high frequency input signal into an intermediate frequency output signal. , It is a coaxial double-connected capacitor, which can make the local oscillator frequency change synchronously with the input frequency, and ensure that the intermediate frequency of the whole band remains unchanged. The advantages of transistor frequency converters are high frequency conversion gain and good isolation between output and input circuits. They are often used in various superheterodyne circuits.
The parametric frequency converter is a kind of microwave frequency converter. The up-conversion parametric amplifier is actually a parametric frequency converter with gain. Its equivalent circuit is similar to that of a negative conductance parametric amplifier, but the idle frequency loop is tuned to f0 = fs + fP, and is used as an output signal loop. The theoretical maximum gain is f0/fs. The characteristic of this parametric amplifier is that the input and output loops have better isolation, the gain is stable, but the gain is low. A parametric amplifier is an amplifier circuit that uses time-varying reactance parameters to achieve low-noise amplification. For example, when a periodic alternating voltage is applied to both ends of a varactor diode, its capacitance parameter will change periodically with time. If this time-varying capacitor is connected to the signal loop, and when the capacitance change and the signal voltage change meet an appropriate relationship, the signal can be amplified. The applied alternating voltage source is called the pump source. Parametric amplifiers can also be constructed using iron core nonlinear inductance coils and the nonlinearity of electron beams. The principle of parametric amplification appeared in the 1930s, but it was not until the late 1950s that semiconductor varactor diodes that could work in the microwave frequency band were developed. This is because varactors have very high Q values and are suitable for making microwave amplifiers with extremely low noise levels. The varactor parametric amplifier is mainly used to amplify weak signals with a frequency of about 1 to 50 GHz. In this frequency range, its noise characteristics are slightly worse than that of quantum amplifiers, but its structure is simple and maintenance is very convenient. Varactor parametric amplifiers are classified according to their working modes, and there are two categories: negative resistance amplifiers and up-conversion amplifiers. The former can be divided into degenerate amplifiers whose signal frequency and idle frequency are roughly equal (the signal frequency loop can also double as an idle frequency loop) and unequal non-degenerate amplifiers. The varactor parametric amplifier is mainly used to amplify weak signals with a frequency of about 1 to 50 GHz. In this frequency range, its noise characteristics are slightly worse than quantum amplifiers, but its structure is simple and maintenance is very convenient. The noise of practical amplifier is very low. For example, in the 4 GHz frequency band, its equivalent input noise temperature can be as low as 50K or less at room temperature. When the operating temperature drops to 20K, the noise temperature can be as low as 10K.