Design of Modulation Amplifier for Instrument Landing System Test Equipment

Amplitude modulation and power amplifier are an indispensable part of the instrument landing system test equipment. This paper makes a preliminary design of them. Through the special application of double-balanced mixer, the amplitude modulation modulator is realized. A power amplifier is added, and a fourth-order low-pass filter is simulated and designed using ADS software. All indicators meet the requirements.

　　0 Introduction

　　Generally, amplitude modulation modulators are implemented using analog multipliers, and mixers are rarely used as amplitude modulation modulators.

　　ILS (Instrument Landing System) is an auxiliary navigation device widely used in aircraft approach and landing internationally, and is also widely used in domestic airports. This system consists of an airborne heading, glide path, and marker beacon receiver and a ground heading, glide path, and marker beacon transmitter. It provides the aircraft with a heading signal aligned with the runway and a glide path signal to guide the aircraft's descent, plus appropriate distance indication signals, so that the aircraft can land safely in low visibility and bad weather conditions with the help of the signals provided by these instruments, and achieve safe blind landing of the aircraft. The test of the instrument landing system is one of the functions of the ICNI equipment. ICNI (Integrated Communication/Navigation/Identification), the full name is Integrated Communication Navigation Identification System. The modulation amplifier is an indispensable part of the transmitting unit in the ICNI equipment instrument landing system test.

　　1 Basic principles of amplitude modulation

　　Assume that the modulation signal is a cosine wave of a single frequency, then

　　Ua(t)=(Uam*Cos(2πfa*t) (1)

　　The carrier signal is

　　Uc(t)=(Ucm*Cos(2πfc*t) (2)

　　Since the carrier frequency remains unchanged after amplitude modulation, the amplitude of the amplitude modulated wave is proportional to the modulation signal. Assuming the initial phase of the carrier signal is 0, the expression of the modulated wave is:

　　UAM=Ucm*［（1+Ma*Cos（2πfa*t+ψ）]*Cos（2πfc*t）

　　=Ucm*Cos（2πfc*t）+Ma*Ucm*Cos［2π（fc+fa）*t+ψ］/2+Ma*Ucm*Cos［2π（fc-fa）*t-ψ］/2 ( 3)

　　Where UAM is the modulation amplitude, Ucm is the carrier amplitude, Ma is the modulation index, fa is the frequency of the audio signal, ψ is the initial phase of the audio signal, fc is the frequency of the carrier signal, and t is time.

　　If the load carried by the AM voltage is a resistor R, the carrier power is

　　P0=Ucm2/2R (4)

　　The sideband power is

　　P1=P2=（Ma*Ucm/2）2/2R=（Ma2/4）*P0 (5)

　　The average total power of the AM wave output is

　　Ptotal = P0 + P1 + P2 = (1 + Ma2/2) * P0 (6)

　　Formula (6) shows that the average total power of the AM wave output is not only determined by the carrier power, but also related to the modulation index. When the carrier power is constant, the total output power increases with the increase of the modulation index.

　　2 Design and implementation of mixer modulation and power amplifier

　　The double balanced mixer can be used as a diode low-level AM circuit, and its principle can be referred to the diode ring modulator. This is a special use of the double balanced mixer as an AM modulator, which is different from the ordinary use of the mixer and requires special application. When the mixer is used as an AM modulator, the RF signal is input from the RF (radio frequency) port, the audio signal is input from the IF (intermediate frequency) port, and the modulated signal is output from the LO (local oscillator) port. When used as a mixer, the input signal is input from the RF and LO ports, and the IF port is used as the output. This is the main difference. Of course, different interface methods have different functions.

　　This article uses mini-circuits' ADE-1ASK for amplitude modulation. ADE-1ASK is a double-balanced mixer. When the LO power is +7dBm, the input frequency range of the LO and RF ports is 2~600MHz, and the input frequency range of the IF port is DC~600MHz, which completely covers the heading and downlink frequency bands (the heading frequency band is 108~112MHz, and the downlink frequency band is 328~336MHz). The audio signal is a synthetic signal of 90Hz and 150Hz, which also meets the input conditions of the IF port. The 1dB compression point input power of the mixer is +1dBm. The audio signal needs to be superimposed with DC, so it is combined through an adder. For the convenience of debugging, a potentiometer is placed at each input to achieve continuous adjustment. Among them, the op amp in the adder uses MC1458I. Some filtering is done between each port.

　　The power amplifier tube selected is ERA-8SM+ from mini-circuits. This power amplifier tube is a monolithic amplifier with a frequency of DC~2GHz, a gain of +31.5dB, a 1dB compression point output power of +12.5dBm, a 3.7V power supply, and a 50Ω internal matching impedance at both ports.

　　Then, ADS software was used to simulate and design a fourth-order low-pass filter to suppress the second and third harmonics.

　　The frequency bands of the heading circuit and the gliding circuit are different, and their main difference is the filter network. The different component values ​​are: in the heading circuit, L1=L4=L5=L6=L7=68nH, C3=C4=C14=C18=39pF, C15=C16=C17=82pF; in the gliding circuit, L1=L4=L5=L6=L7=18nH, C3=C4=C14=C18=12pF, C15=C16 =C17=27pF.

　　3 Debugging and test results

　　After the circuit board was assembled and some debugging was done, the results measured at room temperature were as follows:

　　Through Agilent's DSO6034A oscilloscope, the audio signal and DC must be matched appropriately to ensure that the modulation waveform is not distorted. The DC input is best between 40 and 60mV, and the audio signal is generally within 200mVp-p (peak-to-peak). Measured by Agilent's E4417A power meter, the output power is +6dBm and the flatness is within 0.5dB. Measured by the unit's self-made ILS/VOR/MKR signal analyzer, the amplitude modulation depth can reach more than 90% without distortion. Measured by Agilent's E4407B spectrum analyzer, the second harmonic suppression reaches below -80dB, and the third harmonic suppression reaches below -90dB.

　　4 Conclusion

　　This paper uses a mixer to realize the function of an amplitude modulation modulator, verifying the feasibility of this solution. The 90Hz and 150Hz audio amplitude modulation of the instrument landing system tester's heading beacon and glide path beacon is realized, and the modulation depth can reach more than 90% without distortion; the power amplification of the heading and glide path beacon modulation signals is realized, and the output power is +6dBm; the flatness of the normal temperature test is within 0.5dB; the harmonic suppression reaches below -80dB. The indicators meet the instrument requirements.