JPH04101537A - Modulating circuit - Google Patents

Abstract

PURPOSE:To obtain modulated wave with a small step frequency from a wide band by selecting a signal obtained by mixing sine wave with a small step frequency using a plurality of local signals while modulating the mixed signal as a carrier signal using a base band signal. CONSTITUTION:Output sine wave that is output from a DDS circuit 2 is divided into two by a hybrid circuit 3 and the divided two waves are input in mixer circuits 4 and 5, respectively. An analog switch 8 is controlled such that if a carrier signal to be sent is within a range of frequency extracted by a band pass filter 6, the analog switch 8 selects an output of the band pass filter 6, and if the carrier wave is within a range of frequency extracted by a band pass filter 7, the analog switch 8 selects an output of the band pass filter 7. A converter 9 uses a selected carrier signal to carry out modulation using a base band signal, thus outputting a modulated wave Fm.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a modulation circuit, and more particularly to a modulation circuit that uses DDS and constitutes a wideband, small-step frequency modulator used in mobile satellite communications.

[Conventional technology]

Conventionally, modulation circuits using this type of DDS have been PLL (Phase Locked) circuits using analog circuits.
Loop) Modulation with a baseband signal was performed using a carrier signal output from a synthesizer.

[Problem to be solved by the invention]

In the modulation circuit using the conventional DDS described above.

It is difficult to design the voltage-controlled oscillator (VCO) and the low-pass filter (LPF) as the loop filter that make up the PLL synthesizer used in the analog circuit, especially due to the limitations of the VCO band and the comparison frequency of the phase detector. If the step frequency is made smaller, the comparison frequency of the phase detector will be lowered, and the phase noise of the output waveform will deteriorate.Therefore, it is difficult to realize a modulation circuit with a wide band and small step frequency increments.

Furthermore, since it is an analog circuit, it is difficult to miniaturize it.
Another drawback is that it is sensitive to environmental conditions such as temperature and vibration.

[Means to solve the problem]

The modulation circuit of the present invention includes a DDS circuit that inputs N-bit binary data supplied from the outside and outputs a sine wave signal of small step frequency by digital synthesis;
an oscillator that oscillates the clock frequency of the circuit; and the DDS.
a hybrid circuit that outputs n-divided sine wave signals output from the circuit; and n local components each having a higher frequency than each of the n-divided outputs of the hybrid circuit and the sine wave signal output from the DDS circuit and different from each other. n mixers that input each of the frequencies and generate n carrier signals by frequency mixing; a local signal generator that generates the n local signals; and filtering the output of the n mixers. n bandpass filters that extract only the carrier signal; an analog switch that selects and outputs one of the n carrier signals; and a modulator that modulates the carrier signal selected by the analog switch with a baseband signal. It is composed of:

〔Example〕

Next, nine aspects of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention.

The embodiment shown in FIG. 1 shows an example in which one of the carrier signals obtained by frequency mixing a sine wave output from a DDS and two local signals is selected and modulated by a baseband signal. In Figure 1, 1 is an oscillator that outputs a sampling frequency of frequency Fs, 2 is a DDS circuit, 3 is a hybrid circuit that divides the output of DDS circuit 2 into two, 4 and 5 are mixer circuits, and 6 and 7 are bandpass circuits. Filter, 8 is an analog switch.

9 is a modulator, and 10 is a local signal generation circuit.

FIG. 2(a) is a block diagram showing the DDS circuit 2 of FIG. 1 in detail. In FIG. 2(a), 11 is an N-bit adder, 12 is a latch circuit, 13 is a ROM, 14 is a D/A converter, and 15 is a low-pass filter.

First, the operation of the DDS circuit will be explained with reference to FIG. The N-bit binary data inputted from the outside is added in the N-bit adder 11 to the output of the latch circuit 12 which internally takes in the output of the N-bit adder 11 at the timing of the sampling frequency F8. A signal that sequentially increases by the amount of input data at intervals of Fs is output as an address signal of the ROM 13.

The amplitude data of the sine wave is stored in the ROM 13 in advance, and the sample values of the sine wave read out sequentially are
The signal passes through the D/A converter 14 and is removed by the low-pass filter 15 to obtain a sine wave output frequency F3.

In the above operation, by multiplying the input data by
A sine wave twice the output sine wave is obtained in the same way.

FIG. 2(b) shows the sampling frequency F8 of the DDS circuit 2.
FIG. 3 is a diagram showing the relationship between the output frequency Fg and the output frequency Fg, and shows an example in which the sampling frequency Fs is eight times the output frequency Fg. According to the sampling theorem, the highest output frequency is Fs/2. Also, the maximum output frequency and step frequency will be the same value.

