JPH09270651A - Automatic gain control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a broad band automatic gain control circuit with comparatively inexpensive components and to prevent adverse effect at a high gain when an input signal has a very small level or no signal is received. SOLUTION: A signal resulting from a broad band base band signal whose frequency band B ranges from a signal around a DC component to 100MHz or over scrambled by a pseudo random signal with a period R is fed to an input terminal 10. The input signal is fed to a filter 13 via a variable gain amplifier 11, from which a signal with a frequency band being B/R or over which is part of the entire frequency band is extracted. An output signal from the filter 13 is detected by a detector 14 and fed to a gain control terminal 17 of the variable gain amplifier 11 via a loop filter 15 and a changeover switch 16. Furthermore, an output signal from the detector 14 is fed to a comparator 18, in which the signal is compared with a reference voltage Vrb for a prescribed low level and its comparison output is used to switch the changeover switch 16. An output from the loop filter 15 is fed to a selected terminal (x) of the changeover switch 16 and a fixed voltage Vc used to control the variable gain amplifier 11 to be a prescribed low gain is fed to a selected terminal (y).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD OF THE INVENTION The present invention relates to
The present invention relates to an automatic gain amplifier circuit for amplifying a wide band signal having a base band of up to 00 MHz and even up to several GHz to obtain an output signal of a constant level.

[0002]

2. Description of the Related Art An automatic gain control (AGC) circuit is for automatically controlling the gain of an amplifier circuit according to the fluctuation of the input signal level in a receiver or the like to always obtain an output signal of a constant level. is there. For this automatic gain control, it is necessary to detect the signal level and send it as an AGC signal to the gain control terminal of the variable gain amplifier.

[0003]

By the way, when handling a wideband signal having a base band in the band from near DC to several hundreds MHz or several GHz, the signal distribution should be regarded as equal in time and frequency. Therefore, it is necessary to design the automatic gain control circuit in consideration of the level of the entire target signal band. In this case, the wider the band is, the higher the performance of the component is required, and the more expensive the component is.

Further, when there is no signal, the automatic gain control circuit becomes full gain (maximum gain) and amplifies the noise component of the entire band of the automatic gain control circuit to the maximum amplification capability. As a result, the wider the circuit, the larger the total noise power becomes, which adversely affects the circuit subsequent to the automatic gain control circuit as an unnecessary noise source. In addition, the wide band and high gain cause an unstable circuit, which causes unnecessary oscillation.

The present invention has been made in view of the above circumstances, and a wide band automatic gain control circuit can be realized with relatively inexpensive parts. Further, when there is no signal, an unnecessary output is not generated and the latter stage is realized. It is an object of the present invention to provide an automatic gain control circuit which does not have an undesired adverse effect on the circuit and does not cause the circuit to become unstable.

[0006]

In order to solve the above-mentioned problems, an automatic gain control circuit according to the present invention scrambles a wideband baseband signal having a frequency band B of 100 MHz or more with a pseudo-random signal of period R. Variable gain amplifier to which the input signal is input, filter means for extracting a signal in a frequency band of B / R or more, which is a part of the entire frequency band of the output signal from the variable gain amplifier, and a signal from this filter means. Is detected, and the gain is controlled by sending the detection output from the detection means to the gain control terminal of the variable gain amplifier.

In this case, it is preferable that the pseudo-random signal having the cycle R is an M-sequence signal having a cycle 2 ⁿ -1 which is composed of n stages of shift registers.

Further, the detection output from the detection means is compared with a predetermined low level reference voltage, and the gain control signal of the gainable amplifier is switched to a predetermined low gain according to the comparison output. Good.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will be described below.

First, FIG. 1 is a block circuit diagram showing a basic configuration of an embodiment of an automatic gain control circuit according to the present invention.

