Direct-frequency modulation's principle of work is: With the modulation signal positive governing self-excited oscillator's circuit value or the active status, cause its oscilation frequency to receive the modulation, the varactor frequency modulation, DVB-T TUNER the reactance tube frequency modulation and the relaxation frequency-modulation oscillator and so on belong to this kind. Takes the frequency modulation component in the microwave region broadcast frequency modulator commonly used klystron.
Built-in digital tuner chart 1CE right side (the T2 part) is the general pure oscillator. Left side (T1 part) the electric circuit is called the reactance tube, it amplifier which and RC shift circuit constitution constitutes by transistor T1, its function equivalent in an electric capacity.
The modulation signal adds in the T1 base electrode, when changes namely along with it change, therefore oscillator's output is a frequency-modulated wave. Reactance tube's enlargement component may be the electron tube, the transistor or the field effect transistor; The shift circuit also has many kinds of patterns (for example RC or RL phase shift network), its function is enables amplifying tube T1 output impedance Ze=U0/IC to have component reactance Xe, when Xe along with changes, then obtains the frequency-modulated signal.
The built-in digital tuner uses the different shift circuit, equivalent reactance Xe may be capacitive character, also, but inductive character.DVB-T MPEG 4 The reactance tube frequency modulator's shortcoming is: The oscilation frequency stability is not high; The frequency deviation cannot be too big, impedance Ze usually also has the active component, this component also along with changes, causes the oscillator to have the parasitic amplitude modulation. This kind of frequency modulator's merit is the electric circuit is quite simple, the early frequency modulation installs often uses this kind of electric circuit, latter has been replaced gradually by the varactor frequency modulator.
In vehicle's frequency modulator realizes frequency modulation's electric circuit with the controllable autoexcitation multivibrator (Figure 2). Transistor T1, T2 and electric capacity C1, C2 constitution multivibrator. The modulation signal through transistor T3, T4 control capacitance C1, the C2 charging and discharging electric current, causes the multi-harmonic vibration frequency to produce the frequency-modulated wave change of along with the high clear digital frequency modulator.
This kind of frequency modulation electric circuit is quite simple, the frequency deviation is big, the modulation linearity is good, moreover does not have the parasitic amplitude modulation nearly; But the output is the rectangular wave, includes the rich harmonic component, must use the low pass filter to filter can obtain the sine wave frequency-modulated signal.DVB T RECEIVER This kind of electric circuit's shortcoming is the frequency stability is bad, the oscilation frequency is also low. Commonly used in take audio frequency as carrier low-speed data transmission facility.
May also constitute the frequency
modulation electric circuit using other pattern's relaxation
oscillator, this kind of electric circuit does not use the
inductance, easy integration.
