JP3793641B2 - Direct conversion receiver - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a distortion prevention technique in a direct conversion receiver.
[0002]
[Prior art]
For example, as shown in FIG. 3, the direct conversion type receiver includes a local oscillator 101 that generates two local oscillation signals whose phases are 90 degrees different from each other, and the two local oscillation signals as reception signals. First and second multiplication circuits 102 and 103 for multiplication, first and second low-pass filters LPF 104 and 105 for removing harmonic components of outputs of the first and second multiplication circuits 102 and 103, and An arithmetic circuit 106 for adding and calculating the outputs of the LPFs 104 and 105 and a demodulating circuit 107 for demodulating the output of the arithmetic circuit 106 were provided.
[0003]
In the direct conversion receiver configured as described above, the components S (0), S (1), S (2), S (3), S (4) as shown in FIG. )... Are converted into baseband, as shown in FIG. 4B, a center frequency component S (0) ′ converted into a DC component having a frequency of 0 Hz, Other channel components S (2) ′, S (3) ′,... Are included.
[0004]
However, if there is an interference wave in the vicinity of the received signal, distortion occurs, and the secondary distortion becomes a signal and a DC component having a frequency twice that of the interference wave, and the DC component affects the signal converted to baseband. Therefore, the harmonic component S _dis due to the second order distortion is generated.
When the interference wave is sin φ, the secondary distortion due to the interference wave is sin ² φ, and this secondary distortion is expressed as sin ² φ = (1−cos2φ) / 2. A DC component and a harmonic component (cos2φ) having a frequency twice that of the interference wave are included.
[0005]
Further, for example, the direct conversion receiver described in U.S. Pat.No. 4,944,025 is configured to add the local oscillation frequency to the offset frequency (for example, 100 kHz) and output from the local oscillator. 1 and 2 have a configuration in which a coupling capacitor for removing the DC component of the output of the second multiplication circuit is provided.
This direct conversion receiver is intended for radio waves having a radio broadcast channel interval of about 400 kHz, and the offset frequency is also relatively large at 100 kHz as described above.
[0006]
[Problems to be solved by the invention]
The technique described in US Pat. No. 4,944,025 is effective for a relatively wide channel interval (about 100 kHz or more), but in a field where the channel interval is very narrow, such as amateur radio. Had the problem of including the cause of interference.
[0007]
Therefore, according to the present invention, the modulation frequency (sound frequency) in the case of modulation with sound is usually in the range of about 300 to 2000 Hz, so even if the frequency component below the modulation frequency is removed, there is little influence on communication. In particular, the present invention has been made for the purpose of providing a direct conversion receiver capable of preventing the occurrence of distortion even when the channel interval is narrow as in the field of amateur radio.
[0008]
[Means for Solving the Problems]
The direct conversion receiver according to claim 1 is:
Branching means for branching the received signal received by the receiving unit into two;
A local oscillator that generates two local oscillation signals that are 90 degrees out of phase with each other;
Offset means for offsetting the oscillation frequencies of the two local oscillation signals by a predetermined offset frequency;
A first multiplication circuit and a second multiplication circuit for multiplying two offset received local oscillation signals by two branched reception signals, respectively;
First and second filter means for removing harmonic components and DC components from the multiplication signals output from these two multiplication circuits, respectively.
An adder circuit for adding signals output from these two filter means to each other;
And a demodulating circuit that demodulates the added signal output from the adding circuit to obtain a demodulated signal.
[0009]
The direct conversion receiver according to claim 2 is:
Detecting means for detecting the center frequency of the addition signal output from the addition circuit or the demodulated signal;
Control means for controlling the offset frequency in the offset means based on the detected center frequency.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a direct conversion system receiver according to the present invention will be described in detail with reference to the drawings showing embodiments thereof.
[0011]
In FIG.
Reference numeral 1 denotes a receiving unit including an antenna, a high-frequency filter, a high-frequency amplifier, and the like. Reference numeral 10 denotes a branch circuit that branches the high-frequency signal S output from the receiving unit 1 into two systems and outputs the two systems.
Reference numeral 2 denotes a local oscillating unit. The first local oscillating frequency f _LO 1 to be output can be controlled by an external signal c, and the phase of the first local oscillating frequency f _LO 1 is delayed by 90 degrees. And a phase shift circuit 22 that outputs the signal as two local oscillation frequencies f _LO 2.
[0012]
Reference numeral 31 denotes a first multiplication circuit that multiplies the first local oscillation frequency f _LO 1 by the branched high-frequency signal S and outputs a signal S11.
Reference numeral 32 denotes a second multiplication circuit that multiplies the second local oscillation frequency f _LO 2 by the branched high-frequency signal S and outputs a signal S12.
[0013]
41 is a first filter circuit composed of a high-pass filter 41H that removes a DC component from the signal S11 output from the first multiplication circuit 31, and a low-pass filter 41L that further removes a harmonic component; The signal S21 from which the DC component and the harmonic component are removed is output.
42 is a second filter circuit composed of a high-pass filter 42H that removes a DC component from the signal S12 output from the second multiplier circuit 32, and a low-pass filter 42L that removes a harmonic component. The signal S22 from which the DC component and the harmonic component are removed is output.
[0014]
An adder circuit 5 adds the two signals S21 and S22 and outputs an added signal S3.
Reference numeral 6 denotes a demodulation circuit that demodulates the addition signal S3 and outputs a low-frequency signal S4.