In FIG. 1, at the input terminal 10, a frequency band B from near DC to several hundreds MHz or even several GHz is used as a base band, and the base band signal has a longest linear coded sequence.
nse) So-called M sequence or so-called PRBS (Pseu
A signal coded by a pseudo-random signal such as do Ramdom Binary Sequence) is input. This means that the information signal sequence is scrambled by using a random sequence such as M sequence, and effects such as suppression of zero continuation of the information signal sequence, securing of timing, and suppression of jitter can be obtained. The M sequence is the longest one of the code sequences generated by a shift register or a delay element having a predetermined length, for example, n stages, and the cycle R is 2
^{It is n} −1.

The wideband input signal from the input terminal 10 is sent to the variable gain amplifier 11 and is gain-controlled to become a constant level signal, which is taken out from the output terminal 12. Further, the output from the variable gain amplifier 11 is sent to a detector 14 through a filter 13 such as a bandpass filter for extracting a signal in a frequency band of B / R or more, which is a part of the wide band, and is detected. The output is sent to the gain control terminal 17 of the variable gain amplifier 11 via the loop filter 15 and the changeover switch 16.

FIG. 2 shows the spectrum of a specific example of the scrambled wide band signal supplied to the input terminal 10. This is a cycle R in which a signal having a baseband bandwidth of 1485 MHz is generated at a clock rate of 1485 MHz.
Is a signal spectrum obtained by scrambling with an M sequence signal of, for example, 2 ⁹ −1 = 511, and is a direct current (D
In the band from the vicinity of C) to the clock frequency, the signal components exist evenly or evenly in terms of time series and frequency distribution. The filter 13 extracts a frequency band of B / R or higher, which is a part of such a wide band signal, for example, about 2.9 MHz or higher. Specifically, for example, a low-pass filter having a bandwidth of about 25 MHz is used. An example of the filter output signal at this time is shown in FIG. The signal passing through the filter 13 is detected by the detector 14, and the detected output voltage is supplied to the inverting input terminal of the differential amplifier of the loop filter 15 and compared with the reference voltage Vra supplied to the non-inverting input terminal. Therefore, the error component is input to the gain control terminal 17 of the variable gain amplifier 11 via the selected terminal x of the changeover switch 16.
Sent to. As a result, the gain of the variable gain amplifier 11 is controlled so that the detected output voltage becomes equal to the reference voltage Vra of the loop filter 15.

Here, the principle that the signal level of the entire band can be detected by only extracting a part of the wide band signal by the filter 13 will be described. FIG. 4 is an enlarged view of the frequency axis on the low frequency side of the spectrum of FIG. As is clear from FIG. 4, a signal obtained by scrambling a wideband baseband signal having a frequency band of B with an M sequence signal having a period R has a periodicity of B / R frequency intervals on the frequency axis. For example, when the frequency band B of the baseband signal is 1485 MHz and the period R of the M-sequence signal is 2 ⁹ −1 = 511, B / R is about 2.9 MHz. Therefore, the signal level of the entire band can be detected by extracting the signal of the frequency band of one or more times of this periodic structure, that is, the signal of the frequency band of about 2.9 MHz or more from the wide band signal and detecting or detecting the level.

FIG. 5 shows the relationship between the input level to the input terminal 10 of the automatic gain control circuit of FIG. 1 and the detection output voltage from the detector 14. The detection voltage characteristic of the detector 14 indicates the input level. However, the detected output voltage is constant above −45 dBm. In FIG. 6, this input level is -45d.
The output spectrum at Bm is shown. FIG.
Shows the output spectrum when the input level is −60 dBm, and in this case, it can be seen that the input level is small and amplification cannot be completed sufficiently.

