| 摘要 |
PURPOSE:To detect the reception state of an image signal securely through simple constitution by comparing the mean DC level of an in-phase demodulation output by PLL synchronous demodulation with a reference DC level, and displaying the result on a meter. CONSTITUTION:An input signal fc to be demodulated is mixed 21 with a local oscillation signal fl from a digital synthesizer part 22 and converted into an intermediate frequency signal, which is inputted to the phase detectors 251 and 252 of a PLL synchronous demodulating circuit 25 through a BPF23 and an amplifier 24. The detectors 251 and 252 generate an in-phase demodulation output and an orthogonal demodulation output through LPFs 253 and 254 by mutually 90 deg. out-of-phase carriers from the Johnson (1/4) counter 258 of the circuit 25. The orthogonal demodulation output is passed through a loop filter 256 to control a reference frequency oscillator 221 by its DC component. The in-phase demodulation output is passed through the LPF261 of an image signal reception state detecting circuit part 26 and its mean DC level is compared with the reference voltage Vr from a reference DC voltag generator 262 to display the image signal reception state on a signal meter 263. |
