Demodulators – Amplitude modulation demodulator – Having plural demodulation outputs
Reexamination Certificate
1999-12-15
2001-07-03
Grimm, Siegfried H. (Department: 2817)
Demodulators
Amplitude modulation demodulator
Having plural demodulation outputs
C329S350000, C329S365000, C329S370000, C375S320000, C455S337000
Reexamination Certificate
active
06255901
ABSTRACT:
The invention relates to a demodulator circuit for demodulating a signal ASK-modulated with pulses equal in duration, and having a small depth of modulation and large dynamic range.
Most recently it has become increasingly popular to equip items such as the so-called “smart cards”, luggage tags, labels on all sorts of goods or also on accessory items moved on conveyor belts with transponders serving to identify the item in each case by being read contactlessly. In swapping data between the transponders and the readers ASK modulation is made use of, among other things, involving keying a carrier generated with constant amplitude, i.e. keying it ON/OFF as a function of the data to be transferred. This keying results in a 100% depth of modulation, meaning that the carrier has either its full amplitude or zero amplitude. Such a large depth of modulation produces, however, a relatively wide frequency spectrum with sidebands whose levels violate FCC regulations in the frequency range provided for this special application. One possibility of reducing the sideband levels consists of reducing the depth of modulation, for example, down to 10% so that only a 10% difference exists between the full amplitude of the carrier and the modulated amplitude of the carrier.
However, the lower depth of modulation to be preferably used causes demodulation problems in the cited applications since it is in these applications that the demodulation needs to handle a very large dynamic range of the received signal on the part of the demodulator in the transponder. The spacing between the reader and the transponder is namely not always constant, it instead possibly being greatly different. When the transponder is located near to the reader, the signal transmitted thereto is received at a very high level, whilst where larger distances are involved the level received may assume very small values. To avoid the demodulation input from being overridden an amplitude limiter is usually provided to ensure that the received signal can be further processed without swamping subsequent stages. In the case of a low depth of modulation of, say, 10%, however, it may happen that the received level lies above the response threshold of the limiter in both the unmodulated portion of the carrier and in the modulated portion so that after limiting the amplitude the system is unable to “see” any difference between the modulated and unmodulated portion of the carrier, the output signal of the amplitude limiter then being namely a signal having constant amplitude.
The invention is thus based on the object of providing a demodulator circuit enabling ASK-modulated signals to be demodulated even when these signals have only a small depth of modulation and are received with a large dynamic range.
This object is achieved in accordance with the invention by a demodulator circuit for demodulating a signal ASK-modulated with modulation pulses equal in duration, and having a small depth of modulation and large dyamic range comprising an amplitude limiter through which an amplitude-dependent current flows when the amplitude of the signal to be demodulated exceeds its limiting threshold value, an envelope detector to the input of which the signal to be demodulated is applied, a differentiating network configured so that it differentiates the output signal of the envelope detector and outputs a signal pulse only when the change in amplitude of this output signal is in one direction, a bandpass filter passing, from a signal derived from amplitude-dependent current from the amplitude limiter, the frequency component attributed to the duration of the modulation pulses, a comparator comparing the output signal of the bandpass filter to a fixed threshold value and outputting a signal pulse when this output signal exceeds the threshold value, and a logic circuit outputting the signal pulse existing in each case as the demodulated signal.
The demodulator circuit in accordance with the invention enables a signal characterizing the presence of a modulation pulse to be generated when the amplitude limiter is already active due to a high input signal level not only when the input signal level is below the limiting threshold value of the amplitude limiter, but also to output such a signal when limiting the amplitude has been activated so that the amplitude limiter furnishes at its output a signal remaining constant in amplitude. This is achieved by making use of the fact that, on activation of limiting, a current flows through the amplitude limiter, the magnitude of which differs depending on the amplitude of the limited signal, i.e. the signal to be demodulated.
Advantageous further aspects of the invention are characterized in the sub-claims.
REFERENCES:
patent: 4065722 (1977-12-01), Francis
patent: 5936462 (1999-08-01), Muraishi
Prexl Franz
Steinhagen Wolfgang
Grimm Siegfried H.
Petersen Bret J.
Telecky , Jr. Frederick J.
Texas Instruments Deutschland GmbH
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