EP1413075B1 - Method and arrangement for digital transmission using am emitters - Google Patents

Abstract

A method for digital transmission using an AM transmitter includes operating an output stage of the AM transmitter in a linear mode and correcting the supply voltage of the output stage in the linear mode as a function of an instantaneous drive so as to improve efficiency. The correcting is performed by operating the modulator of the AM transmitter as a switched-mode power supply unit so as to deliver a corrected supply voltage to the output stage, and scanning an envelope of a complex modulated data signal so as to control the correcting. A time constant during the scanning of the envelope enables immediate following of a rise in the envelope. The complex modulated data signal is delayed after the scanning so as to perform the correcting during the delay and prevent an overdriving of the output stage.

Description

The invention relates to the field of broadcasting stations, which are converted in the course of the digitization of analog amplitude modulation (AM) to digital modulation.

• Die AM-Sender arbeiten intern im Schaltbetrieb und haben demzufolge bis zu einem Faktor 3 bessere Wirkungsgrade als lineare Sender, die sonst üblicherweise für digitale Übertragung, z.B. bei DAB (Digital Audio Broadcasting) und DVB (Digital Video Broadcasting), eingesetzt werden. Dadurch ergibt sich eine Einsparung an Betriebskosten.
• Die Broadcaster sind leichter zur Umstellung von analog nach digital zu überzeugen, wenn keine großen Investitionen im Vorfeld anfallen.

The usual types of transmitters, non-linear AM transmitters with RF input (radio frequency) and audio input, should continue to be used. This has the following reasons:

• The AM transmitters work internally in switching mode and therefore have up to a factor of 3 better efficiencies than linear transmitters, which are otherwise usually used for digital transmission, eg DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting). This results in a saving in operating costs.
• The broadcasters are easier to convince to switch from analog to digital, if no major investments in advance.

The digitization of AM broadcasting is seen as the only chance to preserve these frequency bands and the technology used in the long term. For the implementation of the consortium "Digital Radio Mondiale" was founded, see Rundfunktechnische Mitteilungen, Year 43, 1999, Issue 1, pages 29-35 ,

The use of a non-linear AM transmitter for digital modulation requires a special mode of operation of the transmitter. The generation of the modulated digital signal is effected by means of two mutually orthogonal partial signals (I and Q). The I-signal ("in phase") is modulated to a cosine oscillation with the frequency Ft (carrier frequency). The Q signal ("quadrature") is modulated to a sine wave of the same frequency Ft. The sum of both modulated oscillations gives the complex modulated data signal (cosine 0-180 degrees, sine -90 - +90 degrees). The modulated I / Q signal is shaped by filters to have exactly the prescribed waveform with the desired bandwidth.

For non-linear operation, the modulated I / Q signal must be converted to produce the two signals amplitude signal (A signal) and phase-modulated carrier signal (RF-P) therefrom, which are capable of properly driving the AM transmitter. At the output of the AM transmitter results in turn afterwards the modulated I / Q signal with greater power.

• Amplitudensignal (A-Signal)
• Phasenmoduliertes RF-Signal (RF-P-Signal)

The modulated I / Q signal corresponds to a Cartesian representation. This is converted into a polar representation with amplitude and phase. The amplitude signal (A signal) is thereby obtained for driving the AM transmitter at the audio input. From the initially generated phase signal (P signal), a phase-modulated RF (RF-P signal) is generated. Advantageously, the RF-P signal can be obtained directly without the intermediate step via the P signal. This gives you the signals necessary to control the AM transmitter:

• Amplitude signal (A signal)
• Phase modulated RF signal (RF-P signal)

The A signal is input to the modulator input (audio input) of the AM transmitter, and the RF P signal is used to drive the transmitter in the RF manner. In the transmitter output stage, the two signals A & RF-P are multiplicatively combined and form the high-frequency digital output signal.

Both the A-signal and the RF-P signal obtained due to the required processing process far greater bandwidths than the digital signal initially had and should also have at the output of the transmitter again.

The increased bandwidths (factor 3 - 5) can often not be provided by the older modulators because they were not designed for it. Using only the limited bandwidth that "older" transmitters have available in the modulator part results in significant out-of-band emissions. These have the property that they have only a very small slope in the spectrum and thus disturb quite a few adjacent channels.

In general, the emissions are also above the limits coordinated by the ITU, so that a permit seems questionable.

Nonlinear distortions are particularly problematical if, as digital modulation, multicarrier signals, e.g. B. OFDM (Orthogonal Frequency Division Multiplexing) signals to be transmitted.

The DRM (Digital Radio Mondiale) system for digital transmission in the AM areas, which is currently being recommended by the ITU for standardization, proposes an OFDM method with approximately 200 carriers as the multicarrier method.

Although multicarrier modulations have an almost rectangular spectrum, they are noise-like in the time domain, both for the I component and for the Q component of the time signal. This is the consequence of the superimposition of many, statistically practically independent, subchannels. According to the rules of the "Central Limit Theorem", such a superposition has a distribution density function of the amplitude values of both the I component and the Q component, each of which almost reaches the shape of a Gaussian bell curve. In such a case, the distribution density function of the amplitude values of the sum fiction is in the form of a Rayleigh distribution. This means that small and medium amplitude values occur fairly frequently, whereas large amplitude values are very rare.

