Frequency modulator and frequency modulation method

A frequency modulation method is provided in which the frequency of a carrier signal is modulated on the basis of the frequency of a modulation signal, and in which the modulation signal is integrated. The carrier signal is integrated, and a differential signal is formed from the integrated modulation signal and the integrated carrier signal. The differential signal is phase-modulated in order to obtain the frequency-modulated carrier signal as the output signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Utility patent application claims priority to German Patent Application No. DE 10 2004 027 184.4, filed on Jun. 3, 2004, which is incorporated herein by reference.

BACKGROUND

The invention relates to a frequency modulator and to a frequency modulation method.

A frequency modulator101as illustrated inFIG. 8is known from the prior art.

This frequency modulator101, which is implemented, for example, in the RKE transmitter families TDx51xx and TDA525x as well as PMA5100 of Infineon Technologies AG, Munich, comprises a crystal oscillator102, a phase locked loop106and an electronic switch S which can be opened and closed in time with a modulation signal MEthat is applied to an input EM.

An input of the crystal oscillator102is connected to ground via the series connection of a crystal Q to a series circuit comprising two capacitors C1, C2. The switch S is arranged parallel to one of these capacitors C1, C2(namely C1) in such a manner that it bridges the latter when the switch S is closed.

In the case of a frequency modulator101of this type, a frequency-modulated signal which can be tapped off at an output A and can be emitted by an antenna107is frequency-modulated by frequency-modulating the reference frequency of the phase locked loop106. The reference signal for the phase locked loop (PLL)106is provided by the crystal oscillator102which is frequency-modulated by changing the series capacitance CSof the crystal Q, said series capacitance being formed by the individual capacitances C1, C2.

When the switch S is open, the series capacitance CS=C1×C2/(C1+C2) is small, with the result that the oscillator frequency is high. When the switch S is closed, the series capacitance CS=C2is large, with the result that the oscillator frequency is low.

The disadvantage of a frequency modulator of this type is that the data rate of the modulation signal is limited on account of transient processes when closing and opening the switch S.

Only small frequency shifts Δω are possible since the oscillation frequency of a crystal can be changed only to a slight extent. Since the oscillation frequency of the crystal oscillator must be designed to be variable in the case of a frequency modulator of the type described above, this results in the crystal oscillator having an associated low frequency stability. Finally, there is no shift flexibility whatsoever. In addition, no baseband filtering is possible.

The prior art also discloses using a system having direct digital synthesis (DDS) as the frequency modulator. Frequency modulators of this type are extremely complex and have only low efficiency. Only a frequency-modulated signal having a high parasitic oscillation component can be generated using currently available systems.

So-called IQ modulators are furthermore provided in the transmission path. However, IQ modulators of this type are likewise very complex and have low efficiency.

SUMMARY

One embodiment of the invention presents a low-power frequency modulator which can be implemented in a simple manner, and a simple frequency modulation method. A frequency modulation method is provided in which the frequency of a carrier signal is modulated on the basis of the frequency of a modulation signal, and in which the modulation signal is integrated. The carrier signal is integrated, and a differential signal is formed from the integrated modulation signal and the integrated carrier signal. The differential signal is phase-modulated in order to obtain the frequency-modulated carrier signal as the output signal.

DETAILED DESCRIPTION

One embodiment of the invention achieves frequency modulation—as in the case of a frequency modulator illustrated inFIG. 8and described above using a fixed carrier oscillator and a downstream phase modulator. Since phase modulation mathematically represents the integral of frequency modulation, the modulation signal is integrated at the input of the phase modulator. Details of this can be found, for example, in Köstner, R.; Möschwitzer, A.: “Elektronische Schaltungen” [Electronic Circuits], Carl Hanser Verlag Munich, Vienna, 1993, ISBN 3-446-16588-6, pages 246 and 247.

One embodiment of the invention is a frequency modulation method, in which the frequency of a carrier signal is modulated on the basis of the frequency of a modulation signal, and in which the modulation signal is integrated. The carrier signal can be additionally integrated. A differential signal is formed from the integrated modulation signal and the integrated carrier signal. This differential signal is phase-modulated in order to obtain the frequency-modulated carrier signal as the output signal.

One embodiment includes a frequency modulator, in which, in addition to a first integrator in which the modulation signal is integrated, a further integrator is provided in which the carrier signal is integrated. Provision is made of a comparator in which a differential signal is formed from the integrated modulation signal and the integrated carrier signal. The phase modulator which is known from the prior art and is present is now provided for the purpose of modulating the phase of the differential signal. The frequency-modulated carrier signal is then applied, as the output signal, to the output of the phase modulator.

In one embodiment, it is no longer necessary to frequency-modulate the reference, that is, the crystal oscillator. As a result, the crystal oscillator can be designed for maximum stability in terms of frequency and other parameters. The power consumption of the crystal oscillator can also be minimized. The frequency shift can be set in a flexible manner using the amplitude of the integrated modulation signal.

One embodiment of the invention is suitable for any type of frequency modulation, not only for frequency shift keying (FSK). So-called baseband shaping is also possible in a simple manner.

