JPH10117153A - Frequency conversion circuit and receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the S/N in a reception output by decreasing the C/N of a local oscillation signal leaked to an output of a balanced modulator so as to reduce the C/N of outputs of the balanced modulator and a band pass filter. SOLUTION: A received input is freqtzency-converted by a frequency conversion section (balanced modulator) 2, a band pass filter 3 selects a 1st intermediate frequency signal, which is amplified by an amplifier 4. A level of a local oscillation signal from a 1st local oscillation section 10 is controlled by a level control section 13 in response to a reception signal level detected by a level detection section 12 and the controlled signal is fed to the balanced modulator 2 as a carrier. The level control section 13 decreases an attentuation amount of the local oscillation signal when the level of the received signal is high and increases the attenuation amount of the local oscillation signal when the level of the received signal is low and gives the resulting signal to the balanced modulator 2. The effect of the C/N due to leakage of local oscillation is reduced by decreasing the level of the local oscillation level so as to improve the S/N.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a frequency conversion circuit and a receiver for improving a signal-to-noise ratio of a reception output.

[0002]

2. Description of the Related Art In a conventional receiver, in the process of demodulating a received RF signal into a baseband signal (for example, a voice signal) such as a single superheterodyne or a double superheterodyne, once in the case of a single superheterodyne, In the case of double superheterodyne, a required frequency component is extracted by performing frequency conversion twice, amplified, and then demodulated by a method such as detection to obtain a baseband signal.

FIG. 3 shows a functional block diagram of a conventional receiver. An input signal supplied to the receiver is tuned by a reception tuning unit 1 to extract a desired reception frequency signal (even a receiver without the tuning unit 1). After that, the frequency is converted by the frequency conversion unit 2 using the first local oscillation signal given from the first local oscillation unit 10 as a carrier, and the necessary first intermediate frequency signal is selected and amplified by the bandpass filter 3. Amplify in part 4. In a general receiver, the amplification unit 4 is an amplification unit with an AGC function in order to obtain a dynamic range of a reception input.

The received first intermediate frequency signal obtained by amplification is frequency-converted by the frequency conversion unit 5 using the second local oscillation signal supplied from the second local oscillation unit 11 as a carrier wave. Through the intermediate frequency amplifying unit 7, demodulation unit (detector) 8
Is converted (demodulated) into a baseband signal, amplified to a required level by the low-frequency amplifier 9, and output as a reception output.

FIG. 4 shows a frequency conversion circuit, in which a balanced modulator is used for the frequency conversion unit 2. The received signal f _R is frequency-converted by the balanced modulator 2 using the first local oscillation signal f _L as a carrier, and the first intermediate frequency signal (f _L −
f _R ) and f _L.

[0006]

Here, focusing on the elements constituting the signal-to-noise ratio (SN) of the reception output, the signal component S depends on the component of the desired signal given as the receiver input, and The component N is composed of a noise component (input noise) included in a signal supplied to the receiver input and a component (receiver noise) added in the process of frequency conversion, amplification, and detection of the received signal supplied to the receiver input. Is done.

[0007] Since the receiver noise is determined by the circuit configuration and operation of the receiver, it is generally determined uniquely once the design of the receiver is determined. In particular, noise components due to the CN components of the local oscillation signal cannot be removed even during the frequency conversion process of the receiver, and thus greatly affect the noise amount of the receiver.

In the range where the level of the received signal input is large, the gain of the amplifying section is suppressed by the AGC function of the receiver, so that the CN component is suppressed together with the received signal. When the reception input level is low as in the above, since the amplification gain is not suppressed by the AGC function, the CN of the local oscillation signal directly
It is clear that it affects N.

Generally, the effect of the CN component of the first local oscillation signal on the SN of the reception output in the first frequency conversion using a voltage controlled self-excited oscillator (VCO) as the local oscillation signal will be described below.

