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QPSK transmitter design analysis

In Electronic Infomation Category: Q | on February 02,2012

In the past to use more air to air missile telemetry PCM / FM system, FM transmitter is way more work on the phase-locked frequency modulation. The transmitter and MAX507ACWG datasheet and the signal acquisition circuit digital interface, the use of dual differential output D / A converter circuit AD9761 generates baseband I / Q signals, using ADF4360-1 phase-locked loop generates a differential local oscillator signals for differential quadrature modulation using AD8346 , a differential circuit to improve system performance and MAX507ACWG price and reduce the LO leakage, the use of radio frequency amplification chip HMC478 and MAX507ACWG suppliers and HMC457 will signal amplification to 29 dBm. tested the power output of the signal modulation vector error (EVM) of 5% of the transmitter has been involved in telemetry system test results show stable operation.

1 transmitter used modulation

Transmitter that can transmit signals at certain frequencies out of the device. Is a more general concept. Widely used in television, radio, radar and other civilian and military equipment. Can be divided into FM transmitters, AM transmitters, optical transmitters, Harris transmitters and many other types.

Common transmitter modulation frequency modulation and RF direct modulation in two ways. The direct modulation of the RF transmitter is needed on the RF frequency directly to baseband signal modulated to the RF carrier frequency, no frequency link. Two modulation methods have advantages and disadvantages, the main comparison in Table 1.

As IF modulation is generally conducted at a lower frequency, the modulator selected range, easy to implement, and because of the presence of frequency link, the DC component of the modulation and carrier leakage are better suppressed. But with a frequency parts, so the need to increase relative to direct radio frequency modulation mixer, filter, and a local oscillator to improve the system complexity, increase system cost, reliability decreased. At the same time as the lower IF frequency, it is difficult to achieve a high modulation bandwidth.

RF direct modulation method is simple and reliable system, a wide modulation bandwidth, the device small, low cost, more suitable for high data rate applications where small size and other needs. But there is limited carrier frequency adaptation, a large carrier leakage, vibration requirements for this and other issues to be higher, in the carrier leakage of-band attenuation is difficult to meet the strict requirements of the indicator.

2 transmitter design

The system work in a dedicated S-band telemetry, a limited number of users under normal circumstances, the carrier leakage and band attenuation with less stringent requirements, and the telemetry system is the key issue is the small cabin space available, need a strong anti-shells transmitter overload, so the choice of RF direct modulation scheme. The transmitter LO and I, Q signals are differential output mode, the use of differential interface modulator modulation, better suppression of the circuit common mode interference. Figure 1 is a block diagram of the transmitter. Baseband signal is filtered and modulated RF local oscillator and then amplified by the radio-frequency, low-pass filter in addition to the harmonic output.

2.1 digital interface design

Due to front-end digital circuits can be directly output differential of I, Q digital signals directly from the output of the differential front-end digital circuits I, Q signal of the RF local oscillator for quadrature modulation, but by the actual test, modulation characteristics poor, mainly The reason is that digital circuits and the modulator output is difficult to match. Therefore, it has been improved, added to the transmitter by AD9761 dual DAC differential output consisting of interface circuit. The circuit has a 40 MSPS conversion rate (single channel 40 MSPS), 10 bit DAC, dual differential conversion output, and

And has twice the sampling interpolation filtering. The circuit output current mode, to achieve a good match with the modulator.

2.2 baseband filter circuit design

Symbol rate of 1 MHz spectrum of a random sequence shown in Figure 2. Filtering the signal can be suppressed sideband signal, reducing bandwidth usage. Digital filtering techniques can easily achieve high-level filtering, the proximal band signal rejection, reduces the analog filter requirements. The program uses the digital interface circuit is a 2-fold sample 43 order FIR low-pass interpolation filter circuit, representing an increase of the filter, stop-band rejection to 62 dB, thus modulating the output signal spectral characteristics has been greatly improved through The proximal band spectrum has been greatly suppressed for higher conversion clock frequency, the filter characteristics of the periodic folding. Figure 3 shows the interpolated digital filtering of the spectrum compared with the original spectrum, the digital interpolation filter circuit of the analog filter stop-band rejection have a wide transition zone, the circuit requirements are reduced.

