Modulators – Frequency shift keying modulator or minimum shift keying...
Reexamination Certificate
2000-11-22
2002-04-09
Mis, David (Department: 2817)
Modulators
Frequency shift keying modulator or minimum shift keying...
C375S302000, C375S305000, C375S308000
Reexamination Certificate
active
06369666
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a circuit configuration for modulation with a transmission PLL. Such modulators are particularly suitable for GMSK modulation in mobile radios.
In mobile radios operating in the GSM Standard, data are transmitted using GMSK (Gaussian minimum shift keying) modulation. At the moment, the same type of modulation is used both for voice transmission and for data transmission. However, the data rate is not sufficient for multimedia applications going beyond a pure voice service, so that a more recent standard proposes 8-PSK modulation. 8-PSK modulation allows three times as much data to be transmitted per unit time. The GMSK symbols have a constant envelope curve, that is to say a constant amplitude, while the 8-PSK symbols have a non-constant envelope curve, and have a different amplitude. According to the specification in the Standard, it is possible to switch directly between GMSK modulation and 8-PSK modulation from one time slot to the next. The combined transmission system is called GSM Edge.
A transmission PLL (PLL: Phase Locked Loop) or Up-conversion Loop is used for GMSK modulation. The literature reference G. Irvine et al.: “An Up-conversion Loop Transmitter IC for Digital Mobile Telephones”, 1998 IEEE International Solid-State Circuits Conference, pages 364-65, describes a transmission PLL. A voltage-controlled oscillator (VCO) is actuated by the phase information and produces the modulated output signal for transmission via the antenna, after appropriate amplification. The VCO output is fed back via a down-conversion mixer, to which the output signal from a channel oscillator is supplied, and via a quadrature component modulator to a phase detector in the phase locked loop. The downward-modulated signal is compared with a reference frequency in the phase detector. The output signal from the phase detector is passed via a loop filter in order to produce the frequency control signal for the VCO. In Irvine et al., the quadrature component modulator is actuated by the downward-mixed output signal. The literature reference Yamawaki et al.: “A 2.7-V GSM RF Transceiver IC”, IEEE Journal of Solid-State Circuits, Volume 32, No. 12, December 1997, page 2089-96 describes a transmission PLL in which the output of the down-conversion mixer is fed back without being modulated to one input of the phase detector, and the other input of the phase detector is fed from an oscillator, onto which the quadrature components of the wanted signal are modulated. The quadrature component modulator is located upstream of the phase discriminator, either in the feedback loop of the transmission PLL (Irvine et al.) or in the reference signal phase (Yamawaki et al.).
The loop filter of the transmission PLL is a low-pass filter. At radio frequencies it acts as a narrowband bandpass filter within the transmission PLL, in order to allow the wanted signal to pass. The VCO is of low-noise design. This solution has been adopted in particular because there is no need for any duplex filter downstream of the transmission amplifier and immediately upstream of the antenna. The duplex filter ensures that the transmission signal is attenuated outside the transmission band, and prevents noise and nearby transmissions from being injected into the reception channel. It is relatively expensive and necessitates increased power consumption in the transmission amplifier.
Owing to these advantages, it is desirable to continue to use the concept of the transmission PLL in appliances based on the combined GMSK/8-PSK Standard as well. Since the amplitude of the output signal from the VCO is constant in GMSK modulation, but, in contrast, 8-PSK modulation has a variable-amplitude envelope, it is not possible to use a transmission PLL for 8-PSK modulation without some further action.
SUMMARY OF THE INVENTION
The object of the invention is to provide a modulator circuit assembly which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which is suitable for two modulation types, in particular GMSK and 8-PSK modulation.
With the above and other objects in view there is provided, in accordance with the invention, a modulator circuit configuration, comprising:
a first modulator having
a controllable-frequency oscillator with an output for a first output signal;
a down-conversion mixer connected to the output;
a phase detector having a first input and a second input receiving the first output signal of the oscillator fed back via the down-conversion mixer; and
a mixer device configured to produce a signal modulated with a wanted signal;
a second modulator having
an input connected to the output of the oscillator; and
a mixer device configured to produce a signal, modulated with the wanted signal, as a second output signal;
a first switching device connected to the first and second modulators for switching between the first output signal and the second output signal; and
a second switching device connected to and feeding an unmodulated signal to the first input of the phase detector when the first switching device is switched to the second output signal, and feeding the signal modulated with the wanted signal to the first input of the phase detector when the first switching device is switched to the first output signal.
In other words, the objects of the invention are achieved by the circuit configuration for modulation which comprises: a first modulator having a controllable-frequency oscillator which has one output for a first output signal; a phase detector in which the output of the oscillator is fed back via a down-conversion mixer to the phase detector, and a mixer arrangement in order to produce a signal which is modulated with a wanted signal; a second modulator which is connected on. the input side to the output of the oscillator and which comprises a mixer arrangement in order to produce a signal, which is modulated with the wanted signal, as a second output signal; a first switching device in order to switch between the first and the second output signal, and a second switching device in order to produce an unmodulated signal at one input connection of the phase detector when the first switching device is switched to the second output signal, and in order to produce the signal which is modulated with the wanted signal at the input connection of the phase detector when the first switching device is switched to the first output signal.
In the circuit arrangement according to the invention, a transmission PLL is still used for GMSK modulation, and the advantages obtained from this are retained. When 8-PSK modulation is required, the arrangement switches to a direct modulator. The VCO in the transmission PLL is used as the oscillator for the carrier frequency of the direct modulator. For this purpose, the quadrature component modulator in the transmission PLL is switched to be inactive. The arrangement according to the invention thus combines the principles of the transmission PLL and of direct modulation, with particularly complex or expensive components being used jointly.
Owing to the lower specific transmission power for 8-PSK modulation, the standardization envisages less stringent noise requirements for the transmission path. Corresponding to the transmission power, the signal-to-noise ratio at 10 MHz and 20 MHz from the transmission carrier is around 6 dB lower than for GMSK modulation. Filtering is required after the power amplifier for GMSK modulation, since the noise in the transmission band is convoluted into the receiver band by the nonlinear amplifier (noise conversion gain). When a linear amplifier is used for 8-PSK , this effect occurs only to a very weak extent. It is thus sufficient to provide a surface acoustic wave filter at the output of the direct modulator and upstream of the transmission amplifier, in order to avoid transmission into the reception channel. There is no longer any need for a duplex filter, which would be required if a direct modulator were to be used for G
Simon Martin
Veit Werner
Infineon - Technologies AG
Mis David
LandOfFree
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