Design of Microwave Signal Generator Based on Single Chip Microcomputer and LMX2485

With the development of microwave applications, microwave signal sources have been widely used in communications or instruments. Signal source synthesis technology can be divided into direct synthesis and indirect synthesis according to the synthesis method, and can be divided into direct frequency synthesis, phase-locked frequency synthesis and direct digital frequency synthesis according to the form [1-2]. The characteristics of direct frequency synthesis are short frequency conversion time, low output phase noise, high operating frequency, and the ability to generate arbitrarily small frequency intervals; the disadvantage is that a large number of frequency multiplication, frequency division, mixing and frequency selection filters are used, which is not only large in size and weight, high in cost, but also difficult to suppress output ripple, noise and parasitic frequencies. Phase-locked frequency synthesis mainly uses digital phase-locking method. Its main advantage is that the phase-locked loop is equivalent to a narrowband tracking filter, which has good narrowband tracking filter characteristics and the ability to suppress parasitic interference of the input signal, avoiding the use of a large number of filters, and is conducive to integration and miniaturization. The advantages of direct digital frequency synthesis are high resolution, easy to achieve extremely low frequency, flexible control, etc., but it faces two problems: it is difficult to increase the upper limit of the output frequency and difficult to suppress parasitic output. Therefore, the synthesis of microwave and millimeter wave signal sources should mainly adopt digital phase-locking method and be designed based on large-scale dedicated integrated chips. This paper proposes an intelligent microwave signal source generator controlled by a single-chip microcomputer, which is based on the low-power, high-performance delta-sigma fractional frequency digital phase-locked loop circuit LMX2485[3] and YTO of National Semiconductor Corporation of the United States, and is controlled by a single-chip microcomputer C8051F120. The circuit is used to generate a frequency source of 4~7 GHz, and then the signal required for 8~14 GHz application is realized through a frequency multiplier. The microwave signal source generator realized by this method has low cost, small size, good performance, and high practical value.

1 LMX2485 Function Introduction

LMX2485 is a low-power, high-performance delta-sigma fractional-frequency digital phase-locked loop circuit from National Semiconductor Corporation. Its frequency range can reach 50 MHz~3 GHz. The new delta-sigma structure can shift the spurious and phase noise of its low-frequency band to a higher frequency band, thereby reducing the spurious and noise of the required frequency band of the circuit [4]. The delta-sigma modulator is available in four levels, which can take into account the different needs of the application, such as the requirements for phase noise, false signal suppression capability and locking time, to ensure that the system can fully exert its performance. Only a few low-cost external components are required during development, which helps to shorten the design time and reduce system costs. Its working principle is shown in Figure 1. The output frequency f0 is divided by a fraction (÷NF) to obtain the reference frequency f1. The phase detector controls the output phase detector current or voltage by comparing the phase of f1 with the reference frequency. After low-pass filtering, the voltage-controlled oscillator is controlled to change the output frequency. Finally, the two phases are the same, that is, locked. Thus, f0/NF=f1=fref, that is, the output frequency, is obtained, as shown in formula (1). By controlling NF through a single-chip microcomputer, the frequency required by the user can be obtained.
f ₀ =f _ref ×NF (1)

2 System Design

The system design requires the signal source to generate a microwave source of 8~14 GHz with a frequency resolution of 100 Hz. The LMX2485 fractional frequency digital phase-locked loop is used, and the external tuning oscillator uses YTO (YIG tuning oscillator). YTO has a wide frequency tuning range, good tuning linearity, low phase noise, good temperature characteristics, high detuning isolation, and fast tuning speed, so it is widely used. The overall scheme of the system is shown in Figure 2. The setting of the LMX2485 PLL and the voltage bias control of the YTO are performed by the single-chip microcomputer, the ADC7545 is used to control the pre-adjusted voltage of the YTO, that is, the main coil voltage, and the loop filter output controls the secondary coil voltage of the YTO.

2.1 Divider Design

LMX2485 has a 22-bit fractional modulus register inside. The program frequency divider registers are: RF_N (10:0) represents the integer part of NF, RF_FN (21:0) represents the numerator of the fractional part of NF, RF_FD (21:0) represents the denominator of the fractional part of NF, and RF_R (5:0) is the reference divider. For the signal source generator in this example, the output frequency is required to be 8~14 GHz and the frequency resolution is 100 Hz. A 4~7 GHz YTO is used, a 2-frequency multiplication circuit is added to the output stage, and an HMC433 four-frequency division circuit is added to the loop. The system uses a high-precision temperature-compensated 10 MHz crystal oscillator, and the chip uses frequency multiplication control. RF_R is fixed to 1 and RF_FD is fixed to 4 000 000. According to formula (1), the output frequency of this signal source is formula (2). The multiplication by 8 in the formula is due to the addition of a four-frequency division circuit in the loop and a frequency multiplier at the final output end. When RF_N=50 and RF_FN=0, the phase-locked loop frequency is 1 GHz and the system output frequency is 8 GHz. When RF_N=87 and RF_FN=2 000 000, the phase-locked loop frequency is 1 750 MHz and the system output frequency is 14 GHz. The system resolution of this solution is 20 MHz/4 000 000×8=40 Hz, which meets the application requirements. The selection range of RF_N is 50~87, and the selection range of RF_FN is 0~3 999 999. The microcontroller configuration LMX2485 adopts IO control, and its configuration timing is shown in Figure 3.

