A very low noise preamplifier for extremely low frequency magnetic antenna

    Corresponding author: Shimin Feng, fengshimin_86@126.com
  • Department of Weaponry Engineering, Naval University of Engineering, Wuhan 430033, China

Key words: ELF magnetic antennapreamplifierlow noisefrequency compensation

Abstract: Besides the electrode-pair antenna, the magnetic antenna is also used for the extremely low frequency (ELF) submarine communication. To receive the weak ELF signals, the structure of a small sized magnetic antenna determines its specific electrical characteristics. The ELF magnetic antenna shows high internal resistance, alternating-current impedance, and a resonance frequency near the operating bandwidth. In accordance with the electrical characteristics of ELF magnetic antenna, a low noise preamplifier and frequency compensation circuit were designed and realized. The preamplifier is a three-stage negative feedback circuit, which is composed of parallel JFET, common-emitter amplifier with a Darlington structure and a common-collector amplifier in push-pull connection. And a frequency compensation circuit is cascaded to compensate the characteristic in low frequency range. In the operating bandwidth f=30-200 Hz, the circuit has a gain of 39.4 dB. The equivalent input noise is 1.97 nV/$\sqrt {{\text{Hz}}} $ and the frequency response keeps flat in operating bandwidth. The proposed preamplifier of the ELF magnetic antenna performs well in receiving ELF signals.

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1.   Introduction
  • When receiving extremely low frequency (ELF) signal, an electronic antenna will occupy a lot of space. Under conditions of limited space, a magnetic antenna can be appropriate. Because the received ELF signal is weak under water, a very low noise preamplifier is required for the receiver to process and analyze the signals[1]. Meanwhile, to receive weak ELF signal, a tightly wound copper wire with fine diameter and multilayers is used to build the magnetic antenna[2, 3]. Hence the ELF magnetic antenna is characterized by high internal resistance, high alternating-current (AC) impedance, and a resonance frequency near the operating bandwidth. So a preamplifier and frequency compensation circuit are needed to match its characteristics so that the weak ELF signal can be received. In Refs. [2, 4], the preamplifier was a simple current-to-voltage converter which compensated the frequency response. In Refs. [5-7], an integrated operational amplifier was connected directly to the sensor to amplify the signal and compensate its frequency characteristic. Reference [8] proposed two types of preamplifier with and without feedback flux. The frequency response performs well but the equivalent input noise is about 3 ${\text{nV/}}\sqrt {{\text{Hz}}} $ of the above preamplifiers, which is not sufficiently low for an underwater ELF signal reception. In this paper, a different form of preamplifier is designed which possesses lower equivalent input noise and a flat frequency response. The operation principle and electronic characteristic parameters of the ELF magnetic antenna are introduced firstly. Then, according to the electronic characteristic parameters of a typical designed ELF magnetic antenna, a preamplifier composed of discrete components is designed, which has a better noise figure and frequency response. Finally, a frequency compensation circuit is designed, which works with the amplifier and keeps a flat frequency response in the range of operating bandwidth.

2.   The ELF magnetic antenna
  • The structure of an ELF magnetic antenna is a core wound with turns of coils. With an operating bandwidth located in the extremely low frequency band, an ELF magnetic antenna can receive ELF signal penetrating seawater. With a specific operating bandwidth, an ELF magnetic antenna is quite different from an antenna operating in higher frequency. Then it shows different electronic characteristics.

    The magnetic antenna principle derives directly from the Faraday's law:

    where $\mathit{\Phi }$ is the magnetic flux through the coils of the section. Figure 1 shows the equivalent circuit diagram in terms of the electronic characteristics. Its transfer function is defined by the ratio of magnetic flux density to output voltage.

    where $N$ is the total turns of the coils; $S$ is the cross-sectional area of a coil; ${\mu _{\text{e}}}$ is the effective magnetic permeability of the core; $R$ is the direct-current resistance of the coils; $L$ is the induction of the coils; $C$ is the parasitic capacitance between the coils.