FIG. 5 shows the state of CN of the first local oscillation signal f _L given from the VCO. FIGS. 6 and 7 show how the CN component of the first local oscillation signal is included in the first intermediate frequency signal.

FIGS. 6 and 7 show the frequency spectrum of the signal appearing at the output of the balanced modulator 2 and the bandpass characteristics of the bandpass filter 3 in the frequency conversion circuit shown in FIG. 4 shows a frequency spectrum of an output signal which is finally extracted as a first intermediate frequency signal. (F _L −f _R ) and (f _L +
f _R ) indicates the lower sideband signal and the upper sideband signal obtained by the balanced modulator 2, and in this description, the first and second sideband signals are the first and second sideband signals.
The lower band signal is taken out as the intermediate frequency signal,
For this purpose, the band pass filter shown in the drawing is used as the filter characteristic of the band pass filter 3 provided.

It is needless to say that the bandpass filter 3 may be replaced by a low-pass filter.

The f _L component in the frequency spectrum appearing at the output of the bandpass filter 3 is an unnecessary component as an output. However, the coupling capacity between the local oscillation signal input port and the conversion output port of the balanced modulator 2, etc. , A constant amount is output (leaked), and contains the same relative amount of CN components as the first local oscillation signal f _L input to the local oscillation signal input port of the balanced modulator 2. .

It is an object of the present invention to reduce the CN of the local signal leaking to the output of the balanced modulator, thereby reducing the reception output, that is, the CN of the output of the balanced modulator and the bandpass filter. It is to improve the SN in the reception output.

[0015]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a level detecting means for detecting a level of a received signal and a local level control means for controlling a level of a local signal of a balanced modulator in accordance with the level of the received signal. This is achieved by having

If the above means is used, the level of the received signal is detected by the level detecting means, and the level of the local signal is variably controlled by the local level control means according to the level range. That is, when the received signal is large, the amount of attenuation of the level of the local oscillation signal by the local oscillation level control is reduced,
When the received signal is small, the level of the local oscillation signal is attenuated by the local oscillation level control to increase the local oscillation signal level. As a result, a local oscillator signal having a level corresponding to the level of the received signal can be supplied to the balanced modulator.

When the received signal is large, the frequency conversion output of the balanced modulator has a relatively large received signal level even if the local signal level is high, so that the influence of the CN component due to local leakage is small and the received signal is low. When the level is small, the level of the local oscillation signal is reduced. Therefore, even if the level of the received signal is low, the influence of the CN component due to local oscillation leakage is reduced, and as a result, the SN of the frequency conversion output is improved.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a functional block diagram of a receiver provided with a local oscillation level control function according to an embodiment of the present invention. FIG. 2 is a block diagram of a frequency conversion circuit having a local oscillation signal level control function according to the present invention.

In FIG. 2, the received input is frequency-converted by a frequency converter (balanced modulator) 2, a required first intermediate frequency signal is selected by a bandpass filter 3 and amplified by an amplifier 4. The local oscillation signal generated by the first local oscillation unit 10 is
Is supplied as a carrier, but the level is controlled by the local oscillation level controller 13 during the supply. The reception level is detected by the reception level detection unit 12 from the intermediate frequency amplification unit 4, and the detection signal is supplied to the local oscillation level control unit 13 for control.

The level of the received signal can be detected, for example, by extracting a part of the signal of the received electric field strength (or received signal level) detection circuit for the received AGC, and the local level control unit 13 operates according to the range of this level. When the received signal is large, the attenuation of the local oscillation signal by the local oscillation level control is reduced, and when the reception signal is small, the attenuation by the local oscillation level control is increased. As a result, a local oscillator signal having a level corresponding to the received signal level can be supplied to the balanced modulator 2.

The difference between the case where the level of the received signal is large and the case where the level is small is that FIG.
Shows the frequency spectrum of the output signal of the balanced modulator 2 when the received signal level is small.

