| 摘要 |
An improved frequency modulator apparatus for use in phase-locked loop (PLL) frequency synthesizers is disclosed which makes possible high deviation, low distortion, FM signals for a wide range of modulating signals, such as digital data or low frequency tone modulation. In a first embodiment having the improved frequency modulator, this apparatus advantageously injects the modulation signal into two ports, one at the VCO and the other provided in an unmodulated reference source signal chain. By properly balancing the levels into each of these two ports, a cancellation effect occurs which nulls the modulation signal at the output of the phase detector. In a second embodiment, the improved frequency modulator is incorporated in a PLL frequency synthesizer with an offset mixer to achieve relatively wide percentage deviation at a low frequency RF carrier. In a third embodiment, the improved frequency modulator is incorporated in a PLL synthesizer in its feedback loop so as to cancel the modulation components before they reach the phase detector. Thus, in the first two embodiments, the added phase modulator is inserted in the reference source signal path to the phase detector, while in the alternate third embodiment, the phase modulator is inserted into the feedback path and includes a phase inverter stage to correct for the inherent modulation polarity reversal.
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