JPH1098343A - Receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the error of a received code, due to saturation at the time of the high input of an AD converter in the receiver of a linear demodulation system. SOLUTION: A circuit, detecting a received input level as a DC voltage from the output of an intermediate frequency stage at an AGC preamplifier, is provided, and a detected voltage is inputted to a comparator to be compared with a previously set reference voltage. Thereby with respect to an input over a regulated value, a variable attenuator 4 provided at the preceding stage of an amplifier at a reception front/end stage is continued to instantly reduce an input level to the AD converter 11 for preventing the saturation of the AD converter 11 to reduce error of the code received.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a linear demodulation type receiver.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram of a demodulation section of a conventional linear demodulation type receiver. In order to demodulate a signal of the digital linear modulation method having amplitude information, a linear amplifier for storing and amplifying amplitude information of a received wave is required, and an AGC (automatic gain control) amplifier is generally used.

A arriving incoming wave is input from an antenna 1 and is input via an antenna terminal 2 to a band-pass filter 3 in a reception front end. After passing through the band-pass filter 3, the signal in the predetermined pass band is amplified by the amplifier 5, and is converted to the first intermediate frequency (IF) by the mixer 6. The reception signal changed to the IF frequency is band-limited by an IF filter such as a crystal filter, and is input to the linear amplifier 8. Here, an AGC amplifier is generally used as the amplifier 8 as described above. The received signal is further converted to a second intermediate frequency by a mixer 9 and then a band control filter 1 such as a ceramic filter.
0 is input to the AD converter 11.

The received wave is converted from an analog signal into a digital signal by an AD converter 11, subjected to demodulation processing including a required band limitation by a demodulator 12, demodulated as received data, and output from a demodulation output terminal 13. .

Here, the average value of the output values of the AD converter is calculated by the integrator 14, and the control voltage is fed back to the AGC amplifier 8 via the DA converter 15 to use a feedback type linear amplifier for controlling the gain. A method of controlling the input level to the AD converter 11 so that the input signal level does not exceed the dynamic range of the AD converter is widely known.

In FIG. 2, reference numerals 16 and 17 denote first and second local signals, respectively. In particular, in the case of a mobile radio line, the input level of the antenna terminal 2 of the receiver is, for example, -130 dBm to -40 dBm in a normal communication state due to fluctuations in transmission / reception distance or fading.
It fluctuates by more than dB. As an example, if the number of bits of the AD converter is 12 bits, the dynamic range of the AD converter is about 74 dB. Therefore, the AGC amplifier needs to control the input level of 16 dB (about 20 dB). Here, it is considered that such a receiver is adopted for a base station. At this time, if the mobile station of the other party transmits in close proximity to the base station, the antenna input level of the receiver is expected to exceed the above-mentioned reception level. For example, when the incoming arriving wave is input at -30 dBm to -20 dBm, the AGC amplifier needs to further reduce the input level of the AD converter by 10 to 20 dB and control the AD converter so as not to be saturated. .

[0007]

In this case, the AGC amplifier 8 controls the input level to the A / D converter to lower the gain so that the A / D converter does not saturate for a high input level. Thus, linear demodulation can be performed in principle. However, the control of the AGC amplifier is
The AD converter saturates and inputs a linear signal to the demodulator until the calculation of the average value is completed because of the recirculation type in which the average value of the input level is calculated and controlled from the output level of the converter. There are drawbacks that can not be done. A first object of the present invention is to eliminate such a drawback and reduce the saturation of the AD converter even at a high reception input.

[0008]

According to the present invention, in order to achieve the above object, an output of an intermediate frequency stage before an AGC amplifier is branched, and a reception input level is detected as a DC voltage from one of the signals. Input to the comparator to operate the comparator when the detected voltage exceeds a preset value.
Further, the output of the comparator causes the variable attenuator provided in the preceding stage of the amplifier in the reception front end unit to be turned on and off.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention is shown in FIG.
This will be described below. The incoming arriving wave is input from the antenna 1 and input to the band-pass filter 3 of the reception front-end unit via the antenna terminal 2 as in the conventional circuit. Then, it is input to the newly provided variable attenuator 4. Normally, the attenuation of the variable attenuator 4 is set to be 0 dB. The received wave is further amplified by an amplifier 5 and converted to a first intermediate frequency (IF) by a mixer 6. The reception signal converted to the IF frequency is band-limited by the IF filter 7.
Here, this output is branched into two, one of which is an AGC amplifier 8
And the other is input to the mixer 28. AGC amplifier 8
The subsequent operation is the same as the operation of the linear demodulation shown in FIG. The branch signal input to the mixer 28 is processed as follows.

