High-linear amplitude modulator. Encyclopedia of radio electronics and electrical engineering

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An amplitude modulator having good linearity can theoretically operate at a modulating signal frequency equal to the carrier frequency. Transistor Q1 divides the modulating input voltage into two anti-phase bipolar signals. Switches on transistors Q2 and Q3 pass, respectively, the positive and negative half-cycles of the rectangular carrier. Interrupted modulated signals (points C and D) are summed using resistors R5 and R6.

The amplitude modulator, the circuit of which is shown in the figure, has good linearity and operates when the frequency of the modulating signal changes from zero to half the carrier frequency. The linearity of the circuit is maintained up to a modulation factor of 97,5%. The connection between the individual stages is carried out galvanically without the use of inductances or large capacitances.

(click to enlarge)

Transistor Q1 is a phase splitter of the modulating signal, while the signal at the emitter of Q1 has a phase shift and an amplitude slightly lower than the input level. The DC component of the modulating signal is approximately -5 V at the emitter of transistor Q1 and +5 V at its collector, where the signal phase is shifted by 180° with respect to the input. The high-speed switches on transistors Q2 and Q3 alternate between saturated and off in response to an input carrier signal. This signal, preferably of a rectangular shape, is fed to the bases of transistors Q2, Q3, respectively, through resistors R1, R2 and diodes D1, D2. Diodes protect transistors from increased base-emitter reverse voltage that can occur with a high carrier level. Capacitors C1 and C2 serve to reduce the switching time of transistors Q2, Q3.

The collectors of transistors Q2, Q3 are connected to the outputs of the phase splitter Q1 through resistors R3 and R4. These resistors are used to decouple the baseband and baseband circuits. In each positive half-cycle of the carrier, the modulating signal at the collector of transistor Q1 is switched from its average value of 5 V to zero by transistor Q2. As a result, an intermittent modulating signal is formed at the collector of transistor Q2. Similarly, the modulating signal at the emitter of transistor Q1 is interrupted by transistor Q3, with transistor Q3 transitioning from off to saturation during each negative half cycle of the carrier.

Positive and negative intermittent modulating signals are mixed in a simple summing circuit consisting of resistors R5 and R6. When summed, the chopper frequency components present in the discontinuous baseband signals cancel each other out. Thus, in the case of an ideal balance, there are no components with a modulation frequency in the spectrum of the output modulated signal, and only side components of the modulation are present. Theoretically, in this case, it is possible to increase the frequency of the modulating signal to an upper limit equal to half the carrier frequency without applying complex filtering. The envelope of the modulated signal is in this case in antiphase with respect to the input modulating signal.

The output voltage of the circuit is an amplitude-modulated square wave, which itself contains the odd harmonics of the fundamental frequency. (The output signal spectrum can be written as nwc±wm)m, where wc is the carrier frequency, wm is the baseband frequency, and n=1; 3; 5; ... .) To obtain a sinusoidal carrier, the output must be filtered. A low-pass filter can be used to extract the fundamental frequency of the carrier and its sidebands, since the output signal spectrum does not contain a component with the modulation frequency. However, to isolate any harmonic wc, it is necessary to use a band-pass filter.

The frequency properties of the modulator mainly depend on the speed of the switching transistors. For the transistors shown in the figure, the upper frequency of the modulated output signal is 1 MHz. The modulator itself has a flat frequency response and remains linear up to a modulating frequency of 250 kHz, after which the envelope distortion becomes noticeable even to the eye. With a carrier frequency of 100 kHz and a modulation frequency of 1 kHz, linear modulation with a depth of up to 95% can be obtained.

In open-loop mode, the maximum modulated output swing is 7,4V with a 14V input swing. The minimum carrier swing at the modulator input to produce a square wave output is 2,8V. any unwanted effects. The shape of the modulating signal can be arbitrary.

It is also possible to use a sinusoidal signal as a carrier, however, this worsens the interruption process. The minimum peak-to-peak sine-wave carrier is 4 V. With a carrier frequency of 10 kHz and a modulating signal swing of 14 V, linear modulation up to 97,5% can be achieved.

The minimum carrier drive level does not change much at lower carrier frequencies. At the same time, the technical characteristics of the modulator deteriorate somewhat at higher frequencies - the maximum depth of linear modulation decreases and becomes equal to 94% at 500 kHz and 88% at a frequency of 1 MHz. At higher frequencies, the output level also decreases. To expand the frequency range, you can use faster key transistors and reduce the impedances of the circuit stages.

It is also possible to prevent a decrease in the output signal at high frequencies by increasing the supply voltages. The maximum modulation depth is theoretically limited by the saturation voltage of the chopper transistors; this voltage does not have such a strong effect at high supply voltages. The use of pairs of resistors (R3-R4, R5-R6, R7-R8) selected with great accuracy ensures the equality of positive and negative instantaneous values ​​of the output modulating signals.

Authors: Santa Fe College (Gainesville, Florida); Publication: N. Bolshakov, rf.atnn.ru

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