Using VCO to achieve direct frequency modulation of varactor diode 5

The phase-locked frequency stabilization circuit - phase detector is the core part of frequency stabilization, which consists of a digital phase frequency integration chip MC145170 and a loop low-pass filter, as shown in Figure 6. The standard crystal oscillator uses Morion's temperature compensated crystal oscillator MV68 series (10mHz), with a frequency deviation of less than 5 × 10-6, short-term frequency stability of 10-9/s, diode and phase noise of less than -145dbc/hz/10kHz, fully ensuring that the circuit meets the system's requirements for frequency deviation. ---The phase-locked loop integrated chip uses Motorola's MC145170 to achieve a center frequency variation of the frequency modulation excitation source within 87-108 MHz at intervals of 100 kHz. The MC145170 produced by Motorola is a single-mode CMOS phase-locked loop frequency synthesizer chip that can be used for serial code input programming in the MF, HF, and VHF bands. This chip contains fully programmable ÷ r and ÷ n counters, diode an input decoder, an amplifier built into the fin pin, an external crystal oscillator, a programmable reference output, a single or double ended phase detector with linear transfer function, and an adjustable c register. ---In practical circuits, a phase detection frequency of 10kHz is selected, therefore r=1000 and n=8700 to 10800 are set. At the same time, set the c register to (c7c6c5c4c3c1c0)=(10000000), select the dual output of φ r and φ v, and prohibit the output of ld, fr, fv, and ref to reduce circuit power consumption and minimize the impact of useless ports on the circuit. ---The commonly used loop filters include RC integral filters, passive proportional integral filters, diode and active proportional integral filters. From their transfer functions, it can be seen that active proportional integral filters have two independent adjustable parameters, and more importantly, have hysteresis lead characteristics, which are beneficial for the stability of the loop. Therefore, an active proportional integral filter is adopted in the design, and its electrical schematic is shown in Figure 7.diode