For the rest, a frequency modulator according to one type mentioned above is distinguished by low outlay, low power consumption, low complexity, the absence of spurs, the transmission of high data rates, high deviation and very good integratability.

FIG. 1illustrates a block diagram of a first exemplary embodiment of a frequency modulator1according to one embodiment of the invention.

The frequency modulator1includes a crystal oscillator2, two integrators3and4, a comparator5and a phase locked loop (PLL)6.

The input of the crystal oscillator2is connected to ground via a series circuit comprising a crystal Q and a capacitor C. The output of the crystal oscillator2is connected to the input of the first integrator3. The output of the latter is connected to the non-inverting input of the comparator5.

The inverting input of the comparator5is connected to the output of the second integrator4. The output of the comparator5is connected to the input of the PLL6. The output of the phase locked loop6forms the output A of the frequency modulator1according to one embodiment of the invention, said output A being connected to an antenna7for the purpose of transmitting the modulated output signal TM.

In the case of one embodiment of the invention, the crystal oscillator2is no longer frequency-modulated as in the case of the frequency modulator illustrated inFIG. 8. One embodiment of the invention is inserted between the crystal oscillator2and the PLL6. Specifically, a comparator5, which forms the difference between the integrated carrier signal TIand the integrated data signal MIand has the phase locked loop6connected immediately downstream of it, is provided, according to one embodiment of the invention, between said components—the crystal oscillator2and the phase locked loop6.

In order to describe the operation of the frequency modulator1according to one embodiment of the invention, it is assumed that the crystal Q oscillates at a frequency of fQ=13 MHz.

A square-wave voltage signal is therefore applied, as a carrier signal TEat a carrier frequency of fT,E=13 MHz, to the output of the crystal oscillator2.

It is furthermore assumed that a low-frequency modulation signal MEat a frequency fM,Eand having a square-wave signal amplitude is to be modulated onto the carrier signal ME.

The 13 MHz signal TEand the 10 kHz data signal MEare first of all integrated using the integrators3,4and are then supplied to a comparator5. The output signal D of the comparator5is again a 13 MHz signal but the phases of the edges have been modulated. Since the PLL (phase locked loop) reacts to the phase of an edge of the input signal, the PLL6accordingly tracks the output frequency fMA.

FIG. 2outlines the relationships between the signals TE, ME, D at the comparator5.

The relationship between the angular frequency and phase modulation is as follows:

This integration is carried out in the integrator4for the 10 kHz data signal MI. The comparator5then changes the phase or edges of the 13 MHz carrier signal TI. In the case of a sawtooth-waveform integrated carrier signal TIhaving a sawtooth amplitude, the maximum phase shift which can be achieved using this type of modulation is Δφmax=±90°. From peak to peak, the maximum phase angle is thus φmax=180°.

This state of affairs will be explained once again below with reference to a numerical example:

An RF output frequency fM,Aof 434 MHz with an FSK deviation of Δf′M,A=±50 kHz is desired. The PLL6has a division factor F of 32. The oscillator's signal must therefore have a frequency fT,Eof 13.56 MHz for a quasi deviation ΔfT,Eof ±1.5 kHz.

If the modulation frequency fM,Eis 10 kHz, one data bit lasts for 0.05 ms. The phase angle φ of the reference TEmust be changed by the value

Since the phase shift Δφof 27° that is required in this case is less than the maximum permissible phase shift Δφmaxof 180°, the requirement can be realized.

FIG. 3illustrates a specific circuit arrangement of a frequency modulator11according to one embodiment of the invention that was used to verify the numerical example described above.

For functional testing, the input of the frequency modulator11is connected to function generators12,13and the output is connected to a spectrum analyzer18.

The integration of the carrier signal TEand of the modulation signal MEis simulated by the function generators12,13.

The core components in this case are a comparator15(module MAX961) and a phase locked loop16which is connected downstream of the latter and is in the form of the integrated module TDA5100.

Two respective 10 kΩ and 1 k5 Ω resistors R1, R2, R3and R4are used to set (in terms of DC) the operating point at the inputs of the comparator15. Three 10 nF capacitors C3, C4, C5are used for AC coupling. The 13 MHz generator12and the 10 kHz generator13provide triangular-waveform voltages. A 30 kΩ resistor R5at the output of the comparator15is used to produce a current source having a current of 100 μA in order to drive the crystal oscillator input COSC of the TDA5100 phase locked loop16. The output of the evalboard16is connected to a spectrum analyzer18.

FIG. 4reproduces the measured input and output signals, namely the simulated integrated carrier signal TI, the simulated integrated modulation signal MIand the differential signal D derived therefrom, as a function of time t.

In order to illustrate the respective signal profile TI, MIand D,FIGS. 5 and 6illustrate portions of the diagram illustrated inFIG. 4, on the one hand in the vicinity of the maximum of the modulation signal (FIG. 5) and on the other hand in the vicinity of the minimum of the modulation signal (FIG. 6).

As illustrated in the diagram (illustrated inFIG. 7) provided by the spectrum analyzer having an integrated signal analyzer18, a radio-frequency output signal TM′ at a frequency f′M,Aof 433.92 MHz and having a frequency shift Δf of 50 kHz can be tapped off at the output A of the frequency modulator11.