    In comparison with high frequency signal, the ELF signal has a low decay rate when penetrating seawater. Nevertheless, the horizontal magnetic field component of the ELF signal is extremely weak. Given the limited size of the antenna, according to Eq. (1), ${\mu _{\text{e}}}$ and $N$ should be increased so that the antenna can receive larger signal. So permalloy with high initial permeability is adopted. Meanwhile copper wires with fine diameter and using the multilayer tightly wound method are used to build the magnetic antenna for the tradeoff between high induced voltage and large signal-to-noise ratio (SNR). Based on the above reasons, the resultant ELF magnetic antenna has characteristics of high internal resistance, high AC impedance, and a resonance frequency near the operating bandwidth. Table 1 shows the electronic characteristic parameters of an ELF magnetic antenna, which is designed on the basis of the receiving performance index of an ELF magnetic antenna and its parameters are optimized. The tested frequency characteristic of the ELF magnetic antenna in Table 1 is shown in Fig. 2. The internal resistance is 1.17 ${\text{k}}\Omega $ and AC impedance is 39.76 ${\text{k}}\Omega $ (where $f$ $=$ 100 Hz). A resonance frequency appears at $f$ $=$ 360 Hz.

3.   The very low noise preamplifier
  • The electronic characteristics of the ELF magnetic antenna should be considered when designing the preamplifier. In general, the following points should be considered.

    (1) Because of the high AC impedance, the preamplifier should have high input impedance. More focus is on the high output voltage when signal is received. So the input impedance of the preamplifier should be higher than the AC impedance. Both an integrated operational amplifier (OA) and a field-effect transistor (FET) can be used for the input stage of the preamplifier because of their high input impedance.

    (2) Given that the received signal is quite weak, to acquire an effective SNR, the preamplifier should have a very low noise. A discrete component circuit has about 10-20 dB lower noise than the OA. Considering the above two points (1) and (2), a low noise junction field-effect transistor (JFET) is used for the input stage of the preamplifier.

    (3) The equivalent input noise of the preamplifier will get lower along with the increase of its gain. But the gain of the circuit should not be too high before the signal arrives at the receiver. Hence a negative feedback circuit is adopted. When the gain of the negative feedback circuit reduces, the total output noise will reduce with same multiples, so that the circuit remains with an invariable equivalent input noise when the gain reduces[9, 10].

    (4) The resonance frequency of an ELF magnetic antenna appears near the operating bandwidth. At the resonance frequency, the output voltage of the antenna will be very high; however, it will decline sharply at both sides of the peak. It is a defect for the receiving antenna. So a compensation circuit is required to compensate the frequency characteristic flat below the resonance frequency. According to the operating bandwidth, the compensated frequency characteristic should be flat in the range of $f$ $=$ 30-200 Hz.

    The circuit diagram of the preamplifier is shown in Fig. 3.The preamplifier is a three-stage negative feedback circuit. To increase input impedance, the first stage is a common-source amplifier which is composed by parallel low noise JFETs 2SK389. The second stage is a common-emitter amplifier using low noise PNP transistor 2SA1190 with a Darlington structure, which can increase apparent ${h_{{\text{FE}}}}$ and accordingly increase the total gain ahead of the negative feedback. The third stage is composed of 2SA1190 and its complementary pair NPN transistor 2SC2855. It is a common-collector amplifier in push-pull connection and then ${R_{\text{f}}}$ and ${C_{\text{2}}}$ are connected back to the input stage to form a negative feedback circuit to improve the noise and frequency characteristics.

    The power source of the amplifier is positive and negative power, and the voltage ${V_{{\text{CC}}}}$ $=$ 12 V. In the input stage, V1-V4 is composed of 0-offset circuit. It can omit offset resistor in cascade stage. Thus, the equivalent input noise can be lower and coupling capacitance is not needed in the input stage. The parallel four JFETs can reduce the optimum source resistance. According to the datasheet of 2SK389, the current${I_{\text{D}}}$ is set to 5 mA, which keeps the equivalent input noise sufficiently low and meanwhile the gain does not fall too low. Based on the value of${I_{\text{D}}}$, the resistor V2 is set to 620 $\Omega $ and V1 is 100 $\Omega $ to make the voltage ${V_{{\text{DS}}}}$ approximately 10 V which maintains a low circuit resistance while decreased the equivalent input noise. The equivalent input noise is minimized as a result. Since the voltage ${V_{{\text{DS}}}}$ $\approx$ 10 V, the offset voltage of the post-stage circuit is close to positive power voltage. If the input signal is large, flat-top distortion appears in the upper half of the output signal wave. However the received signal value is about several nanovolts. So the method that merely adjusts operating point to minimize the noise is available.

