Main circuit design of VHF transmitter

Abstract: VHF transmitter is a key device in the monitoring terminal and a typical digital communication RF wireless transceiver. This paper elaborates on the indicators, links and module circuit working principles of VHF transmitters, and gives the design methods of each module circuit of VHF transmitter with carrier frequency of 156.025~162.025 MHz.
Keywords: VHF transmitter; VCO; mixer; power amplifier; filter

0 Introduction
The main advantage of superheterodyne transmitter-receiver is that it can realize relatively narrow bandwidth and high rectangular coefficient intermediate frequency filter in relatively low intermediate frequency band. Such intermediate frequency filter can improve the selectivity of receiver and obtain greater gain from intermediate frequency stage, thereby reducing the difficulty of achieving high gain in RF stage. When the frequency of RF signal rises to microwave or even millimeter wave, the secondary frequency conversion method can be used to further reduce the difficulty of filter implementation and ensure the selectivity of receiver. In this VHF transmitter-receiver, the signal frequency is around 160 MHz, and the bandwidth is only 15 kHz. In this way, in order to achieve signal filtering, if secondary frequency conversion is not used, the corresponding filter design will become very complicated.
However, superheterodyne circuits often have image frequency interference. If the image frequency is within the passband of the input loop, the image signal and the nearby radio signal will be moved to the intermediate frequency band through heterodyne frequency conversion, thereby interfering with the received signal. In order to suppress image interference, the difference frequency of 161.975 MHz (261.975 MHz and 100 MHz) can be selected as the carrier of the CH8 7B channel transmission signal during design. At the same time, 162.025 MHz (262.025 MHz and 100 MHz difference frequency) is selected as the carrier of the CH88B channel transmission signal. The transmitter circuit in this article contains almost all the commonly used circuits and RF communication system solutions for RF communication equipment, and can also be used to build a wireless RF monitoring system.

1 Transmitter Introduction
The structural block diagram of the transmitter is shown in Figure 1. In the figure, the operating frequency of the transmitter is 156.025~162.025 MHz. The system is mainly divided into three parts: baseband modulation, modulation transmission, and phase locking. The modulated transmission part has only one channel, which can pass 156.025~162.025 MHz signals. The transmission is controlled by the single-chip microcomputer to transmit intermittently. The transmission cycle is affected by factors such as time, location, and communication distance, and is about 3 to 10 seconds. According to the reception requirements, the transmitter adopts TDMA mode and can transmit signals of different frequencies in different time slots. Usually, the time interval for switching the transmission channel is less than 25ms. The specific specifications are listed in Table 1.

2 Implementation of the transmitter module
2.1 Frequency deviation control circuit
Figure 2 shows the frequency deviation control and relay selection circuit of the transmitter. The GMSK signal can be divided into two paths through the operational amplifiers U13A and U13B. VR501~VR504 in the figure are potentiometers. Adjusting the potentiometer can change the amplification factor of the operational amplifier for GMSK and the amplitude of the modulation signal, thereby changing the magnitude of the modulation frequency deviation of the VCO. The SWITCH1 and SWITCH2 signals passing through the two operational amplifiers can be controlled by the microcontroller to control the relay and select one of them.

2.2 Design of frequency-modulated voltage-controlled oscillator
Figure 3 shows the frequency-modulated voltage-controlled oscillator circuit of this system. This VCO circuit belongs to a direct frequency modulation circuit. MODULE (U32) is the baseband GMSK signal, which can control the variable capacitance diode D1 so that the capacitance of D1 changes with the amplitude of the signal. Therefore, the output frequency of the VCO is controlled by the baseband GMSK signal, and the signal (2162.025MHz or 261.975 MHz) at the frequency output terminal Fin (U32) of the VCO contains the information of the baseband GMSK signal, that is, the signal is a frequency-modulated signal. The baseband GMSK signal can control D1, thereby changing the frequency deviation of the VCO's transmission frequency. VFDC (U32) is the control voltage input terminal of the phase-locked loop. The PLL can control the VCO through this control voltage so that it works at the set center frequency. The LG (2SC2712) part can form an active power filter, which can effectively filter out signal components below tens of KHz (especially low). In this way, the circuit can work reliably by connecting an active power filter between the voltage regulator and the VCO. K52 (2SK508) is the oscillator tube of the VCO. This tube, capacitors C54 (5.1pF), C57 (5.1pF) and several varactor tubes can form a Crapo oscillator. R25 (2SC3356) is used as an output buffer to prevent the Crapo oscillator from being affected by the low input impedance of the next stage. In the VCO, two varactor diodes are usually connected back to back. When one of the diodes conducts AC, the bias potential is embedded at a low level, and the other diode is reverse biased at this time, which can reduce the distortion component. However, this will also reduce the capacitance of the varactor diode by half. In order to improve the Q value of the VCO, the inductor L2 should be wound with enameled wire (wire diameter is 0.4 mm, inner diameter is 2.3 mm, and wound 3 times). Another function of using wire-wound inductors is that they are more convenient for debugging. If the PLL is not locked, the coil can be loosened appropriately until the PLL is locked. The gain of the VCO is about 15 MHz/V, and as the control voltage increases, its output frequency also increases, so it is a linear increasing process.

