Amplitude modulation by selective transistor amplifier
In this article, I will explain Amplitude modulation by selective transistor amplifier. In this I explained the selective transistor amplifier along with the circuit diagram, mathematical derivation and frequency spectrum.
The circuit shows an AM modulator. The two signals, LF and RF are additively superimposed before the modulation on the base resistance (1). The amplitude of the sum signal is sufficiently large in the positive range, and controls the transistor characteristic in the entire curved portion of the base-emitter path of.
The highly distorted collector output signal is at a tuned to the RF carrier frequency oscillation circuit, filtered out. The output of the resonant circuit, the AM signal.
This circuit has been tested under laboratory conditions simulated in the simulation program. To better illustrate the AM modulated with a single tone frequency constant. The following graph shows for each measurement point, the line graph and the Fourier analysis. At the measurement point (2), first without, then with NSHAPE measured resonant circuit.
The beat signal at the measuring point (1), the addition signal of the modulating signal and the carrier to the effective resistance. At any time, the amplitude values of LF and HF are only added. Within a 10 millisecond oscillating characteristics of the carrier are visible. The maxima and minima of the signal, the frequency bands of the AF signal. Superposition of the signal from the output signals can be recovered by suitable filters. The frequency spectrum shows the frequency curve of the audio signal of 1 kHz and at 10 kHz of the carrier. There are no new frequencies added.
The middle line chart is amplified by the transistor collector output without an activated resonant circuit. Since the transistor has no preset operating point, it is controlled by only the positive signal components of the sum signal, the entire curved line portion of the characteristic base-emitter diode will be run. On this curve, the sum signal is potentiated. This multiple multiplication creates various modulation products. Additional frequency lines in the frequency spectrum adjacent to the single, double and triple carrier frequency can be seen. They are arranged symmetrically with respect to the support line at a distance of ± 1 kHz, the modulation or signal frequency. In accordance with the power series shown above, the amplitude of the high integer multiple of the carrier frequency is decreased rapidly.
The bottom two graphs show the AM signal after filtering by the switched resonant circuit. The resonant frequency of the parallel resonant circuit is equal to 10 kHz with the carrier frequency. When properly sized bandwidth of the resonant circuit to filter out all about low and high frequency bands from the overall spectrum.
In the AM signal, the original information signal is no longer included. Right and left to the carrier frequency line, two side lines of equal height at a distance of frequency information. The information is encoded or modulated there. The left side line is located at 9 kHz (carrier frequency - frequency information) and the right side line at 11 kHz (carrier frequency information + frequency).
In technical applications, the carrier frequencies are much higher. The broadcasting operation used for medium wave RF carrier between 510 ... 1605 kHz. In the FM broadcast band, the stereo audio signal is modulated at 38 kHz and on television the chrominance signal to a carrier frequency of 4.43 MHz.
At the two other sides of this web project the interference signals such as sinusoidal modulation superimposed and can be interactively evaluated by varying the frequency and amplitude. The second flash step the film illustrated by the model of AM rotating pointer. The explanations are illustrated by line diagrams, phasor diagrams and the corresponding spectra of the Fourier analysis.
Some parameters can be quite easily determined from the timing diagram of an AM signal. These include the carrier frequency, with a single-tone AT, the signal frequency and the degree of modulation.
In the timing diagram with a suitable resolution may be determined for each time period, the carrier frequency. The amplitude of the carrier varies symmetrically with the rhythm of the frequency information. For the LF frequency, there is no closed curve. It is implicit in the period of the so-called envelope. The oscillogram the side frequencies are not apparent. Is determined in the timing diagram, the maximum and minimum deflection of the AM signal, then, as described above, the degree of modulation m to be calculated from the ratio of the distance to the track difference sum.
In transmitter operation, the quality of the modulation signal is controlled by the trapezoidal modulation. This oscillogram obtained in the A / B operation, where the AM signal, the channel A and channel B on the low-frequency information signal is connected.
Here, too, can be calculated from the two left and right vertical deflection derived using the above equation (5) the degree of modulation.