    In the first stage, the utilization of JFETs will decrease the gain, which generates less feedback. Hence the performance of the output impedance and noise as well as frequency characteristics will get worse. To correct these defects, the second stage is designed as a common-emitter amplifier composed of V5 and V6 with a Darlington structure and capacitance C3 bypassing the resistor of the emitter to the ground. According to the 2SA1190 datasheet, component values are chosen as following: ${I_{\text{C}}}$ $=$ 3 mA, R3 $=$ 260 $\Omega $ and R4 $=$ 3 ${\text{k}}\Omega $. Diodes D3-D5 generate 1.8 V compensation voltage to eliminate switch distortion of the following push-pull circuit. V7 and V8 is a common-collector amplifier in push-pull connection. Component values are set as following: ${I_{\text{C}}}$ $=$ 3 mA, R5 $=$ $R_6$ $=$ 100 $\Omega $. In the end, the output is connected to the source of the JFET to form a feedback circuit. Besides, the positive and negative powers are both bypassed by a large value capacitor and a small one so that the AC impedance between power source and ground is low.

4.   Performance of the preamplifier

    4.1.   Gain

  • Because resistor R2 is bypassed by the capacitor C1, the gain is determined by the capacitors ${R_{\text{f}}}$ and ${R_{\text{s}}}$. Smaller values of ${R_{\text{f}}}$ and ${R_{\text{s}}}$ will generate lower noise. So the values are the following: ${R_{\text{f}}}$ $=$ 510 $\Omega $ and ${R_{\text{s}}}$ $=$ 5.1 $\Omega $. The AC gain $A{\text{v}}$ of the preamplifier:

    where $A$ is the open loop gain of the circuit.

    Provided $A$ is assumed to be infinite, then

    The actual frequency characteristic is shown in Fig. 4. The actual gain is 39.4 dB, that is 91.3 in linear. The actual gain is slightly lower than theoretical because that the open loop gain $A$ is not infinite.

  • 4.2.   Frequency response

  • The first stage is a common-source amplifier composed of four JFETs. Because operating bandwidth is between $f$ $=$ 30 Hz and 200 Hz, the low pass response caused by the Miller effect and R3, C3 can be ignored. (The low pass response in high frequency band is omitted in Fig. 4.)[11, 12]. The capacitor C3 bypasses the R3 and ground. So the value of $C_3$ should be same with C5, which is 220 $\mu F$.

    The operating frequency is very low, so the high pass response of the circuit should be of concern. The high pass cut-off frequency ${f_{\text{h}}}$ should be lower than 30 Hz. There is no need for an input coupling capacitor and thus high pass response caused by the coupling capacitor and input impedance can be avoided. ${R_{\text{s}}}$, C1 and ${R_{\text{f}}}$, ${C_{\text{2}}}$ determines high pass frequency ${f_{\text{h}}}$1 $=$ 1/2$\pi$${R_{\text{s}}}$C1, ${f_{\text{h}}}$2 $=$ 1/2$\pi$${R_{\text{f}}}$${C_{\text{2}}}$. Since the value ${R_{\text{s}}}$ is small, to lower the high pass frequency, the value of C1 is set to be as large as 2200 $\mu F$. Thereby one of the high pass frequency is ${f_{\text{h}}}$1 $=$ 14.5 Hz (Figure 4 also shows the frequency response when $C_1$ $=$ 220 $\mu F$ and $C_1$ $=$ 880 $\mu $F). Meanwhile, the value of ${R_{\text{f}}}$ is large. Then the value of ${C_{\text{2}}}$ can be as small as 220 $\mu F$ and another high pass frequency ${f_{\text{h}}}$2 $=$ 1.45 Hz. ${f_{\text{h}}}$1 is about ten times that of ${f_{\text{h}}}$2. In Fig. 4. there is a knee point in $f$ $\approx$ 1.5 Hz. Apparently the theoretical values of ${f_{\text{h}}}$1 and ${f_{\text{h}}}$2 are highly approximate with the actual values. Hence, in general the high pass frequency ${f_{\text{h}}}$ $=$ 14.5 Hz, which is lower than 30 Hz.

  • 4.3.   Input and output impedance

  • In order to improve the noise characteristic, four parallel JFETs are used in the circuit to lower the input impedance. However, in the meantime, four parallel JFETs are composed of the 0-offset circuit. The input impedance of the circuit is still high. Through experimental measurement, the input impedance is 5.8 M$\Omega $, which is much higher than the AC impedance 39.76 k$\Omega $ (where signal frequency is 100 Hz).

