Demodulation of FM signals in detail
In this article, I will explain demodulation of FM signals considering Slope detector, Foster-Seeley discriminator, Ratio detector, Counting discriminator with a detailed circuit diagram and frequency spectrum.
In the frequency-modulated signal, the low frequency information is encoded in the zero crossings of the radio frequency FM signal. The demodulation circuit must be able to convert this into a frequency change amplitude curve. This task can perform a frequency-dependent quadrupole who works optimally only in the area of ??the demodulated FM. FM demodulation to a number of circuits have been developed. The best known are described below.Slope detector
The traditional FM demodulator using a resonant circuit. It converts the FM signal to be demodulated in an amplitude modulated signal, which is then converted by a demodulation in the NF-AT. The FM-AM converter discriminator (Selection, separating circuit) called and referred to the operation as Slope detector. The resonant frequency of the resonant circuit is above or below the FM carrier frequency. The resonant circuit should have a high quality, so that the work area has a linear ramp as long as possible. The FM carrier frequency is set to the edge center.
The image shows a simple FM demodulator according to the principle of Slope detector. The resonant frequency of the resonant circuit is 159.155 kHz, the bandwidth of 160 Hz, corresponding to the high Q factor of nearly 1000th The following AM demodulator charged to the resonant circuit and thus reduces the quality factor and increases the bandwidth. The resonant circuit is tuned to the IF frequency, where the FM-modulated carrier, is located on the edge at -6 dB. The passage of the discriminator curve shows that even to -9 dB there is no ideal linear characteristic curve in the narrow working range between -3 dB. This circuit can convert only FM signals with low bandwidth distortion in AM signals. The downstream AM demodulator generates the low-frequency signal.Quadrature or Foster-Seeley discriminator
The changes in frequency of the FM signal to be transferred only in phase angle, and then changes in voltage change representing the demodulated AF signal. The input circuit is tuned to the FM center frequency 2-circuit bandpass filter. With inductive coupling of the phase angle between the secondary and primary circuit at resonance is exactly 90 °. The converter was designed by Foster and Seeley and is named after them as Foster-Seeley discriminator.
The final IF amplifier which carries out the feed in the primary circuit should be a limiter amplifier. The two resonant circuits of the filter are tuned to the IF frequency. The coupling capacitor does not affect the resonant frequency, but passes the FM signal on the center tap of the secondary coil of the RF choke L 3rd It is a short circuit for low diode currents. The inductively coupled to the secondary circuit FM signal is divided on the basis of the center of phase of the two winding halves.
For an unmodulated FM carrier signal, the secondary circuit behaves like an ohmic resistance at resonance. The two sub-voltages across the diodes resulting from the geometric addition of the secondary half-circuit voltage and the voltage across the inductor, which form a phase angle of 90 °. The capacitors provide an RF short circuit represents the voltages across the diodes u 1 and u 2 are the same size and load on the filter capacitors. Related to the mass sum circuit voltage is the demodulated AF signal and is 0 V.
Above the resonant frequency of the secondary circuit acts inductively. Based on the voltage across the inductor to the power running. The part of tensions in the secondary circuit and remain out of phase with respect to the center tap of the same size. The geometric addition u L3 leads to different sized diodes partial voltages. The filter capacitors are charged differently. Your generates a voltage difference AF output voltage. Below the resonance to reverse the phase angles. The secondary circuit behaves more capacitive, the current lags the voltage at the throttle before it creates an opposing LF output signal.Ratio detector
In ratio detector, the diodes are connected in anti-parallel in series. The AF signal is picked up at an additional resistor between the two diodes to ground. An electrolytic capacitor at the output in parallel with the load resistors is noise blanking and generates a control voltage. This is not necessary for the demodulation of FM. The circuit shows the balanced ratio detector.
The origin of the LF signal is explained by the current sum, the three leads to the resistor R NF-voltage. For the currently drawn polarity of the voltages on the part of the secondary circuit flows through the upper diode, a positive and a negative lower through the diode rectifier current. Outside the FM center frequency of the currents vary in size and the resistor R 3, the FM demodulated be measured.
The operation of the newly added Capacitor C ratio can be described as follows. The voltage across the Capacitor C ratio results from the sum of the two peak values of the rectified voltages of the RF section of the secondary circuit. It is thus a measure of the reception field strength of the FM-modulated transmitter, or fed into the primary circuit signal strength. The modulation has no influence on the charging voltage, because decreases in accordance with a larger current through the current through R 1 R 2nd The charging voltage at Capacitor C ratio is used to gain control of precursors.
If the RF-IF-voltage superimposed with a positive noise amplitude as an AM component, as to the conductivity of the upper diode and its differential resistance decreases. The load on the secondary resonant circuit, its circular damping increases and the circuit voltage. The larger current through the upper diode charges the Capacitor C ratio to positive. The effluent charge current can thereby also be lower diode conducting and supporting the damping circuit. This process counteracts the perturbation and can be referred to as noise blanking.
A negative error is corrected in the reverse direction as well. If the secondary circuit again without problems, so the capacitor discharges by shifting the diode operating points in a more negative range, to its original state of charge is restored. The control constant t should be 100 ms for the VHF FM reception. t = r D · C ratio = 100 ms.
The circuit variant shown in the picture is called unbalanced ratio detector, because the NF is not released as a bridge voltage. There are fewer components but requires the AM interference can not compensate as well. The capacitors C 1 and C 2 are working as an RF short circuit. The two diodes in series to the resistors of the same resistance and diodes differential so symmetrize the operation of the demodulator. The following work on the resistance RC low-pass is responsible for the de-emphasis in FM radio reception.Counting discriminator
The basic idea is to charge and discharge of a capacitor through a constant current. The current through C is dependent on the frequency. Frequency changes in the FM signal can thus be converted into electricity flow changes. Labor resistance resulting voltage proportional changes. The circuit is called counting discriminator can be built and integrated is widely used in televisions for audio demodulation.
The FM signal can be mixed down to a lower IF, and is converted by a limiter upstream in a rectangular FM signal. The FM signal is differentiated and limited by a diode. The needle-shaped pulses at (I) control a transistor-based circuit switching stage. Labor resistance, rectangles form (II) with variable pulse-pause times. The waveform in (II) is only measurable without charging capacitor. The low-pass counts and integrates the pulses. The voltage-time areas are balanced to the reference voltage, which adjusts for the FM center frequency. The voltage changes at C2 in the low-frequency signal and be further strengthened.Source of images
Own created images using CAD software