JPH0450767B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a gain control circuit using a power detection circuit;
In particular, the present invention relates to the configuration of a circuit that measures the input power to the input amplifier circuit of a modem in communication, starts the modem, and automatically controls the gain of the amplifier circuit.

[Conventional technology]

The basic configuration of a conventional gain control circuit is shown in FIG. An input analog signal passes through an input terminal 1, is amplified at a predetermined amplification degree by a variable gain amplifier 3, and is applied to a signal processing circuit (not shown) such as a modem through an output terminal 2. The power of a portion of the output of the amplifier 3 is detected by a power detection circuit 4. The detected power is used to generate a necessary gain control signal in the gain setting circuit 5, and the control signal is supplied to the variable gain amplifier. A portion of the gain control signal is applied to the power detection circuit 4 and used for power detection calculations. A desired reference power is applied to terminal 6 as a signal.

[Problem that the invention seeks to solve]

In the gain control circuit of FIG. 1, if the variable gain amplifier 3 can be varied linearly in decibels by digital signal input, it is desirable that the output of the power detection circuit 4 also be a digitized power signal expressed in decibels. In this case, the decibel conversion circuit is often configured with a read-only memory (ROM) in order to reduce calculation time.
This method has the disadvantage that if the dynamic range of the received power is wide, the ROM capacity or calculation time increases accordingly.

Therefore, an object of the present invention is to maintain the ROM even when the received power has a wide dynamic range.
It is an object of the present invention to provide a gain control circuit capable of obtaining a digitized power signal with high-speed gain control and accurate decibel display without increasing capacity or calculation time.

[Means for solving problems]

In order to achieve the above object, the present invention implements the power detection circuit 4 using a floating point arithmetic method, effectively expresses a power value with a wide dynamic range using a mantissa and an exponent, and uses this value to effectively express a power value with a wide dynamic range. Perform decibel conversion, subtract the gain indicated by the current gain control signal from the output power of the variable gain amplifier indicated by this decibel display value to find the amplifier input power, and calculate the input power determined from the required output power. The feature is that the gain control signal is obtained by subtraction.

[Effect]

According to the above configuration, since the input power detection value that determines the accuracy of output power control is obtained as a floating point number, the quantization error is small even though the input power has a wide dynamic range. Since the difference from the detected input power is directly the gain of the amplifier, the gain can be controlled quickly and accurately.

〔Example〕

Hereinafter, an embodiment in which a gain control circuit according to the present invention is applied to a data transmission modem will be described in detail with reference to FIG. The analog signal applied to the input terminal 1 is amplified by a variable gain amplifier 3 whose gain can be varied linearly in decibels, and converted into a digital signal by an analog/digital A/D converter 7. This signal is applied to the demodulation circuit of the modem from terminal 2, and the other end is applied to the squaring circuit 8, after which it is converted by the averaging circuit 9 into a signal corresponding to the average power from the multiple sample values, and then the decibel conversion circuit 1
0, it is converted to a signal expressed in decibels. The present invention is characterized in that a signal corresponding to the average power input to the decibel conversion circuit 10 is displayed in a floating point format and decibel conversion is performed.

That is, if the signal corresponding to the average power is expressed in a floating point format as Po'=a×2 ^b (mw), the output Po of this decibel converter 10 will be as follows.

Po=10logPo′(mw)/1(mw)=10loga+3b(dBm) Based on this result and the gain control signal applied to the variable gain amplifier 3, the signal power calculation circuit 11 calculates the following:
The power value of the input signal can be determined as a digital value. In other words, the gain of the variable gain amplifier is A
(dB), the power value Pi of the input signal can be found as Pi = Po - A (dBm).

Furthermore, if the required output power Po is Pref, the gain setting circuit 5 determines Pref−Pi=Aref from the detected Pi, and the necessary gain control signal is applied to the variable gain amplifier to obtain the required power.
An amplified input signal corresponding to Pref can be obtained, and an automatic gain control system can be constructed. Pref is input to the gain setting circuit 5 from the terminal 6.

〔Effect of the invention〕

Next, the effects of the floating point system according to the present invention will be described. FIG. 3 is an example of a comparison between the case where the power value of the digital input to the decibel conversion circuit 10 is based on the fixed-point system with an 8-bit mantissa, and the case where the digital input power value is based on the floating-point system of the present invention with a 5-bit mantissa and 3-bit exponent. It is.

This comparison is performed when both inputs to the decibel conversion circuit 10 are 8 bits.
As is clear from FIG. 3, the floating point system can handle power values over a wider range.
Furthermore, when the power value is large, the error caused by the minimum bit of the mantissa part is better when the fixed point method is used, but when the input power is small, the error caused by the fixed point method increases significantly. On the other hand, the floating point method has a nearly constant error and is very effective as a circuit used for gain control.

Furthermore, in order to make the power value conversion limit in the fixed point system equivalent to that in the floating point system described in FIG. 3, it is necessary to further increase the number of bits of the digital input to the decibel conversion circuit 10 by 4 bits.
This increase in the number of input bits means that, for example, when performing decibel conversion using read-only memory (ROM), if the decibel power value is expressed in 8 bits (1 byte), the ROM capacity will increase from 256 bytes to 4096 bytes. will increase dramatically. Furthermore, if it is implemented using other hardware, there is a drawback that the circuit scale becomes extremely large.

Arithmetic processing using this floating point system can be used not only for the power value decibel conversion circuit described here, but also for other arithmetic circuits in this gain control system.

Although the system has been described above with reference to embodiments, the present invention is not limited to the above embodiments. The squaring circuit, averaging circuit, decibel conversion circuit, signal power calculation circuit, gain setting circuit, etc. can be configured with dedicated circuits, or can be configured in combination with a signal processing device such as a microcomputer or other digital processing device. it is obvious.

[Brief explanation of drawings]

Figure 1 is the basic configuration diagram of the gain control circuit, Figure 2 is the basic configuration diagram of the gain control circuit.
This figure is a block diagram of an embodiment of the gain control circuit according to the present invention, and FIG. 3 is a diagram showing the effect of the floating point system according to the present invention. DESCRIPTION OF SYMBOLS 1...Input terminal, 2...Output terminal, 3...Variable gain amplifier, 4...Power detection circuit, 5...Gain setting circuit, 6...Gain setting signal input terminal, 7...
A/D converter, 8... Square circuit, 9... Averaging circuit, 10... Decibel conversion circuit, 11... Signal power calculation circuit.

Claims (1)

[Claims] 1 comprising a variable gain amplifier, a gain setting circuit for controlling the gain of the amplifier, and a power detection circuit for detecting the input power of the amplifier from the output signal of the amplifier and the output signal of the gain setting circuit, The detection circuit includes an A/D converter that converts the output signal of the amplifier from analog to digital, and an arithmetic circuit that calculates the average value of the output power of the amplifier as a floating point digital signal from the output of the A/D converter. It consists of a decibel converter that converts the output of the arithmetic circuit into a decibel signal, and a circuit that subtracts the output signal of the gain setting circuit from the output of the decibel converter, and the gain setting circuit receives the output signal of the amplifier. An automatic gain control circuit characterized in that the automatic gain control circuit is configured to obtain a gain control signal for the amplifier by subtracting the output signal of the power detection circuit from the power value required for the automatic gain control circuit.