Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
2000-03-21
2004-11-09
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Automatic frequency control
C375S146000, C375S272000
Reexamination Certificate
active
06816559
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a frequency shift keying (FSK) transceiver. Such circuits are generally used to transmit, especially by radio, digital data.
2. Discussion of the Related Art
The drawing schematically shows an FSK transceiver of a type usable according to the present invention. The transmit portion Tx and the receive portion Rx are integrated in the same circuit.
The transmit portion includes a phase-locked loop
10
used to obtain an adjustable carrier frequency. This phase-locked loop includes a reference oscillator
12
, the output signal of which, of fixed frequency, is provided to a phase comparator
14
. Phase comparator
14
receives the output of a controlled oscillator
16
via a divider
18
. Oscillator
16
is controlled by a low-pass filter
20
that receives the output of phase comparator
14
.
The binary signal to be transmitted Tx
in
is received on a pin
22
of the integrated circuit. Signal Tx
in
is added to the control signal of oscillator
16
at
24
via an amplifier
26
. The gain of amplifier
26
determines the “deviation” of the FSK modulation, that is, the frequency shift undergone by the carrier provided by oscillator
16
when signal Tx
in
switches between logic levels 0 and 1. This deviation must in some applications be precisely determined. For this purpose, the gain of amplifier
26
is adjustable, for example, externally by means of an adjustable resistor
28
connected to a pin
30
of the circuit.
The output of oscillator
16
provides the modulated signal to be transmitted Tx
out
on a pin
32
, possibly via an amplifier
34
.
In receive portion Rx, a received modulated signal Rx
in
, provided for example by an antenna
36
, is applied on a pin
38
of the integrated circuit. An amplifier
40
transmits this signal to two mixers
42
and
43
. The two mixers also respectively receive two carriers of same frequency in phase quadrature.
Since the circuit is generally used in a single way, either to transmit, or to receive, the controlled oscillator
16
of phase-locked loop
10
is used in receive mode to provide the two carriers in phase quadrature. Then, no modulating signal is provided to the phase locked-loop, so that oscillator
16
operates at the frequency defined by reference oscillator
12
and divider
18
, which frequency is no other than the carrier frequency.
The outputs of mixers
42
and
43
are provided to respective low-pass filters
45
and
46
. Two signals in phase quadrature, the frequency of which is that of the above-mentioned deviation, are thus obtained. Further, state 1 corresponds to a 90° phase advance of the first channel with respect to the other, and state 0 corresponds to a 90° phase lag of the first channel with respect to the other.
The output signals of filters
45
and
46
, of sinusoidal shape, are provided to respective clipping amplifiers
48
and
49
intended for converting these signals into rectangular signals. These rectangular signals are provided to an FSK demodulator
51
that extracts logic values 0 or 1 by analyzing the phase of the signals provided by clipping amplifiers
48
and
49
. Demodulator
51
provides the binary extracted signal Rxout to a pin of the integrated circuit.
Generally, since the number of circuit pins is high and the circuit is used in a single way, signal Rxout may be provided, as shown, to pin
22
which is used, in transmit mode, to receive signal Rx
in
to be transmitted. A switch K enables choosing the required function of pin
22
.
As previously mentioned, the gain of amplifier
26
must be precisely adjusted to obtain a correct deviation in the modulation of the transmitted signal. For this purpose, the circuit is set to the transmit mode, a test signal is applied on pin
22
, and the output signal sampled from pin
32
is analyzed. The analysis generally consists of performing a frequency demodulation followed by a spectrum analysis. Resistor
28
is adjusted until the spectrum corresponds to the desired deviation. This adjustment solution requires particularly expensive laboratory equipment.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a solution enabling adjustment of the deviation with low cost laboratory equipment.
To achieve this and other objects, the present invention provides an FSK transceiver circuit, including a controlled oscillator used, in a transmission mode, to modulate a carrier by frequency shifts according to a binary transmission signal present on an input pin of the circuit, the shift amplitude being determined by an adjusting element connected to an adjusting pin of the circuit; two mixers connected to receive a reception signal present on a receive pin and, respectively, two carriers in phase quadrature of the controlled oscillator that, in receive mode, receives no modulating signal; and a demodulator connected to the outputs of the mixers via respective filters, the output of one of the filters being accessible from the outside by a circuit test pin.
According to an embodiment of the present invention, the controlled oscillator is part of a phase-locked loop.
The present invention also aims at a method for adjusting a circuit as described hereabove, including the steps of:
setting the controlled oscillator to the transmit mode;
applying a carrier on the receive pin;
applying a binary test signal on the input pin;
measuring the frequency of the signal present on the test pin; and
acting upon the adjusting element to obtain the required frequency of the signal present on the test pin.
REFERENCES:
patent: 2640106 (1953-05-01), Wilson et al.
patent: 3944742 (1976-03-01), Cunningham
patent: 4466106 (1984-08-01), Serrano
patent: 4481642 (1984-11-01), Hanson
patent: 5633898 (1997-05-01), Kishigami et al.
patent: 5912926 (1999-06-01), Koenck et al.
patent: 5945885 (1999-08-01), Schwartz et al.
patent: 6275542 (2001-08-01), Katayama et al.
patent: 6411646 (2002-06-01), Walley et al.
patent: 24 55 698 (1976-08-01), None
French Search Report from French Patent Application 99 04200, filed Mar. 30, 1999.
Patent Abstracts of Japan, vol. 017, No. 039 (E-1311), Jan. 25, 1993 & JP 04 258062 A (NEC Corp.).
Kise T. L. et al., “Half-Duplex FSK Modem” IBM Technical Disclosure Bulletin, US, IBM Corp. New York, vol. 21, No. 11, Apr. 1979, pp. 4461-4462.
Dugas Christophe
Sirito-Olivier Philippe
Chin Stephen
Jorgenson Lisa K.
Morris James H.
STMicroelectronics S.A.
Williams Lawrence
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