| 摘要 |
A system of frequency-shifting of frequency modulated signals, continuously above or below a pre-established frequency, the shift being possibly also null. The system includes a source of frequency modulated signals, the frequency of which is to be shifted; a reference oscillator; a first phase locked loop to which is applied the signals from the signal source and the output of the reference oscillator, the arrangement being such that the first phase locked loop will be locked to a conversion frequency above or below the frequency of the signal source. The output of the first phase locked loop is applied to a second phase locked loop together with a variable frequency signal produced by an oscillator which can be frequency controlled, the arrangement being such that the second phase locked loop will be locked to a conversion frequency above or below that of the first phase locked loop so that the output frequency of the second phase locked loop will be greater than, equal to, or less than the frequency of the signal source according to the value of the frequency of the frequency controllable oscillator.
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