The detection output voltage from the detector 14 is the comparator 1
8 is also sent to the inverting input terminal of the comparator 8. The reference voltage Vrb is applied to the non-inverting input terminal of the comparator 18.
The reference voltage Vrb of the comparator 18 is set so as to correspond to a small signal level such that the input signal of the input terminal 10 cannot be sufficiently amplified by the variable gain amplifier 11, and the input signal level lower than such a specification is set. At this time or when it is determined that there is no input, the changeover switch 16 is controlled to change over to the selected terminal y. A fixed voltage Vc from the voltage source 19 is supplied to the selected terminal y of the changeover switch 16, and the fixed voltage Vc is a voltage for setting the gain of the variable gain amplifier 11 to the minimum. Therefore, when the input level is lower than the specification or when there is no signal, the changeover switch 16
Is switched to the selected terminal y, the fixed voltage Vc is supplied to the gain control terminal 17 of the variable gain amplifier 11, and the gain of the variable gain amplifier 11 is controlled to be set to the minimum. As a result, when the input signal level becomes low or when there is no signal,
The problem that the gain of the variable gain amplifier 11 is controlled to the maximum gain and the noise component in the entire region is amplified to adversely affect the circuit (not shown) in the subsequent stage of the automatic gain control circuit in FIG. 1 is prevented. be able to.

In FIG. 8, the level of the input signal at the input terminal 10 is lower than, for example, -60 dBm, the detection output voltage from the detector 14 becomes lower than the reference voltage Vrb of the comparator 18, and the changeover switch 16 is operated. It shows a state in which the fixed voltage Vc is supplied to the gain control terminal 17 of the variable gain amplifier 11 and is controlled to the minimum gain by being switched and connected to the selected terminal y side. At this time, the output signal becomes the lowest level, and the adverse effect on the circuit in the subsequent stage is eliminated. In addition, it is possible to prevent the variable gain amplifier 11 itself from becoming the maximum gain and becoming unstable and causing unnecessary oscillation and the like.

Such an automatic gain control (AGC) circuit is used in a wide band transmission system as shown in FIGS. 9 and 10, for example.

FIG. 9 shows a wideband serial data transmission system using a coaxial cable 51. At the input terminal 52, M series of wideband baseband signals from near DC to several hundreds MHz or several GHz are shown. An input signal obtained by scrambling with the signal is supplied. This broadband input signal is input to the cable driver 53, which is a transmitter, and is sent to the coaxial cable 51. At the receiving end of the cable, the signal is input to the equalizer 54, the cable equalization circuit 55 compensates for the loss incurred during the cable transmission, and then it is amplified to an appropriate level by the AGC (automatic gain control) circuit 56, and output from the output terminal 57. Taken out. The signal from the output terminal 57 is subjected to descrambling processing (not shown) to restore the original baseband signal data.

FIG. 10 shows a wideband serial data transmission system using an optical fiber 61, and an input terminal 62 is supplied with an input signal obtained by scrambling a wideband baseband signal with an M series signal. This broadband input signal is input to an electric / optical conversion circuit 63 such as a laser driver on the transmission side, converted from an electric signal to an optical signal such as laser light, and sent out to an optical fiber 61. At the receiving end of the optical fiber 61, it is input to the optical / electrical conversion unit 64,
After being converted from an optical signal to an electric signal by a light receiving element 65 such as a PIN photodiode, it is amplified to an appropriate level by an AGC (automatic gain control) circuit 66 and taken out from an output terminal 67. This output terminal 67
Descramble processing (not shown) is applied to the signal from the above to restore the data of the original baseband signal.

Each AG having the configuration shown in FIG. 9 or FIG.
As the C circuits 56 and 66, the automatic gain control (AGC) circuit as shown in FIG. 1 described above is used.

According to the automatic gain control circuit as described above, the detector 14, the loop filter 15, or the comparator 18 shown in FIG. 1 does not require a wideband circuit up to several 100 MHz or several GHz. 25M in the example
Since it suffices to use a circuit that normally operates in the Hz band,
A broadband automatic gain control circuit can be realized with relatively inexpensive parts.