f ₀ =10×2/RF_R×(RF_N+RF_FN/RF_FD)×4×2 (MHz)
=10×2×(RF_N+RF_FN/4 000 000)×4×2 (MHz) (2)

2.2 Digital Phase Detector

The phase detector is integrated inside the LMX2485 chip and uses fractional frequency division. The maximum phase detection frequency is limited to 50 MHz, and 20 MHz is actually used. The design of the phase detection frequency requires compromise. If the phase detection frequency is too high, although the phase noise can be reduced, the locking time will be greatly extended, and the frequency resolution performance will be reduced. After the phase detector circuit is a charge pump, its output is a high-resistance current, which outputs a frequency control signal through an external filter circuit, and then drives the YTO to generate the required frequency through the YTO drive circuit. There is a digital lock detection circuit and detection algorithm in the chip. When the loop is locked, the output lock indication is 1.

2.3 YTO and driver

YTO is widely used in microwave instruments because it has better performance than VCO. YTO has main and auxiliary coils inside, and correspondingly requires main coil drive circuit and auxiliary coil drive circuit outside. The main coil causes a large range of frequency changes, and the auxiliary coil drives a small change in frequency, thereby obtaining better performance. The control voltage of the main coil drive circuit is calculated by the microcontroller according to formula (3), and then set by DAC7545, where k and f0 are constants determined by the characteristics of YTO.

f=kV+f ₀ (3)

The secondary coil of YTO is for slight changes in the output frequency of YTO. Therefore, the control voltage of the secondary coil is controlled by the phase difference of the two frequencies output by the phase detector and then by the voltage after loop filtering, so as to achieve a fixed phase relationship between the frequency of the output signal source and the reference crystal frequency, that is, the phase-locked loop is locked at a fixed frequency.

3 Hardware Design

The signal source generator hardware system mainly includes two parts: the single chip microcomputer control system and the phase-locked loop system.

3.1 Single-chip microcomputer control system

The single-chip microcomputer mainly realizes the human-machine interface and phase-locked loop control. It uses C8051F120, whose core is a 100 MIPS 8051 microcontroller. The input frequency value is obtained through the SPI interface and the human-machine interface chip ZLG7289, and the register value corresponding to the phase-locked loop LMX2485 is calculated according to the frequency value. Then, the IO pin is used to perform the three-wire configuration of LMX2485 according to the timing shown in Figure 3. LMX2485 automatically performs phase-locked tracking and finally locks to the set frequency value. The frequency value and locking result are displayed by ZLG7289.

The single-chip microcomputer calculates the control voltage corresponding to the YTO main coil according to the input frequency value and outputs it through the D/A chip AD7545. AD7545 is a 12-bit resolution single voltage control CMOS digital-to-analog conversion chip. The reference voltage is set to 12 V. The single-chip microcomputer can easily control it through the parallel interface. The voltage of the YTO secondary coil is realized by the loop output control of the phase-locked loop.

The human-computer interaction circuit mainly realizes information input, data display and warning functions, and is implemented using ZLG7289. It contains digital tube display driver and keyboard scanning management circuit, which can directly drive 8-bit common cathode digital tube or 64 independent LEDs, and can also scan and manage up to 64 keys, using SPI serial bus and single-chip microcomputer interface. The maximum frequency of this system is 14 GHz, so two ZLG7289 are connected in parallel.

3.2 Phase-locked loop circuit

The phase-locked loop, four-way frequency division and other circuits are shown in Figure 4. LMX2485 is connected to the microcontroller through three wires, and the reference frequency is provided by a high-stable temperature-compensated crystal oscillator. The YTO output frequency is divided by four through the four-way frequency division circuit HMC433 and then enters the RF input of LMX2485. The two-way signal passes through the internal phase detector, the charge pump output, and then passes through the external loop low-pass filter and operational amplifier OP07 to control the YTO small coil drive.

In specific implementation, due to the high operating frequency, the circuit board needs more than four layers.

4 Software Design

The system software mainly receives the input of the signal source frequency, and configures the LMX2485 fractional frequency digital phase-locked loop circuit and the D/A control of the YTO main coil drive voltage after calculation by the single-chip microcomputer. Then, the phase-locked loop circuit tracks and locks the YTO to output the required frequency. The software block diagram is shown in Figure 5.

The system output spectrum is shown in Figure 6(a). When a four-layer circuit board design is adopted and the relevant amplifier input and output matching issues are adjusted, the effect is better, as shown in Figure 6(b).

The microwave signal source generator introduced in this article uses a single-chip microcomputer to control a low-power, high-performance delta-sigma fractional-frequency digital phase-locked loop and a corresponding drive circuit to control the output of a tuned oscillator (YTO). The signal source generator implemented with this technology can bring advantages such as high frequency accuracy and stability, small error, and convenient operation and control, so it has broad application prospects.