Class C frequency doubler

Frequency multipliers are widely used in electronic equipment such as radio transmitters and frequency synthesizers. Its function is to transform the input signal with frequency fc into the output signal with frequency nf c (n is a positive integer). The advantages of using a frequency multiplier are:

1. It can reduce the main vibration frequency of electronic equipment, which is beneficial to improve the frequency stability of the equipment. Because the higher the frequency of the oscillator, the worse the frequency stability. Generally, the frequency of the main oscillator should not exceed 5MHz. Therefore, when the transmitter frequency is higher than 5MHZ, a frequency doubler is usually used.

2. Under the condition that the working band of the main vibrator of the communicator does not expand, the working band of the output stage of the transmitter can be expanded by a frequency multiplier. For example, the main vibrator works at (1.5 ~ 3) MHz, and then uses an amplification and doubling stage, which is under the control of the band switch, which can work in both the amplified state and the double or quadruple frequency state. . In this way, with the change of the band switch, the transmitter output stage can obtain (1.5 ~ 3) MK (3 ~ 6) M Yihe 6 ~ 12) MHZ three band output.

3. In frequency modulation and phase modulation transmitters, the use of frequency multipliers can increase the frequency shift or phase shift, which can deepen the modulation depth.

The frequency doubler can be divided into three categories according to its working principle. The first type is the harmonic multiplied by the harmonics in the current pulse of the Class C amplifier. The second category is the use of analog multipliers to achieve frequency multiplication. The third type is to use the nonlinear change of the PN junction capacitance to obtain the harmonics of the input signal frequency, and obtain the frequency multiplier through the frequency selection loop, which is called the parametric frequency multiplier. When the operating frequency is tens of MHz, the triode type C frequency doubler is mainly used, and when the operating frequency is higher than 1000MHz, the parametric frequency doubler composed of a varactor diode and a step diode is mainly used. The frequency multiplier composed of multipliers is mainly limited by the upper limit operating frequency of the multiplier. At present, the upper limit working frequency of the multiplier can be 1000MHz. This section only introduces the basic principle of the class C frequency multiplier.

In class C operation, the collector current pulse of the transistor contains rich harmonic components. If the collector resonance circuit is tuned to the second harmonic or third harmonic frequency. Then, the amplifier has only the second harmonic voltage or third harmonic voltage output. Such a class-C amplifier becomes a doubler or tripler. The instantaneous value of the input and output voltage of the frequency doubler can be written as

u b = U bm cosωt

u c = u cmn cosnωt

And the instantaneous voltage between transistor poles can be written as

U bee = V bb + U bm cosωt

uce = V cc -U cnm cosωt

In the formula, U cnm is the amplitude of the nth harmonic voltage across the loop.

Using the analysis results of the high-frequency power amplifier, the output power and efficiency of the n-th frequency multiplier is

Pon = 1 / 2UcnmIcnm = 1 / 2UcnmI cM α n (θc)

ηcn = Pon / P == UcnmIcMαn (θc) / 2VccIco1 = Ucnmα n (θc) / 2Vccα 0 (θc)

It can be seen from the cosine pulse decomposition coefficient that regardless of the value of the pass angle θc, α (θc) is less than α 1 (θc), that is, under the same conditions, the output power of the class C frequency doubler is lower than the output of the class C amplifier Power and efficiency.

In order to improve the output power and efficiency, the selection of the pass angle θc of the class C frequency doubler must meet

θ c = 120 / n

Because n = 2, θc = 60, α 2 (θc) is the maximum; when n = 3, θc = 40 · α 3 (θc) is the maximum. Therefore, the θc of the frequency doubler should be 60. The θc of the tripler should be 40. . In this way, the corresponding output power and efficiency are maximized.
Single-stage Class C frequency multipliers are generally only used as double or triple frequency multipliers. If you want to increase the number of frequency doubling, you can use a multi-stage class C frequency multiplier to achieve.