A silicon diode is connected in series with the input of an operational amplifier having a feedback resistor, and the modulated ac signal to be detected is fed to the input side of the diode. The signal current through the diode is an accurate function of only the voltage across the diode, and the output voltage of the operational amplifier is proportional to such diode current. Preferably, the frequency pattern has a triangular shape. Such a frequency characteristic is advantageous in that the output signal as a function of frequency has a much more uniform i.e. trapezoidal pattern than in the case of a conventional detector where a substantially rectangular frequency characteristic occurs. This is especially important to FSK modulation with a large frequency deviation for which the modulated signal spectrum has two relatively discrete power peaks.
1 is a schematic diagram of a typical conventional detector. Needs to review the security of your connection before proceeding. Detector as claimed in Claim 7, characterized in that a pair of parallel-arranged diodes having opposite polarities are included. 4 shows the well-known equivalent-circuit diagram of an emitter-follower. Figure 1 shows the circuit of a square-law detector. The charging time constant RsC should be short compared to the carrier period 1/fc.
Hence, this diode current i containing all these frequency terms is passed through a low-pass filter which allows to pass the frequencies below or up to modulating frequency ωm and rejects the other higher frequency components. Thus, the modulating or baseband signal with frequency ωm is recovered from the input modulated signal. The anode 6 of diode Dlis connected directly tothe input of an operational amplifier 7. Asillustrated, this may be a differential amplifier, with the diode being connectedto the minusinputterminal 9.
Course,a verysmall voltagemustappearrat this amplifier input to make the amplifier work, but the operational amplifier 7 has a very high gain so that the anode 6 of diode D1 remains essentially at zero potential. Therefore, the signal current through diode D1 is a function of only the applied voltage at the cathode 5. The output signal taken between the amplifier output terminal 11 and ground is the .desiredsignal which is representative of the current through diode D1. Operational amplifier 7 functions as a currentto-voltage converter.
This process of detection is exactly opposite to that of modulation . The invention relates to a square-law detector having an input, an output and means for providing a square-law relationship between the amplitude of the signal applied to the input and the amplitude of the signal produced on the output. In the positive half cycle of AM wave, the diode conducts japanese father in law forcing and the capacitor charges to the peak value of AM wave. When the value of AM wave is less than this value, the diode will be reverse biased. Thus, the capacitor will discharge through resistor R till the next positive half cycle of AM wave. When the value of AM wave is greater than the capacitor voltage, the diode conducts and the process will be repeated.
Download more important topics, notes, lectures and mock test series for Defence Exam by signing up for free. Briefly, my invention comprises a square-law diode having one lead connected to an input of an operational amplifier and the other lead adapted to be connected to a source of modulated ac signal to be detected. Feedback means areconnected around the operational amplifier to maintain the said input thereto at an essentially constant potential during operation. Bias resistance means are preferably connectedfroma suitable power supply to opposite sides of the diode.
But, this time constant should not be too long which will not allow the capacitor voltage to discharge at the maximum rate of change of the envelope . An envelope demodulator produces an output signal that follows the envelope of the input AM signal exactly . It is used in all the commercial AM radio receivers . A further object is to provide a square-law detector in which means are included to bias the diode at a desired dc operating point. Detector as claimed in Claim 5, characterized in that the frequency characteristic shows a substantially triangular pattern. Surprisingly, the invention has shown that an emitter-follower circuit having a capacitance load approaches the desired frequency characteristic extremely well.