Next, the relative relationship between the level of the leakage of the local signal to the output of the balanced modulator 2 and the level of the desired sideband will be described. As can be seen from FIGS. _{If R} is large (FIG. 6), the desired signal component (lower band component) (f _L −
Although the level of f _R ) is sufficiently higher than the CN level of the leakage of the f _L signal, when the received signal f _R is small (FIG. 7), the desired signal component (at the balanced modulator port) is obtained. f _L -f _R) is the level of it can be seen that a state where it can not be said that f _L a signal sufficiently in comparison with the CN-level leakage amount large. In extreme cases, it can be assumed that these levels are almost the same.

FIG. 8 shows a frequency spectrum of a signal obtained by selectively extracting a desired signal from the output of the balanced modulator shown in FIGS. 6 and 7 by the bandpass filter having the filter characteristics shown in FIG. It is clear from the figure that when the reception level is low, the influence of the CN component due to local leakage is large, and the SN of the converted output is deteriorated by the CN component due to local leakage.

In general, a balanced modulator has a signal input (received signal input) in which the level of a required local oscillation signal is to be converted.
It is known that a value that is at least slightly larger (about 10 dB) than the level of the received signal input is required. Conversely, the minimum value of the local oscillation signal level required for the conversion operation is obtained, and if the minimum value is supplied, it is useless.

On the other hand, since the amount of local leakage at the output port of the balanced modulator depends on the intrinsic performance of the balanced modulator, the absolute value of the amount of local leakage is proportional to the level of the input local signal. I understand. Therefore, in order to reduce the level of the CN component included in the local leakage, it is apparent that the level of the local signal should be reduced.

From this, it is clear that it is effective to reduce the amount of local component signal leakage during frequency conversion in order to reduce the amount of noise components that determine the performance of the reception SN. In FIG. 2, the received signal level is detected by the level detection unit 12, and when the received signal level is low,
By changing the level of the local oscillator signal continuously or stepwise in proportion to the received signal level by the local oscillator level controller 13, the local oscillator signal level is reduced according to the received signal level,
When the reception signal level is low (the reception electric field is low), the local oscillation signal level can be reduced to reduce local leakage and improve SN. When the received signal level is high, the amount of attenuation of the local oscillation signal level can be reduced, and the required local oscillation signal can be supplied to the balanced modulator 2 to perform a required conversion operation.

[0028]

As described above, according to the present invention, it is possible to reduce the extent to which the CN component of the local oscillation signal affects the reception output, and particularly to the reception near the reception sensitivity point where the reception input level is low and noise is a problem. Can be improved. As a result, the same effect as improvement of the sensitivity of the receiver can be obtained, and a high-sensitivity receiver can be obtained without selecting components and materials constituting the receiver.

[Brief description of the drawings]

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

FIG. 2 is a block diagram of a frequency conversion circuit according to an embodiment of the present invention.

FIG. 3 is a block diagram of a conventional receiver.

FIG. 4 is a block diagram of a conventional frequency conversion circuit.

FIG. 5 is a frequency spectrum of a first local oscillation signal.

FIG. 6 is a frequency spectrum of a first frequency conversion output.

FIG. 7 is a frequency spectrum of a first frequency conversion output.

FIG. 8 is a band filtered signal frequency spectrum of a first frequency conversion output.

[Explanation of symbols]

1: reception tuning unit, 2: frequency conversion unit (balanced modulator), 3
... Band filter, 4 ... Intermediate frequency amplifier (AGC amplification), 5
... Frequency converter, 6 band filter, 7 intermediate frequency amplifier, 8 demodulation detector, 9 low frequency amplifier, 10 first local oscillator, 11 second local oscillator, 12 Reception level detector, 1
3. Local oscillation level control unit.

Claims (2)

[Claims] In a frequency conversion circuit using a balanced modulator of a receiver, a level detection means of a received signal and a local oscillation level control for controlling a level of a local oscillation signal of the balanced modulator in accordance with the received signal level And a frequency conversion circuit. 2. A receiver, wherein the frequency conversion circuit according to claim 1 is provided in a first frequency conversion unit.