If the amplitude signal is amplitude-limited in an AM transmitter which is operated in this non-linear mode, non-linear distortions result which on the one hand lead to increased out-of-band radiation and on the other hand also cause internal band disturbances, which due to the mode of operation of the transmitter can considerably exceed the out-of-band radiations. The internal band disturbances reduce the achievable supply area, since a signal which is already intrinsically disturbed can tolerate fewer interferences in the radio channel in order to reach a critical threshold at the receiver.

EP-A-0431201 discloses an AM transmitter for digital signals operated in linear mode.

The present invention describes a method and arrangement for digital transmission with conventional AM transmitters, with which unwanted spurs by nonlinear distortions are largely avoided.

Non-linear distortions can be prevented if the operating point of the transmitter is shifted in such a way that a linear mode of operation arises. For linear operation, the transmitter output stage is driven by the complex modulated data signal (I / Q signal) as known from digital systems DAB and DVB.

The linear operation of the transmitter is advantageous with respect to the noise emissions. These have spectrally much larger slopes than in the previously described nonlinear mode, so that the ITU spectrum mask can be maintained with good alignment of the transmitter. Only the efficiency of the transmitter is very low in linear operation and causes high electricity costs.

The efficiency in linear operation of the AM transmitter is so bad because even with small modulation of the transmitter output stage, the full supply voltage is applied to this stage and due to the quiescent current of the transmitter output power is converted into heat. An improved efficiency can be achieved that the supply voltage is not made much larger than the current level of the final stage requires it.

For tracking the supply voltage for the transmitter output stage as a function of the instantaneous modulation, the envelope of the complex modulated data signal is sampled by an amplitude detector (envelope rectifier or peak rectifier) and the supply voltage or anode voltage of the output stage is controlled by means of the modulator acting as a clocked power supply.

Special emphasis should be placed on the fact that in the context of the tracking no - albeit only temporary - override occurs. An override could be caused by the fact that the envelope of the digital signal rises faster than it provides the tracking of the supply voltage. This is to be assumed as a rule, since the modulator just not the required bandwidth. This disadvantage can be remedied by delaying the complex digital signal after sampling its envelope in a delay stage so that in the meantime the supply voltage of the transmitter output stage can be tracked. Amplitude detector and delay stage are to be retrofitted on the occasion of the conversion to digital operation in the transmitter (see Fig. 1 ).

The time constant of the envelope detector must be such that an increase in the envelope can be followed immediately, so that no overdriving with the resulting distortion and noise emissions occurs. Different, however, as it is z. B. in "dynamic amplitude modulation" is common, the time constant for the waste can be chosen as large as for the increase, since it does not have to be taken to a "hearing" consideration. The smaller waste time constant additionally increases the efficiency of the transmitter.

Transmitters using Pulse Duration Modulation (PDM) or Pulse Step Modulation (PSM) have such modulators in the form of clocked power supplies. The voltage obtained from the sampled envelope of the digital signal is used to drive these PDM or PSM modulators, thereby accurately tracking the supply voltage for the transmitter output stage according to the envelope of the digital signal. This achieves both objectives: linear operation and increasing the efficiency of the transmitter to an acceptable value.

List of reference numbers used

1

Amplitude detector for envelope sampling

2

Delay stage for the complex modulated data signal

3

RF preamplifier

4

transmitter output stage

5

Driver stage of the modulator for tracking the supply voltage

6

Power level of the modulator for tracking the supply voltage

7

Smoothing low pass of the modulator

8th

Output filter of the AM transmitter

Claims (3)

1. A method for digital transmission using AM transmitters, in which nonlinear distortions resulting in inband interference and out-of-band emissions occur during digital transmission, due to the nonlinear mode of operation, the output stage of the AM transmitter being operated in linear mode, the linear mode being operated in conjunction with a correction of the supply voltage of the transmitter output stage as a function of the instantaneous drive in order to improve efficiency, characterized in that
   the modulator of the AM transmitter operates as a switched-mode power supply unit and supplies the corrected supply voltage for the transmitter output stage; the envelope of the complex modulated data signal is scanned, and this signal controls the correction of the supply voltage for the transmitter output stage; the time constant during the scanning of the envelope is such that a rise in the envelope is immediately followed; the time constant during the scanning of the envelope is equal for the rise and decay of its envelope; and the complex modulated data signal is delayed after the scanning of an envelope in such a way that the correction of the supply voltage is effective in the meantime, thus preventing even short-duration overdriving of the transmitter end stage.
2. The method according to Claim 1, characterized in that
   the modulator operating as a switched-mode power supply unit can also be a pulse duration modulator or a pulse step modulator; and if AM transmitters having class B push-pull modulators are used, replacement with one of these modulators must be carried out.
3. A system for digital transmission using AM transmitters, in which the transmitter output stage is operated in linear mode to avoid non-linear distortions, and whose supply voltage is corrected by the complex modulated data signal as a function of the drive in order to improve efficiency, characterized in that
   an amplitude detector (1) which scans the envelope of the complex modulated data signal is connected upstream from the modulator (5 and 6) operating as a switched-mode power supply unit; a delay stage (2) for the complex modulated data signal is installed upstream from the high-frequency preamplifier stages (3) in the signal path to the transmitter output stage (4), the time constant during scanning of the envelope by the amplitude detector (1) being equal for the rise and decay of the envelope.

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