Telecommunications – Transmitter and receiver at same station – With frequency stabilization
Patent
1997-07-22
1999-11-23
Bost, Dwayne D.
Telecommunications
Transmitter and receiver at same station
With frequency stabilization
455118, 455260, 455262, H01Q 1112, H04B 106
Patent
active
059916052
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method and circuit for creating a modulated signal with a frequency synthesizer based on a phase-locked loop. A fixed-frequency reference signal and a signal created by down-converting from the output signal of the frequency synthesizer are input to the phase comparator of the loop.
A known method of creating a modulated transmitting frequency is shown in FIG. 1. The transmission frequency f.sub.TX is generated in a closed phase-locked loop by controlling a voltage-controlled oscillator 109 (VCO) with a control voltage V.sub.CMOD which is formed from the loop's internal control signal V.sub.C and an external modulation signal MOD, which are summed in a summing circuit 110. As is characteristic of a phase-locked loop, the control voltage V.sub.C is formed with a phase comparator, which is part of the phase-locked circuit 107 (PLL), and a loop filter 108 (LPF) by creating a signal that is proportional to the phase difference between the output signal of the VCO 109 and a reference signal preprogrammed for circuit 107, and then filtering the signal in the loop filter 108 (LPF). Only those phase or frequency changes caused by the modulating signal MOD that are faster than the settling time of the loop can be detected in the output signal f.sub.TX of the VCO, which means that the loop has a high pass type frequency response to the modulating signal MOD.
FIG. 1 also shows a conventional heterodyne receiver in which the receiver signal f.sub.RX is mixed with a local frequency f.sub.1 in a mixer 3 and filtered with a passband filter 4 to form an intermediate frequency f.sub.IF. A local frequency f.sub.1 is generated in an oscillator 5, which may be a phase-locked frequency synthesizer or a crystal oscillator, for example. The antenna 1 of the transceiver, a duplex filter 2 and the power amplifier 106 of the transmitter are also shown in the figure for clarity.
The frequency adjustment range of voltage-controlled oscillators used in mobile phones is about 30 MHz (e.g., when the carrier frequencies are 890-915 Mhz) and the control voltage range is less than 5 V, whereupon a one kilohertz deviation in the output frequency of the VCO corresponds to a 200 .mu.V variation in control voltage and, as a consequence, noise in modulation signal MOD easily cause inaccuracy in output signal f.sub.TX. Furthermore, the modulation coefficient defined as the ratio between the frequency change of VCO 109 and the corresponding change in control voltage V.sub.CMOD is not constant but varies as VCO output frequency f.sub.TX changes. As a result, the modulation coefficient changes easily from device to device and in proportion to frequency or temperature. The solution shown in FIG. 1 is advantageous from the point of view of the transmitter's energy efficiency, because the output power of the VCO 109 is applied almost directly to the transmission signal f.sub.TX. The solution is simple but not suitable for DC modulation or digital phase modulation due to the high pass nature of its frequency response and its sensitivity to noise.
The problems of the solution shown in FIG. 1 can be partially eliminated with the solution shown in FIG. 2, in which the transmission frequency f.sub.TX is formed by mixing a common local frequency f.sub.1 of the transceiver unit and a modulated offset frequency f.sub.OFF generated in an oscillator 209. The modulation coefficient can be realized according to practical requirements, whereupon, for example, a frequency deviation of about 1 KHz can be created with approximately a one volt variation in the modulating signal MOD. Either a frequency f.sub.1 +f.sub.OFF or f.sub.1 -f.sub.OFF at the output of mixer 208 is selected with passband filter 207 as the transmission frequency f.sub.TX. The signal is attenuated approximately 10 dB in the mixer 208 and the filter 207, thus reducing the overall efficiency of the circuit when compared to the solution in FIG. 1. The signal outputted by the filter 207 is applied by amplifier 206 to the duplex filter 2.
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Bost Dwayne D.
Nokia Mobile Phones Ltd.
Persino Raymond B.
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