In the mixer 28, the mixed signal is mixed with the branched second local signal 17, converted into a second intermediate frequency, and band-limited by a filter similar to the ceramic filter 10. Next, this signal is amplified by a logarithmic amplifier 21 and converted into a DC output via a detection diode 23. At this time, by appropriately selecting the values of the resistor 22 and the capacitor 24, the time constant τ ₁ can be reduced.
For example, by setting τ ₁ to be about 1/10 of the processing time of the integrator 14 for averaging the output value of the AD converter 11,
It is possible to know the change in the input signal level in a sufficiently short time with respect to the transmission rate. As an example, the symbol rate of data is set to 4.8 ksps (kilo symbol / second).
And then, assuming that calculates an average reception level of a few symbols, constants about 1ms (~ 0.208ms / Symbol × 5 symbols) time until the AGC amplifier is controlled
About. If the input level of the arriving received wave is very high for several symbols during this time, the AD converter will saturate, increasing the received code error. In contrast, assuming that outputs a control voltage of the detector diode 23 with a time constant of about 1/2 symbol (~ 0.1 ms),
Assuming that the value of the resistor R of 22 is 10 kΩ, it can be realized by setting the capacitance value of the capacitor 24 to 0.01 μF, and the level of the received signal can be detected with a time constant sufficiently fast with respect to the transmission rate. Can be.

As described above, the DC output proportional to the received electric field input is detected and input to the comparator 26. In the comparator 26,
Compared with a predetermined reference voltage 27, the output is set to High when a reception electric field input exceeding the reference voltage is input.

The variable attenuator 4 receives a High control voltage input from the comparator 26 and selects a variable attenuator having a constant attenuation. Such a variable attenuation attenuator can be easily obtained as a commercial product. For example, the attenuation is 2
If 0 dB is selected, the arriving wave is -20 to -30 dB
At Bm or more, the variable attenuator is inserted by 20 dB, so that the input level after the amplifier 5 is -40 to -50 dBm
The input level can be instantaneously reduced in a time sufficiently faster than the time constant of the AGC amplifier 8. Therefore, the saturation of the AD converter 11 can be reduced instantaneously. After that, the AD converter is controlled by the time constant of the feedback loop by the AGC amplifier 8, so that it can perform linear demodulation.

The circuit indicated by the broken line block 25 in FIG. 1 is a commercially available IC having an RSSI (reception field strength display) function, and can be easily obtained at relatively low cost.
The required amount of attenuation of the variable attenuator, the switching operation condition of the reception input level (set value of the reference voltage 27), and the like may be appropriately selected depending on the communication system. Further, since the variable attenuator operates only when the level of the received incoming wave is high, the reception sensitivity in the normal state does not deteriorate.

[0014]

According to the present invention, the input level of the AD converter can be controlled faster than the response characteristic of the AGC amplifier. It is possible to prevent the AD converter from being saturated with a sufficiently fast time constant. For this reason, it is possible to reduce an increase in received code errors due to saturation of the AD converter.

Further, since the variable attenuator is inserted in the front stage of the amplifier in the receiver front end section, the input level of the amplifier is reduced by the predetermined attenuation of the variable attenuator even at an extremely large reception input level. Therefore, it is possible to suppress the occurrence of nonlinear distortion due to the saturation of the amplifier, and this is also effective in reducing received code errors at high input. Further, since the circuit for detecting the reception electric field level can be constituted by a general-purpose IC, it is possible to improve the reception performance by inexpensive and simple means.

[Brief description of the drawings]

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

FIG. 2 is a block diagram of a conventional linear demodulation receiver.

[Explanation of symbols]

1: antenna, 2: antenna terminal, 3: front-end band limiting filter, 4: variable attenuator, 5: amplifier, 6:
Mixer, 7: band limiting filter for first IF, 8: AGC
Amplifier, 9: mixer, 10: second IF band limiting filter, 11: AD converter, 12: demodulator, 13: demodulation output terminal, 14: integrator, 15: DA converter, 16: first local signal, 17: second local signal, 20: second IF
Band limiting filter, 21: logarithmic amplifier, 22: resistor,
23: detection diode, 24: capacitor, 25: RS
SI IC, 26: comparator, 27: reference voltage, 28: mixer.

Claims (2)

[Claims] 1. A linear demodulation type receiver having a variable gain amplifier or an amplification circuit having a function of varying a gain and an AD converter, a circuit for detecting a received electric field strength of an incoming wave as a DC voltage, and a reference voltage and a reference voltage. A comparator for comparing the voltage and an attenuator in the front-end unit can be used to turn on / off the limited amount setting of the attenuator provided in the front-end unit in a time period different from the time constant of the variable gain control circuit of the intermediate frequency stage. A receiver characterized by the following. 2. The receiver according to claim 1, wherein a plurality of comparison levels of the comparator are provided, and an attenuation amount of the attenuator is varied stepwise according to a detected received electric field intensity.