Frequency modulated radio transmitter

摘要

The transmitter has a reference source (8) fed to a first phase lock loop (4), the loop VCO (14) being supplied with an information signal (SM) to produce an FM output signal (SI). The loop filter of the first PLL is of relatively narrow passband to enable frequency modulation to occur. The modulated output signal is applied to a second PLL (5), with a relatively high cut-off frequency loop filter and a VCO (15) providing an output (SE) to feed a frequency translation circuit (55). The latter also receives an unmodulated signal (ST), pref. output by the VCO (16) of a third PLL (6). The translation circuit includes two mixers (57, 58) and a summing circuit (61) arranged to combine the two input signals (SE and ST) and 90 degrees phase-shifted versions of them, so as to eliminate the sum component of the two frequencies from the output spectrum. The third loop may include a variable ratio divider (46) to allow duplex operation on a number of channels.