2.3 Design of Transmitting Phase-Locked
Loop The transmitting phase-locked loop circuit in this system is shown in Figure 4. In the figure, the LMX1501 phase-locked loop controls the frequency modulation VCO. After the baseband signal (GMSK modulated signal packaged by HDLC) is frequency modulated by the VCO, it will be locked by the phase-locked loop at 262.025 MHz and 261.975 MHz, with a frequency deviation of ≤5 kHz. The reference frequency of the phase-locked loop uses the temperature-compensated crystal oscillator (TCXO) of NDK Company's FUA31 77A, with a reference frequency of 12.8 MHz and a frequency stability of 2.5 ppm. The TCXO has a VC function, which is a function of self-adjusting the frequency to ensure that the mobile terminal and the base station signal frequency are the same. In other words, VC is a frequency adjustment function of the phase-locked loop circuit that can make the local oscillation frequency and the base station frequency reach the frequency locking state. Under normal circumstances, the CDMA mobile terminal requires the VC frequency adjustment function of the VCTC-XO to have a frequency change range of ±8 to ±16 PPM and a VC voltage change range of 1.5±1 V. Fin (U32) is the output frequency of the VCO, and VFDC (U32) is the locking voltage of the PLL output that controls the VCO. When the VCO is locked, the voltage is between 0 V and Vp (the power supply of the charge pump circuit). The voltage Vcc of the PLL is lower than the voltage Vp of the charge pump (the DC control voltage input to the VCO is always a few tenths of a volt less than Vp, so Vp must have a suitable amplitude sufficient to drive the DC control terminal of the VCO). PLLC, PLLD, and PLLE1501 are signals sent from the microcontroller to control the phase-locked loop.

2.4 Transmitting link design
The modulated signal Fin (U32) (262 MHz) and the local oscillator Fin (U29) (100 MHz) are mixed after passing through the LC bandpass filter to obtain the transmitting signal. The transmitting signal is amplified by two stages of R25 and buffered by the MMIC circuit, and then sent to the input end of the power integrated amplifier circuit M57719, which can amplify the power to 12.5W and then transmit it from the antenna. Figure 5 shows the transmitting mixing and amplification circuit.

Figure 6 shows its microwave integrated pre-amplifier circuit. The gain of IC2 in the figure can reach 15 db and is resistant to high temperatures. Its input and output impedances are both 50 Ω. The typical application frequency of this circuit is 500 MHz. It is an NPN type circuit. The circuit pins are all composed of microstrip lines. Two pins of different lengths correspond to the input and output pins respectively. When the chip is working normally, the on-voltage at the input end is about 0.58 V, and the output end needs to add a 14 V DC voltage. The other two ends are equal in length and are grounded. When IC2 is working normally, it will generate very high temperatures, and long-term operation will reduce the chip function. In severe cases, it will break through the PN junction of the internal circuit and damage the chip. The above phenomena all occurred during the experiment. The on and off of IC2 is determined by the TDD time slot, so it can play a protective role.

The M57719 in the circuit of Figure 6 is an integrated high-power amplifier with a frequency range of 145-175 MHz, a maximum output power of 14 W, and an input and output impedance of 50Ω. The signal is input from pin 1 and output from pin 4. Pins 2 and 3 are the power supply terminals of the two-stage amplifier. M57719 has certain requirements for the power of the input signal, that is, the pre-amplifier gain relative to the local oscillator signal must be 20 dB to meet the output power requirement. At this time, the input level of the amplifier is about 10 dbm or higher, and can reach 16 dbm during the experiment, otherwise the op amp will not work. It should also be noted that the radio cannot transmit for a long time when it is working, otherwise the preamplifier circuit and the amplifier module will be damaged.
The two-stage amplifier of M57719 is powered by 14 V DC. The second-stage amplifier is directly powered by DC (pin 3). The power supply voltage of the first-stage amplifier is controlled by the power amplifier control circuit. The antenna detection voltage signal V1 and the reference voltage V2 controlled by the single-chip microcomputer are connected to the inverting and non-inverting input terminals of the operational amplifier respectively. The output voltage Vo=2(V2-V1) of the operational amplifier can provide the base of the switch tube V36 with a conduction voltage after the resistor voltage divider. When there is no transmission, V1 is very small. At this time, the switch tube V36 is turned on, the BAQL tube is turned on, and the collector outputs a voltage of about 14 V to provide a DC working voltage to the output terminal of the 2nd pin of M57719 and IC2; when transmitting, it can be detected that V1 suddenly becomes very large. At this time, the voltage after the voltage division of V0 is less than the conduction voltage of V36, and V36 changes from conduction to non-conduction. M57719 and IC2 are powered off and do not work, and the transmitter stops transmitting.
The circuit signal is amplified to 12.5 W by M57719 and output from pin 4. After passing through the coupling capacitor and the switching diode D1, it can be transmitted to the antenna filter for transmission. The antenna filter is a low-pass filter that can be designed using ADS2004. The low-pass filter is a common part for receiving and transmitting, and its turning point frequency is 167MHz. The function of the two diodes D1 and D2 is to add voltage and short-circuit the high-frequency signal during transmission, so that the transmitted high-power signal can pass through D1 to enter the low-pass filter and be transmitted, and at the same time, D2 is turned on to protect the receiving circuit. During reception, since there is no DC voltage, D1 is not turned on (similar to an open circuit), so that the received signal enters the high-pass filter without attenuation. From the ADS simulation effect, the antenna filter's transmission frequency points of 162.025 MHz and 161.975 MHz are almost at the center of the circle of the original Smith diagram, and its in-band insertion loss (S21) is less than 0.5dB.

3 Conclusion
This transmitter has been tested with the baseband and receiver, and its receiving bit error rate can reach 0.05%. Therefore, it can be used in the field of industrial control to realize digital communication in the community.
This system can transmit data to the baseband board through the serial port terminal, and the receiver can receive the demodulation of the returned data, so as to transmit the radio signal to the baseband board, receive the demodulated signal, and receive the returned data.