Phase-locked loop design based on V600ME14-LF

The phase-locked loop is a closed-loop control system that can track the phase of the input signal. It is widely used in many fields, such as modulation and demodulation, frequency synthesis, precision instrument measurement, FM stereo decoding, etc. The wide application of the phase-locked loop is determined by its unique and excellent characteristics. It has a carrier tracking characteristic. As a narrowband filter, it can extract the signal submerged in the noise and lock it with a highly stable reference oscillator. It can also provide a series of highly stable frequency sources for high-precision frequency and phase measurement. This design gives the application and design method of the phase-locked loop in the frequency source.

1 Phase-locked loop design

Figure 1 shows a block diagram of a scheme designed according to the basic principle of a phase-locked loop and specific needs. The reference frequency REF is generated by a 10MHz crystal oscillator. Considering that the frequency output of the VCO is 2~3GHz and the input frequency range of the phase detector is limited, the frequency is first divided by the frequency divider, then sent to the DDS, and finally divided by the DDS to meet the requirements of the input frequency range of the phase detector. The voltage tuning range of the VCO chip V600ME14-LF is 0~24V, so the loop filter in this scheme uses an active filter, and the output of the phase detector is amplified and sent to the VCO, so that the voltage tuning range can meet the requirements of the output frequency.

Figure 1 Block diagram of phase-locked loop design

This design uses DDS as a programmable frequency divider, and its frequency division coefficient can be changed between 1/2 and 1/2n, so as to achieve high-frequency fractional frequency division. As a frequency divider, the frequency of DDS cannot be too high, so a frequency divider should be added in front of it. The frequency divider can be set to 4, 8, 16, or 32 divisions. This frequency divider uses 32 divisions, and by controlling the DDS frequency division coefficient, the VCO's range of variation can reach the required 2 to 3 GHz.

The loop filter adopts the design of active filter. The active filter can not only filter out the interference signal, but also has the effect of amplification, which can make up for the shortcoming of the small output signal of the phase detector, so as to meet the requirements of the VCO input voltage.

2 V600ME14-LF Introduction

2.1 Feature Description

V600ME14-LF is an S-band and RoHS-compliant VCO launched by Z-COMM. The operating frequency range of this chip is 2000MHz ~ 4000MHz, and the tuning voltage range is 0 ~ 24V. At the same time, this VCO has the following characteristics:

◇ Phase noise at 10kHz is -89dbc/hz;

◇ Harmonic suppression (2nd): Typical value is -15dbc, maximum value is -10dbc;

◇ Typical output power: 2～11dbm;

◇ Driving capability: 10MHz/V;

◇ Tuning sensitivity (avg.): 110MHz/V;

◇ Operating temperature range: -40-85℃;

◇ Power supply voltage: 5V;

◇ Power supply current (Icc): Typical value is 44mA.

FIG2 is a schematic diagram showing the relationship between phase noise and frequency deviation, where the units are dbc/Hz and Hz respectively.

Figure 2 Phase noise vs. frequency deviation curve

2.2 Pin Introduction

The V600ME14-LF chip uses the MINI-16-LOW package. Figure 3 shows its pin diagram. The functions of each pin are as follows:

Figure 3 Pin diagram of V600ME14-LF

P1: Tuning voltage input terminal. After loop filtering, the tuning voltage can be input into the resonant circuit to provide an adjustable voltage; P2: RF output terminal; P3: Power supply voltage input terminal, used to input the power supply voltage required by VCO; REST: Ground terminal. Except for the above three pins marked in the figure, the remaining pins are ground terminals.

2.3 Peripheral Circuit

The output standing wave ratio of the V600ME14-LF chip is less than 1.67:1 (RL=12.0db). In order to prevent impedance mismatch caused by load chips (such as mixers, etc.), some form of isolation should be added to the P2 end during design. The circuit shown in Figure 4 uses a 10db PAD and a buffer amplifier to meet the above requirements. If the isolation does not meet the requirements, it will cause three problems:

Figure 4 External circuit connection diagram

One is the fluctuation of power output; the second is the degradation of phase noise performance; and the third is that the load will vary depending on the frequency.

The bypass capacitor and coupling capacitor in the circuit of Figure 4 are related to the frequency. The distance between the 10db PAD and the RF output terminal in the figure should be as close as possible, and the layout and wiring must strictly comply with the specifications of RF design. For example, a 50Ω microstrip line is a must, and careful selection of components is also essential.

3 Application Notes

The VCO is a surface mount package. The ground of the VCO must be directly connected to the copper ground layer on the PCB board, and the copper must cover the entire bottom of the VCO.

When in use, the inner diameter of the 16 half holes located on the perimeter of the board is mainly used to provide a connection surface for the VCO and the motherboard. The shielding cover around the device is tin-plated CRS or alloy. Adding bypass capacitors or an emitter follower at the power input can suppress power supply noise, and placing some vias under the device can reduce unwanted ground reactance. In fact, it is necessary to add some additional vias during design depending on the output frequency.

4 Conclusion

V600ME14-LF is a VCO with good performance, which can be widely used in test instruments, CATV front-end equipment, satellite communication systems, etc. However, in the application process, it is necessary to lay out and wire according to the requirements of the device-related data, so as to achieve better results.