Radio transmitter

Details Radio transmitter Radio transmitter

1999-09-13

2004-11-30

Vo, Don N. (Department: 2631)

Pulse or digital communications

Transmitters

Angle modulation

C375S308000

Reexamination Certificate

active

06826237

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a radio transmitter having particular, but not exclusive, application in digital communication systems such as GSM.
BACKGROUND OF THE INVENTION
The GSM specification for spurious emissions from a mobile station transmitter, as defined in the European Telecommunication Standards Institute (ETSI) document “GSM: Digital Cellular Telecommunications System (Phase 2) Radio Transmission and Receptions (GSM 05.05 version 4.19.0)”, is summarised in
FIG. 1
of the accompanying drawings. The figure plots the permitted levels of unwanted noise N in a 1 Hz bandwidth, referenced to a carrier power level of +33 dBm at 902 MHz, against frequency f in MHz. Also shown are the positions of the GSM transmit (Tx) and receive (Rx) bands. In the portion of the GSM receive band between 925 and 935 MHz the noise must be held below −150 dBc, and in the portion between 935 and 960 MHz the noise must be held below −162 dBc. Such low levels of noise are difficult to achieve with a fully-integrated transmitter, and to meet this specification with a conventional architecture it is necessary to use an expensive RF filter after the final stage of power amplification, with a consequent loss of transmitter efficiency.
A block diagram of a conventional transmitter architecture, which performs dual up-conversion in analogue circuitry, is shown in
FIG. 2
of the accompanying drawings. Digital data for transmission is provided as an input
202
to a Gaussian Minimum Shift Keying (GMSK) modulator
204
which produces as output analogue in-phase I and quadrature phase Q signals on a zero-frequency carrier. The I signal is supplied to a first IF mixer
206
, and the Q signal is supplied to a second IF mixer
208
. An output signal from a first Voltage Controlled Oscillator (VCO)
210
is supplied via a first 90° phase shifter
212
to the local oscillator port of the first IF mixer
206
, and directly to the local oscillator port of the second IF mixer
208
. The resultant output signals from the mixers
206
,
208
are added together by a combiner
214
and filtered in a bandpass filter
222
to produce a signal at the required IF frequency, for example 100 MHz. As well as removing unwanted mixing products the bandpass IF filter
222
reduces levels of out-of-band noise. The filter
222
is commonly implemented off-chip.
The first VCO
210
is driven by a signal produced by an IF synthesiser
216
which derives its output using a 13 MHz reference oscillator
218
under the control of instructions passed on a control bus
220
to produce a fixed IF output.
The filtered IF signal is split into two parts. The first part has its phase shifted 90° by a second phase shifter
224
and is then up-converted by a first RF mixer
226
, the second part of the IF signal is up-converted by a second RF mixer
228
. An output signal from a second VCO
230
is supplied directly to the local oscillator port of the second RF mixer
228
, and via a third 90° phase shifter
232
to the local oscillator port of the first RF mixer
226
. The resultant output signals from the mixers
226
,
228
are added together by a combiner
234
to produce a combined RF signal including a product at the required frequency in the GSM transmit band between 880 and 915 MHz.
The second VCO
230
is driven by a signal produced by an RF synthesiser
236
which derives its output using a 13 MHz reference oscillator
218
under the control of instructions passed on a control bus
220
to produce a variable output frequency.
Without extra filtering, noise from the second VCO
230
would fall into the GSM receive band at an unacceptably high level. The RF signal therefore passes through a first RF bandpass filter
238
before being amplified for transmission by a power amplifier
240
. The amplifier
240
is normally operated under heavy compression for best efficiency, and this has the effect of removing the AM component of single-sideband noise on the input signal. Without the AM component, the residual FM component produces noise at equal levels on the two sides of the signal, largely restoring noise in the unwanted sideband. Hence the signal must be filtered by a second RF bandpass filter
242
before transmission via an antenna
244
. The second RF filter
242
is much less desirable than the first filter
238
both in terms of cost (because of the higher power levels it must handle) and because of the resultant loss in transmitter power due to losses in the filter
242
. These transmitter losses can amount to more than 1 W, requiring the use of a bigger power amplifier
240
and a larger battery.
Such an architecture therefore has a number of disadvantages for use with current digital cellular communications standards. It is difficult to use for telephones operating in accordance with two or more standards unless the schemes are compatible (in the sense of having similar requirements for bandwidth and modulation schemes, for example). This is because only the baseband circuitry is digital, and the analogue IF and RF circuitry is inherently less flexible. Also, as mentioned above, it is difficult to meet the GSM requirements for spurious emissions without additional filtering after the power amplification stage.
SUMMARY OF THE INVENTION
An object of the present invention is to address the problems described above.
According to the present invention there is provided a radio transmitter comprising modulation means for producing quadrature modulated signals, first frequency-translation means for translating said signals to a variable intermediate frequency (IF) signal in digital form, digital to analogue conversion means for converting said variable IF signal to analogue form, second frequency-translation means for translating said analogue IF signal by a fixed frequency to a radio frequency (RF) signal, and power amplifying means for amplifying said RF signal for transmission.
The present invention is based upon the recognition, not present in the prior art, that digital up-conversion to a variable IF provides a more flexible transmitter architecture that does not require expensive RF filtering.
An advantage of the described transmitter architecture is that it is extremely versatile, giving the possibility of changing modulation methods, frequencies, sampling rates or bandwidths to accommodate a variety of communication standards.
Advantageously, error correction means are provided between the first and second frequency translation means, to correct for the imbalance between in-phase and quadrature signals in the second frequency translation means.
Provision of such means enables automatic calibration of the transmitter during manufacture to take account of the imbalance between signal paths in the second frequency-translation means, after which calibration no further attention is required.


REFERENCES:
patent: 4262361 (1981-04-01), Hauer
patent: 4809203 (1989-02-01), Wilson et al.
patent: 5483695 (1996-01-01), Pardoen
patent: 5548541 (1996-08-01), Bierman et al.
patent: 5673291 (1997-09-01), Dent
patent: 5748623 (1998-05-01), Sawahashi et al.
patent: 5812605 (1998-09-01), Smith et al.
patent: 6072994 (2000-06-01), Phillips et al.
patent: 6091780 (2000-07-01), Sointula

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