Further, when the input level is very low or there is no signal, the automatic gain control circuit does not generate an unnecessary output. That is, the automatic gain control circuit operates so that the smaller the input level is, the larger the gain becomes, and the maximum gain (full gain) is achieved at the time of minute input or no signal, and the circuit at the subsequent stage is adversely affected by the amplified noise. Also, it is possible to prevent the problem that the amplifier itself of the automatic gain circuit becomes unstable due to the high gain and causes unnecessary oscillation.

The present invention is not limited to the above-described embodiment. For example, in the circuit of FIG. 1, the filter may be a low pass filter or a band pass filter, and the loop filter 15 compares the reference voltage with the reference voltage. It may not be necessary, and various other configurations are possible. Also, the frequency band of the baseband, the clock frequency of the M series, and the M
The number n of shift register stages for sequence generation or the frequency band of a filter for extracting a part of the entire band can be arbitrarily set.

[0025]

As is apparent from the above description, according to the automatic gain control circuit of the present invention, the frequency band B is 10
A wideband baseband signal having a frequency of 0 MHz or higher is scrambled by a pseudo-random signal with a period R, and is used as an input signal of the variable gain amplifier, and becomes a part of the entire frequency band of the output signal from the variable gain amplifier. A wide band automatic gain control circuit is realized with relatively inexpensive parts by controlling the gain by extracting the signal in the above frequency band with filter means, detecting it, and sending the detected output to the gain control terminal of the variable gain amplifier. it can.

Further, the detected output is compared with a predetermined low level reference voltage, and the gain control signal of the variable gain amplifier is controlled to be switched to a predetermined low gain in accordance with the comparison output, so that a small input level is obtained. Alternatively, when there is no signal, the gain of the automatic gain control circuit is forcibly reduced to prevent the adverse effect of the automatic gain control circuit becoming the maximum gain (full gain) at a minute input level or when there is no signal. it can.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing a basic configuration of an embodiment of an automatic gain control circuit according to the present invention.

FIG. 2 is a diagram showing a specific example of a spectrum of an input signal.

FIG. 3 is a diagram showing a specific example of a spectrum of an output signal from a filter 13.

FIG. 4 is an enlarged view showing the low frequency side of the spectrum of the input signal of FIG.

FIG. 5 is a diagram showing a relationship between an input level and a detection voltage.

FIG. 6 is a diagram showing a specific example of a spectrum of an output signal when the input level is −45 dBm.

FIG. 7 is a diagram showing a specific example of a spectrum of an output signal when the input level is −60 dBm.

FIG. 8 is a diagram showing a specific example of a spectrum of an output signal when the input level is very low or when there is no signal.

FIG. 9 is a diagram showing an example of a wideband signal transmission system using a coaxial cable.

FIG. 10 is a diagram showing an example of a wideband signal transmission system using an optical fiber.

[Explanation of symbols]

10 input terminals, 11 variable gain amplification amplifier, 12
Output terminal, 13 filter, 14 detector, 15
Loop filter, 16 changeover switch, 17 gain control terminal, 18 comparator, 19 voltage source

Claims (3)

[Claims] 1. A frequency band B from near DC of a signal is 1
A wideband baseband signal of 00 MHz or higher has a period R
Variable gain amplifier which is scrambled by the pseudo random signal and inputted, and a filter means for taking out a signal in a frequency band of B / R or more which becomes a part of the entire frequency band of the output signal from the variable gain amplifier, and this filter. An automatic gain control circuit for controlling the gain by sending a detection output from the detection means to a gain control terminal of the variable gain amplifier. 2. The automatic gain control circuit according to claim 1, wherein the pseudo-random signal of the cycle R is an M-sequence signal of a cycle 2 ⁿ -1 which is composed of shift registers of n stages. 3. The detection output from the detection means is compared with a predetermined low level reference voltage, and the gain control signal of the variable gain amplifier is controlled to be switched to a predetermined low gain according to the comparison output. The automatic gain control circuit according to claim 1.