[0015]
Reference numeral 7 denotes a detection circuit that detects a change in the center frequency of the addition signal S3 and outputs a detection signal c1 corresponding to the change.
Reference numeral 8 denotes an offset frequency control circuit, and 9 denotes an offset circuit. The offset frequency control circuit 8 controls an external signal c output from the offset circuit 9 to the local oscillation circuit 2 based on the detection signal c1.
[0016]
In the above configuration,
As shown in FIG. 2A, the high-frequency signal S output from the receiving unit 1 includes components S (1), S of other channels centered on the center frequency component S (0) of the frequency f0. (2), S (3), S (4)... Are distributed.
The multiplication circuits 31 and 32 multiply the high frequency signal S by the local oscillation frequency f _LO 1. Here, since the local oscillation frequency f _LO 1 is a frequency obtained by adding an offset frequency to the down-conversion frequency, the frequency of the center frequency component S (0) is 0 Hz as shown in FIG. Instead, the offset frequency f is _off . Note that a DC component S _DC and a harmonic component S _dis due to secondary distortion are generated, and unnecessary components S (2) ′, S (3) ′... Are included.
[0017]
When the DC component is removed by the high-pass filters 41H and 42H having the cut-off frequency fc (<f _off ), the DC component S _DC is removed and the cause of distortion is reduced. However, as the component of the offset frequency f _off The obtained center frequency component S (0) ′ is not removed.
[0018]
Subsequently, the harmonic component S _dis is removed by the low-pass filters 41L and 42L in which the cutoff frequency is set to a frequency slightly higher than the offset frequency f _off .
Accordingly, the components of the two local oscillation signals whose phases are shifted by 90 degrees are canceled out by the adder circuit 5, and only the frequency-converted signal component is output from the adder circuit 5 as the add signal S3.
[0019]
This addition signal S3 is FM detected in the demodulation circuit 6 to obtain a low frequency signal S4.
[0020]
In the detection circuit 7 to which the addition signal S3 is branched and input, if the center frequency fluctuates during reception, the detection signal c1 corresponding to the frequency shift due to the fluctuation is output, so that the control is performed by the offset frequency control circuit 8. The external signal c output from the offset circuit 9 that has been changed also follows. For this reason, the offset frequency f _{off of the} local oscillation signals f _LO 1 and f _LO 2 output from the local oscillation circuit 2 also fluctuates, so that the center frequency component is removed by the filter circuits 41 and 42 or the DC component remains. It can be prevented. In addition, since the cut-off frequency can be set to a marginal frequency without considering a margin, interference can be reliably removed even when the frequency difference with other adjacent channels is small.
[0021]
By configuring the filter means with a high-pass filter and a low-pass filter, it is possible to provide a direct conversion receiver that can obtain the above-mentioned effects at low cost by using general-purpose circuits and components.
[0022]
【The invention's effect】
As described above, in the invention of claim 1,
Since the oscillation frequency of the two local oscillation signals is offset by a predetermined offset frequency,
By removing the harmonic component and the DC component from the multiplication signals output from the two multiplication circuits, it is possible to reduce both the distortion applied to the harmonic component and the distortion applied to the DC component.
[0023]
Further, in claim 2, since the center frequency of the addition signal or demodulated signal output from the addition circuit is detected and the offset frequency is controlled based on the detected center frequency, the frequency of the reception signal varies. Can be tracked, and it is possible to prevent the signal component from being removed or the direct current component from remaining and causing distortion, and the cutoff frequency of the first filter means and the second filter means can be set to a desired value. Since the frequency can be set at the limit, distortion can be prevented even when the channel interval is narrow as in the field of amateur radio.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing the configuration of an embodiment of a direct conversion receiver according to the present invention.
FIG. 2 is a frequency distribution diagram of signals at various parts in the direct conversion receiver.
FIG. 3 is a block diagram of a conventional direct conversion receiver.
FIG. 4 is a frequency distribution diagram of a signal of a conventional direct conversion receiver.
[Explanation of symbols]
2 Local oscillator (local oscillator)
31 First multiplication circuit
32 Second multiplication circuit
41 First filter circuit (first filter means)
42 Second filter circuit (second filter means)
41H high-pass filter
41L low-pass filter
42H high-pass filter
42L Low-pass filter 5 Adder circuit 6 Demodulator circuit 7 Detection circuit (detection means)
8 Offset frequency control circuit (control means)
9 Offset circuit (offset means)
10 Branch circuit (branch means)

Claims (2)

Branching means for branching the received signal received by the receiving unit into two;
A local oscillator that generates two local oscillation signals that are 90 degrees out of phase with each other;
Offset means for offsetting the oscillation frequencies of the two local oscillation signals by a predetermined offset frequency;
A first multiplication circuit and a second multiplication circuit for multiplying two offset reception signals by two branched reception signals;
First and second filter means for removing harmonic components and DC components from the multiplication signals output from these two multiplication circuits, respectively;
An adder circuit for adding signals output from these two filter means to each other;
And a demodulating circuit for demodulating the added signal output from the adding circuit to obtain a demodulated signal. Detecting means for detecting the center frequency of the addition signal output from the addition circuit or the demodulated signal;
2. The direct conversion system receiver according to claim 1, further comprising control means for controlling the offset frequency in the offset means based on the detected center frequency.

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