Demodulators – Frequency modulation demodulator – Input signal converted to and processed in pulse form
Reexamination Certificate
1999-02-26
2001-05-08
Kinkead, Arnold (Department: 2817)
Demodulators
Frequency modulation demodulator
Input signal converted to and processed in pulse form
C329S336000, C375S324000, C455S214000
Reexamination Certificate
active
06229386
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital FM demodulation circuit which uses an arctangent circuit having ROM tables.
2. Description of the Related Art
FIG. 1
is a schematic block diagram showing a conventional digital FM demodulation circuit. The digital FM demodulation circuit includes delay circuits
1
,
2
, and
6
, adders
3
and
7
, a divider
4
, and an arctangent circuit
5
. The operation of the digital FM demodulation circuit will be described. In the demodulation circuit, there is inputted a digital FM signal into which an analog FM signal is converted with a sampling frequency that is four times higher than a frequency of the analog signal.
When the sampling frequency is set to be four times as large as the frequency of the original signal, it is possible to set a phase difference between the sampling frequency and an non-modulated digital FM signal at 90 degrees. Each time when a digital FM signal is passed through a delay circuit, the phase of the signal is delayed at 90 degrees. The sampled digital FM signal passes through a delay circuit
1
to be converted to a second signal that is delayed for a constant time. The second signal is referred to as sin&ohgr;t.
On the other hand, the sampled digital FM signal is converted also to a third signal which is always phase-shifted at 90 degrees with respect to the second signal, by a Hilbert conversion circuit including the delay circuits
1
and
2
, the adder
3
, and the divider
4
. The third signal is referred to as cos&ohgr;t. A FM signal C which is phase-shifted by 90 degrees, i.e., cos&ohgr;t, and A FM signal S which is not phase-shifted, i.e., sin&ohgr;t are inputted to the arctangent circuit
5
. The arctangent circuit
5
performs a division of the two signals to obtain tan&ohgr;t (=S/C=sin&ohgr;t/cos&ohgr;t), and obtains arctan(S/C) from ROM tables. As a result, a phase &ohgr; of the FM signal is detected.
An output of the arctangent circuit
5
is delayed for one sampling period by the delay circuit
6
, and then inverted by an inverting circuit which is not shown. The inverted output is supplied to one input of the adder
7
. On the other hand, to the other input of the adder
7
, the output of the arctangent circuit
5
is directly inputted. In the adder
7
, the two inputs are added to each other, so that a FM detector output as a demodulated signal is obtained.
As described above, a general digital FM demodulation circuit requires an arctangent circuit in which the phase of a sampled signal is calculated from the amplitude of the signal. Usually, it is often that such a calculation circuit itself is not configured and the phase is obtained by using ROM tables corresponding to an input value.
In most of A/D converter circuits, the output digital value is expressed by one bit in code information and n bits in amplitude information. In the following description, therefore, a 4-bit A/D converter in which code information consists of 1 bit and amplitude information of 3 bits is used as an example, and it is assumed that an analog FM signal is converted to a digital FM signal by the A/D converter and the digital FM signal is inputted to a digital FM demodulation circuit. In the 4-bit A/D converter, the amplitude of the input signal is expressed by one of digital values of 15 kinds from −7 to 7. Consequently, each of the above-mentioned S and C can have one of the values from −7 to 7, and 113 ROM tables of arctangents of S/C are necessary (when S/C≧0, 64 ROM tables, and when SIC<0, 49 ROM tables). When no countermeasure is taken, therefore, there arises a problem in that the cost of preparing ROM tables is too high.
SUMMARY OF THE INVENTION
In view of the problem, it is an object of the invention to reduce the number of ROM tables used in an arctangent circuit without lowering the performance.
According to the present invention, there is provided a digital FM demodulation circuit sampling an input analog FM signal and digitally detecting the sampled input signal, comprising:
an arctangent circuit into which a second signal that is delayed from the sampled input signal for a constant time and a third signal that is different in phase at 90 degrees from the second signal are inputted, and which outputs a corresponding arctangent value based on a result of a division of the two input signals, and
the arctangent circuit including:
roundoff means for sequentially rounding the division result to one of a plurality of typical values; and
ROM address unit for selecting an arctangent value corresponding to the rounded typical value from a ROM table containing typical arctangent values, and outputting the selected arctangent value.
According to the present invention, the arctangent circuit may further comprise:
polarity detection means for detecting whether the division result is a negative value; and
a polarity correction unit which changes a polarity of the outputted arctangent value, and
wherein if the polarity detection unit judges that the division result is a negative value, the polarity correction unit changes the polarity of the output arctangent value to negative.
According to the present invention, the arctangent circuit may further comprise:
a level comparison unit comparing a level of the second signal with a level of the third signal;
an exchange unit exchanging the second signal with the third signal; and
a correction unit correcting the arctangent value,
wherein if the level comparison unit judges that the second signal is larger in level than the third signal, the exchange unit exchanges the second signal with the third signal, and selects a corresponding arctangent value from the ROM table based on the exchanged value, and the correction unit corrects the arctangent value outputted from the ROM table to the arctangent value to be obtained in the case where the second signal is not exchanged with the third signal.
REFERENCES:
patent: 4470147 (1984-09-01), Goatcher
patent: 4560941 (1985-12-01), Welles, II et al.
patent: 4875017 (1989-10-01), Sakazaki
patent: 5848739 (1999-04-01), Doi
patent: 0 526 747 (1993-02-01), None
patent: 2 113 930 (1983-08-01), None
Kinkead Arnold
Pioneer Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
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