It is expressed as fs/2N (where N is the number of pits in the N-bit adder 10).

If the number of bits N of the N-bit adder 10 is a sufficiently large value, the variable range of the frequency output from the DDS circuit 2 is fs/2N-fs/2, and an oscillator with a wide band and small steps is realized. can.

Next, referring back to FIG. 1 again, the operation of the embodiment will be explained. As mentioned above, in FIG.

It is output from the DDS circuit 2. The output sine wave, which is variable in frequency in small steps according to the value of input data, is split into two by the hybrid circuit 3 and input to the mixer circuit 4 and the mixer circuit 5, respectively.

Local signals f LOa and f LOb of different frequencies are input from the local signal generation circuit 10 to the mixer circuits 4 and 5, respectively. These local signals f L
Oa and fLOb have frequencies shown in FIG. Namely.

f LOa > f LOb r f LOb f
LOa=f2 f-.

Figure 3 (a) and (b) respectively show the mixer circuit (4)
and (5), as seen on the frequency axis. The frequency range output from the DDS circuit 2 is f1~
If f2, the mixer circuit 4 is centered around f LOa (
ft, o-f2) ~(ft, o-f,) and (f
Frequency conversion is performed over the range from Lo, -f, ) to (fL,, -f2). The bandpass filter 6 has (fLo, -f2) to (fL, -f.

).

Similarly, in the mixer circuit 5, (fLob-f2) to (fLob-fl) and (t'L
, b-f, ) to (f tobf 2 ) by frequency conversion and a bandpass filter (fLobfQ )
It is retrieved between ~(fLob - f, ).

The analog switch 8 selects the output of the band pass filter 6 when the carrier to be transmitted is within the frequency range taken out by the band pass filter 6, and selects the output of the band pass filter 6 when the carrier to be transmitted is within the frequency range taken out by the band pass filter 7. It is controlled to select the output of the bandpass filter 7.

The converter 9 performs modulation with a baseband signal using one selected carrier signal, and outputs a modulated wave F. In this way, by combining the sine wave output from the DDS circuit 2 with two types of local signals and selecting one of them as a carrier signal, a band twice the signal band output from the DDS circuit 2 is obtained. It is possible to obtain a carrier signal with In this case, if the number of local signals is multiplied by m, a carrier signal with m times the band can be obtained.

In this way, an output modulated wave can be obtained with small frequency steps over a wide frequency range. A modulator using this DSS circuit does not have a feedback loop unlike a PLL circuit, so there is no pull-in operation, and nine frequencies can be changed extremely quickly in one clock from the time the input data is changed.

〔Effect of the invention〕

As explained above, according to the present invention, by modulating a baseband signal as a carrier signal while selecting a signal obtained by mixing a sine wave with a small step frequency obtained by a DDS circuit with a plurality of local signals, A modulated wave with a small step frequency can be obtained, and the PL
Since it does not use a VCO and low-pass filter like the L synthesizer, the design conditions are easier, and because there are fewer analog circuits, the modulator can be made smaller and lighter, and it has the advantage of being highly resistant to temperature changes and vibrations. be.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, FIG. 2(a) is a block diagram showing details of the DDS circuit 2 in FIG. ), Fig. 3(a) and (
b) is a diagram of the signals combined by the mixer circuits 4 and 5 in FIG. 1 viewed on the frequency axis. 1... Oscillator, 2... DD8 circuit, 3... Hybrid circuit, 4, 5... Mixer circuit, 6, 7...
Band pass filter, 8... Analog switch, 9.
...Modulator, 10...Local signal generation circuit, 11.
...N-bit adder, 12...Latch circuit, 13...
・ROM, 14...D/A converter. 5...Low pass filter.

Claims (1)

[Claims] A DDS (Direct Digital Synt) that inputs N-bit binary data supplied from the outside and outputs a sine wave signal with a small step frequency by digital synthesis
an oscillator that oscillates the clock frequency of the DDS circuit, a hybrid circuit that outputs n-divided sine wave signals output from the DDS circuit, each of the n-divided outputs of the hybrid circuit, and the DDS circuit. n mixers which generate n carrier signals by frequency mixing by inputting each of n local frequencies having a higher frequency than the sine wave signal outputted by the sine wave signal and which are different from each other; a local signal generator that generates a signal; n bandpass filters that filter the outputs of the n mixers to extract only the carrier signals; and an analog switch that selectively outputs one of the n carrier signals; A modulator comprising: a modulator that modulates a carrier signal selected by the analog switch using a baseband signal.