Design of RF Signal Generator Based on GHz band

Design of RF Signal Generator Based on GHz band

In modern wireless communication systems, high-capacity and high-speed data wireless transmission is increasingly demanding. Many manufacturers have also released rf ic Based on the 802.11 series protocol, in addition, wireless routers, Bluetooth and other technologies are widely used, and the demand for GHz band is increasing. However, in addition to some high-end signal generators with GHz band, most common signal generators do not involve the 2.4GHz band, and a RF Signal Generator Based on the 2.4GHz band is developed to meet the needs of scientific research and teaching instruments. Based on this, this paper designs a 2-4 GHz RF signal generator with low cost and reliable performance.

System Solution

The system scheme uses the man-machine control on the instrument panel to set the operating frequency and baseband modulation mode, and uses FPGA to directly control and generate four basic modulation modes, that is, the two channels of the baseband I/Q signal are sent to the AD9856 through the serial string and converted, and the signal is modulated to the intermediate frequency signal of 70 MHz, then, the RF signal from the upper mixer MAX2671 to the 2450MHz is sent to the RF filter, and then the signal is output by the controllable gain amplifier. The diagram of the RF signal generator in the GHz band is shown in Figure 1.

Circuit Design

The signal modulation circuit is designed to use the EP1C20 chip of ALTERA Corporation in FPGA circuit design. The control information is output by the man-machine interface through VHDL programming, and then the control information is mapped to the signal to be generated, output the signal to AD9856. AD9856 is a 12-bit integrated digital uplink converter with a single-chip hybrid signal of ADI. The sampling rate is 200 MSPS, which generates 80 MHz digital output and 80 dB narrowband dynamic range of non-stray signal. The AD9856 has an internal clock of MHz and is integrated with 4 ~ The 20x programmable clock multiplier provides high-precision system clock, single-ended or differential input reference clock, and can output data clock. The internal 32-bit orthogonal DDS can implement the PSK modulation function; 12-bit DDS, DAC, and data path structure, can accept composite I/Q input data; 32-bit frequency control words, using interfaces compatible with SPI, reliable and convenient with FPGA control, the serial clock is 10 MHz. With the SINC function, the desired signal envelope is restored before the DAC conversion. The circuit diagram for generating Modulation Signals Using AD9856 is shown in figure 2.

Figure 1 system diagram

Figure 2 AD9856 modulation signal generation circuit

As shown in figure 2, the two I/Q signals produced by encoding and modulation in FPGA are sent to AD9856 after being passed through serial and converted. There is a DDS kernel in AD9856, an orthogonal base-vibration signal is generated by FPGA control and sent to an orthogonal modulated device. Each channel is multiplied by an I/Q signal by two levels to generate an orthogonal modulation signal, the specific modulation mode can be implemented through the baseband signal encoding ing Design of FPGA, and the 12-bit DAC can be used to convert the analog differential signal output to orthogonal modulation, then, a coupled RF transformer is used to convert the output differential signal to a single-ended signal, which is filtered by the SAW Filter at 70 MHz. Finally, the intermediate frequency amplifier is used to enlarge the signal, and the mixer can be sent for mixing.

Mixer circuit

The mixing circuit is extremely important for the implementation of the GHz band. It mainly completes the modulation of the 70 m intermediate frequency signal to the GHz RF, and requires the frequency band of the mixing circuit, here we use the MAX2671 mixing chip dedicated to the GHz band of MAXIM. MAX2671 allows an intermediate frequency to be between 40 MHz and MHz, and the RF output frequency is between GHz and GHz. The single-ended signal is integrated with a single-channel multiplier. The MHz RF signal mixing output has a dB gain. Therefore, the intrinsic signal can be between-10dBm and + 5dBm. When the input and output match, only a few peripheral devices are needed, as shown in circuit structure 3.

Figure 3 MAX2671 Circuit

RF vibration signal circuit design

In the signal generator design, to mix a 70 MHz intermediate frequency signal to a 2380 MHz ISM frequency band RF signal, the RF Local Vibration signal must be generated at a frequency of MHz. This Zhenxin signal circuit uses the PLL + vco pll to provide the local vibration signal, which has the advantages of precision, high stability, and variable frequency, so as to facilitate the adjustment or expansion of frequency resources in the future. The frequency stability of the vibration signal is very important. This part is designed with the integrated circuit as the core, and adopts the ADI Frequency Synthesizer ADF4113 and MAXIM company's voltage controlled oscillator MAX2750. Its principle is shown in Figure 4.

Figure 4 RF vibration signal circuit diagram

In order to control the frequency synthesizer, the three-line serial interface signal timing is simulated by FPGA to control the frequency synthesizer ADF4113, the frequency and pressure control oscillator (VCO) of the Reference crystal oscillator are completed based on the ADF4113) after the low-pass filter (LPF) removes high-frequency interference, obtain a relatively stable voltage, control the VCO oscillation frequency output, and obtain the required 2380MHz vibration signal.

ADF4113 is a high-performance frequency synthesizer of ADI with a maximum operating frequency of 4 GHz. ADF4113 consists of A low-noise digital phase detector (PFD), A precision charge pump, A programmable reference divider, programmable A (6bit), and B (13bit) A crossover counter and a dual-mode divider (P/P + 1. MAX2750 is a voltage controlled oscillator used by MAXIM Corporation for the ISM frequencies from GHz to GHz ~ The GHz ISM frequency band is shown in the circuit structure 5.

LPF is used to remove high-frequency voltage interference to obtain a relatively stable voltage. The Design of LPF can be implemented using a dedicated program-controlled filter chip. For example, MAX297 can be used to adjust the low-pass cutoff frequency. The cutoff frequency is 0 ~ 50 KHz, so the design is more flexible; another method is to use the standard third-order passive loop filter, that is, the LRC circuit design.

Figure 5 MAX2750 circuit diagram

Design Test

Determine the testing frequency and input and output dB values at all levels, and complete system debugging. The intermediate frequency signal power entering MAX2671 is-30dBm ~ -20dBm, RF Local Vibration Signal Power:-10dBm ~ + 5dBm. Through the test, the output spectrum of the RF Local Vibration signal circuit is 6. As shown in the figure, the output power of the signal source reaches-9dBm, which can fully meet the requirements of the upper mixer MAX2671 RF vibration signal input between-10dBm and + 5dBm. As shown in the figure, the output power of the signal source reaches-24.5dBm, which can fully meet the requirements of the upper mixer MAX2671 IF signal input between-30dBm and-25dBm.

Figure 6 rf lc signal spectrum at 2380MHz

Figure 7 shows the signal spectrum output by the mixer MAX2671. After mixing the 70 MHz intermediate frequency signal with the 2380MHz rf lo signal, the upstream and downstream frequency signals of the 2450 MHz and MHz are output, by filtering out the lower frequency component of 2310MHz through the RF medium filter, you can obtain the useful signal of 2450MHz. After setting the output power through the adjustable power amplifier, you can send it to the N-type header for output.

Figure 7 output signal spectrum of MAX2671

Conclusion

The 2.4GHz RF signal generator is designed to meet the wide application of wireless communication and wireless LAN. It has low cost, simple peripheral circuits, stable operating frequency band, and adjustable parameters, friendly man-machine interface, as convenient as ordinary signal generators, can meet the application requirements of testing instruments, teaching and scientific research.