    The output stage of the circuit is a common-collector amplifier in push-pull connection. This type of circuit structure can avoid the imbalance of voltage drop between two diodes and two transistors. There are two resistors of 100 $\Omega $ between the two transistors, which increases the output impedance to 100 $\Omega $. However, the influence of increased output impedance can be ignored for the frequency compensation circuit followed.

  • 4.4.   Equivalent input noise

  • The input stage of the preamplifier is four parallel JFETs, which makes the noise voltage be 1/$\sqrt 4 $ times and noise current $\sqrt 4 $ times than single JFET respectively. Then the optimum source impedance becomes 1/4 times than the former. The negative feedback circuit reduces the noise with same multiples to its gain, which improves the noise characteristic greatly.

    When testing the circuit, the input signal is an attenuated sine wave where ${V_{{\text{rms}}}}$ $=$ 4 nV and $f$ $=$ 80 Hz. A standard 30 dB gain amplifier is cascaded following the preamplifier to make the amplified signal larger than the background noise of the frequency spectrograph. Figure 5 shows the spectrum tested on the frequency spectrograph. The lower curve shows the noise and amplified signal of the amplifier proposed in this paper. The amplified signal is -98.4 dBV and the output noise is -105.1 dBV. The calculated equivalent input noise is 1.97 nV/$\sqrt {{\text{Hz}}} $. The higher curve shows the noise and amplified signal of the amplifier proposed by OA given in Refs. [2, 8]. The equivalent input noise of amplifier proposed in this paper is about 6 dBV lower. In addition to the input signal frequency $f$ $=$ 80 Hz, there are $f$ $=$ 50 Hz and $f$ $=$ 100 Hz power-line interference, which can not be completely eliminated in the test.

5.   Frequency compensation circuit
  • The resonance frequency near the operating bandwidth of the magnetic antenna will decrease the gain at lower frequency. Because the gain varies with frequency, it influences the signal receiving performance. Hence a frequency compensation circuit, shown in Fig. 6, is cascaded following the preamplifier.

    Two time constants 1/R3C and 1/V2C are in the circuit. The smaller time constant 1/R3C determines the frequency point which starts to compensate the low frequency response. Below this frequency the frequency response is compensated. The larger time constant 1/V2C keeps the gain from being overly high when the frequency is near 0 Hz. To stabilize the circuit, 1/V2C is set to 1 Hz. The values are set as follows: V2 $=$ 3.7 ${\text{k}}\Omega $, R3 $=$ 30 M$\Omega $, and $C$ $=$ 3.3 nF. The frequency response of the circuit is shown in Fig. 7.

    When the preamplifier is cascaded with the frequency compensation circuit, the resultant frequency response is shown in Fig. 8. The compensated frequency response is flat between the frequency 30 Hz and 200 Hz, which can satisfy the requirements.

    In Fig. 8 the curve without resonance frequency is the frequency response of the OA amplifier directly connected with the ELF magnetic antenna[2, 8]. It shows a better performance only if the frequency response is considered. However, as shown in Fig. 5, its equivalent input noise is about 3 nV/$\sqrt {{\text{Hz}}} $, which for the preamplifier proposed in this paper is 1.97 nV/$\sqrt {{\text{Hz}}} $, also the compensated frequency response satisfies the requirement, the preamplifier and compensation circuit proposed in this paper leads to better performance.

6.   Conclusion
  • A preamplifier is designed for the ELF magnetic antenna to receive extremely weak underwater ELF signal. Given the ELF magnetic antenna has high internal resistance, alternating-current impedance and a resonance frequency near the operating bandwidth, the preamplifier adopts a three-stage negative feedback circuit, which is composed by parallel JFETs, a common-emitter amplifier with a Darlington structure and a common-collector amplifier in push-pull connection. Then a frequency compensation circuit is cascaded to compensate the characteristic in low frequency range. In operating bandwidth $f$ $=$ 30-200 Hz, the circuit possesses a gain of 39.4 dB. The equivalent input noise is 1.97 nV/$\sqrt {{\text{Hz}}} $. On premise of satisfying the receiving requirements, the preamplifier of the ELF magnetic antenna has a better noise figure and its frequency characteristic keeps flat in the range of operating bandwidth.

Figure (8)  Table (1) Reference (12) Relative (20)

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