| 摘要 |
1,268,414. Measuring signal noise ratio. INTERNATIONAL STANDARD ELECTRIC CORP. Aug.5, 1969 [Aug.9, 1968], No.39086/69. Heading G1U. To measure the signal to noise ratio of an input signal spectrum of given amplitude located within a given frequency baseband, the input signal is translated in frequency to occupy a given frequency band. This translated signal is coupled to a device having a passband adjacent to but spaced from the given band so that only noise in the passband is passed. This noise is coupled to a logarithmic amplifier and detector providing a D.C. output proportional to the logarithm of the noise. This D.C. output is applied to a recorder calibrated to read signal to noise ratio of the input signal. The input signal, which may lie within a given frequency baseband 1 determined by an FDM communication system in which the signal is to be used, is applied via an attenuator 2 to a mixer 3, where it is heterodyned with a carrier signal from a crystal oscillator 4, to provide an output centred on a frequency close to the centre frequency of a crystal band pass filter 5. This may be e.g. 9À8 MGz. The frequency of crystal oscillator 4 is chosen so that the translated frequency band of the input signal lies just outside the passband of the filter 5. With a constant amplitude input signal, the logarithm of the noise detected in the filter output corresponds with the signal-noise ratio in the input. The D.C. signal from the detector 7 following the logarithmic amplifier is applied to a recorder 8 and may be applied to control an alarm relay 9 associated with the FDM system to protect it from excessive noise. |
