Modulators – Frequency modulator
Reexamination Certificate
2001-06-21
2003-07-01
Tokar, Michael (Department: 2819)
Modulators
Frequency modulator
C332S123000
Reexamination Certificate
active
06587011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to frequency modulation and, in particular to frequency modulation of a carrier by radio signals.
2. Discussion of the Related Art
The frequency modulation (FM) of carriers by audio signals is very common. It is involved in various fields, such as the radiofrequency transmission of sounds and video cameras that transmit by hertzian means a composite audio and video signal to a video recorder or a television receiver.
Generally, in television systems, the audio signal, coming from a microphone or any processing circuit, is first used to modulate in frequency a carrier of frequency equal to 4.5, 5.5, 6 or 6.5 MHz, according to the desired television standard (PAL, NTSC). The obtained FM signal is then added to the baseband video signal and the combined signal is used to modulate a carrier for transmission.
FIG. 1
illustrates in a simplified manner an FM audio modulation chain usable in such a system. In
FIG. 1
, an analog baseband audio input signal IN, typically having a frequency under 20 kHz, drives an analog-to-digital converter (ADC)
2
. Converter
2
provides a digital audio signal coded over 16 bits, sampled at a frequency on the order of 50 kHz. The digital audio signal then generally passes through an automatic gain control unit (AGC)
4
, which clips possible signal peaks and increases the level of very low sounds.
The output of unit
4
is connected to a first output terminal OUT
1
. Output OUT
1
provides a digital audio signal coded over 16 bits, that can, for example, be used for mixing or sent for processing on the USB bus of a universal computer.
The audio signal at the output of the AGC unit is also provided to a digital frequency modulator (FM)
6
, connected to a second output OUT
2
. Output OUT
2
provides the desired signal, that is, frequency-modulated by the audio signal.
FIG. 2
illustrates in greater detail the FM modulation chain of FIG.
1
.
In
FIG. 2
, a signal IN drives a circuit
10
acting as an analog-to-digital converter and ensuring the automatic gain control. Output OUT
1
of circuit
10
provides a digital audio signal, coded over 16 bits, here at the 52.6 kHz frequency. Circuit
10
is connected, via various processing units, to an actual FM modulator
30
that provides, on output OUT
2
, a carrier modulated in frequency by the audio signal.
Circuit
10
includes a delta-sigma modulator (&Dgr;&Sgr;)
11
that turns the analog input signal into a signal coded over one bit at a sampling frequency, here equal to 6.75 MHz. This signal drives a comb filter
12
followed by a decimator
13
. The comb filter behaves as a low-pass filter and enables sub-sampling of the signal. Decimator
13
is a decimator by sixteen. Decimator
13
replaces sixteen successive samples of the signal with a single sample, of a value equal to the sum of the values of the replaced samples. Thus, at the output of decimator
13
, the frequency is equal to 421 kHz (6.75 MHz divided by 16) and the signal is coded over sixteen bits.
Then, the signal passes through an automatic gain control unit (AGC)
14
, having the same function as unit
4
of FIG.
1
. Then, the signal passes through a finite impulse response filter (FIR)
15
, followed by a decimator by four
17
. At the output of decimator
17
, the signal is sampled at a frequency of 421 kHz divided by four, that is, approximately 105 kHz. The signal, still over 16 bits, then passes through a second finite impulse response filter (FIR)
18
and a decimator by two
19
. At the output of decimator
19
, the signal is sampled at approximately 52.6 kHz. It is coded over 16 bits and supplies output OUT
1
. Filters
15
and
18
behave as low-pass filters. These are filters with steep sides, which enable sub-samplings
17
and
19
.
The output signal of circuit
10
crosses a pre-emphasis filter (PREEMP FILT)
22
. This filter is intended for emphasizing the high frequencies of the audio signal for a joint transmission with the video signal (in receive mode, a de-emphasis filter, symmetrical to the pre-emphasis filter, is used to restore the original audio frequencies). Then, the signal passes through a gain multiplier unit
23
. Unit
23
multiplies the signal by a constant G and aims at establishing an amplitude of the audio signal appropriate to the desired transmission standard. Then, the signal passes through an over-sampler
26
followed by a comb filter (COMB)
28
. Over-sampler
26
multiplies the number of samples by
512
. It conventionally operates by inserting zeroes, here
511
, between two samples, and the assembly is smoothed by filter
28
, used as a low-pass filter. At the output of filter
28
, the signal, still coded over 16 bits, is sampled at a frequency of 27 MHz (512 times 52.6 kHz).
The signal then drives the actual FM modulator
30
. The modulator conventionally includes a phase loop formed by an adder
32
and a shift register (REG)
33
. Adder
32
has three inputs. On a first input, it receives the audio signal coded over 16 bits. On a second input, it receives a constant P
0
, coded over 25 bits. On a third input, it receives the output of shift register
33
, also over 25 bits. The output of adder
32
is connected to the input of shift register
33
. Register
33
is driven by a clock of frequency FS equal to 27 MHz. The output of register
33
is connected to a sinusoidal shaper (LUT)
34
.
Unit
34
is formed of a look-up table, which retains at its input the 12 most significant bits from among the 25 output bits of register
33
. The table provides, on output OUT
2
, a sinusoidal signal corresponding to a carrier modulated in frequency by the input audio signal.
The operation of modulator
30
is the following.
Shift register
33
is a 25-bit register providing at its output a number ranging between 0 and (2
25
−1). Assume, to begin with, that the audio signal is absent. Upon each clock signal, output REG′ of register
33
is: REG′=REG+P
0
(modulo
25
), REG being the register output at the preceding clock pulse. REG′ follows an amplitude ramp between 0 and (2
25
−1). The frequency of this ramp is a frequency F
0
equal to F
S
.P
0
/2
25
. The choice of constant P
0
determines the carrier frequency of modulator
30
. For example, a constant P
0
equal to 6,835,162 will provide a carrier of a frequency F
0
equal to 5.5 MHz. The generated ramp may easily be transformed into a sinusoidal signal by unit
34
, the phase of the generated sinusoidal signal being proportional to the output signal of register
33
.
When an audio signal is present, its 16 bits are aligned on the 16 most significant bits among the 25 bits of the adder. The audio signal is added to constant P
0
, and a ramp having a frequency oscillating according to the audio signal around a central frequency determined by P
0
is obtained at the output of register
33
. Since the audio signal is sampled at the 27-MHz frequency, the output value of the shift register changes upon each pulse of clock CK. If the audio signal was sampled at a lower frequency, it would remain constant during several clock signals and a signal having a frequency varying by steps would be obtained as an output, which is not desirable. Unit
34
then receives the ramp at the register output and provides on output OUT
2
a signal corresponding to a carrier of frequency F
0
modulated by the input audio signal.
The modulation chain of
FIG. 2
has many disadvantages.
For example, modulator
30
is bulky and relatively expensive. The shift register indeed occupies a non-negligible space in the circuit, and so does the adder. The bus connecting the output of the shift register to the adder input is also bulky.
Further, to bring the audio signal to the operating frequency of the modulator, prior art provides an interpolator, which multiplies the sample frequency by
512
. This interpolator by
512
is an expensive and bulky component. Also, finite impulse response filters
15
and
18
are expensive components.
SUM
Morris James H.
Nguyen Khai
STMicroelectronics S.A.
Tokar Michael
Wolf Greenfield & Sacks P.C.
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