To simplify the circuit, the program uses a relatively simple RC filter sidelobe suppression to 40 dB.

2.3 LO design

RF local oscillator circuit to generate local oscillator signals. Since the LO quadrature phase modulation signal high performance requirements, and this system uses direct RF modulation for up to a certain carrier leakage control should be selected differential output of the oscillator circuit. The program used ADF4360-1 Integrated phase-locked loop (PLL) circuit to generate the RF local oscillator. ADF4360-1 is the ADs complete with an integrated VCO PLL chip, small size, easy to use and reliable performance characteristics. Package is only 4 mm 4 mm, to output 2 050 ~ 2 450 MHz RF signal, the 3-wire serial interface.

LO frequency by the oscillator circuit, the oscillation frequency of the first principle of selecting either hinder the work of other radio frequencies. Frequency stability should be in at 25 : 1MHz (this is typical parameters) within the required local oscillator frequency stability is very important, otherwise it will generate the local oscillator frequency drift caused by not viewing the consequences of the local oscillator circuit in plus a phase-locked loop circuit, thus ensuring a high stability. Satellite receiver automatic frequency control (AFC) circuit, the oscillation frequency change for the elimination of antenna down by the high frequency radio and television signals by satellite low-noise microwave amplifier into the mixer, while the high local oscillator circuit frequency local oscillator signal into the mixer. Two different frequency signals into the mixer, due to the mixer is a nonlinear device, the antenna sent a signal to the LO signal is sent within the mixing in the mixer, resulting in a series of IF signals of different frequency (the vibration amplitude is the principle of selecting the output of the mixer IF signal distortion subject to a minimum) should reduce the frequency of these signals to the satellite receiver system in the first intermediate frequency range.

2.4 modulation circuit

Modulation (modulation) is the source of the information processing added to the carrier, so that it becomes suitable for transmission in the form of the channel process is to make the carrier signal with the change of technology. In general, the source of information (also known as source) with a DC component and lower frequency of frequency components, called base-band signal. Baseband signal is often not as a transmission signal, it is necessary to base-band signal into a baseband frequency in terms of a relatively very high frequency signals suitable for transmission channel. This signal is called the modulated signal, and baseband signal is called modulation. Carrier modulation is that message by changing the frequency of the carrier signal amplitude, phase or frequency to the baseband signal with the magnitude of change achieved.

Modulation circuit was completed for direct radio frequency modulation function. It uses filtered dual differential modulation of the RF local oscillator is modulated. The design choice of AD produced by quadrature modulator AD8346, it is a wideband quadrature modulator, frequency band of 800 ~ 2 500 MHz, with modulation of high precision, and low noise level, the phase error 1 , I, Q amplitude imbalance of 0.2 dB, the noise level of -147 dBm / Hz. This project uses a frequency of 2.2 ~ 2.3 GHz, through the rational layout and port matched to achieve good modulation effect, vector modulation error of the measured 2% ~ 3%.

2.5 amplifier As the RF modulated signal

a lesser extent, so the need for effective amplification, in order to achieve the required power. In this scenario, using two radio frequency amplifiers, the first stage using HMC47 8MP86, the second stage using HMC457QS16G, two amplifier input and output matching to 50 , HMC478 the high gain, low noise figure, linearity; HMC457 output power, the 1 dB compression output of up to 30.5 dBm. two-stage amplifier gain a total of 41 dB, net of inter-stage matching loss, the final output of 29 dBm. two amplifiers are Hittites industrial-grade products to meet -40 ~ +85 temperature environment, and both surface-mount package, shock vibration capability. Increase the signal amplitude or power electronic circuits. Applied to achieve amplification of the amplifier device as amplifiers. It is the core of electronic active devices, such as tubes, transistors, etc. In order to achieve amplification, the amplifier must provide energy. Energy is commonly used in DC power supply, but some also use high-frequency power amplifier as the pump source. Amplification is the essence of the power of the energy transferred to the output signal. Input signal is to control this shift, so that the amplifier output signal changes reflect the input signal repetition or change. Modern electronic systems, electrical generation, transmission, reception, transformation and processing, almost all of the amplifier circuit is based.

2.6 RF filter circuit design

RF amplifier inevitably there must be non-linear, will produce frequency harmonics, the lower-band radiation, reducing the interference to other systems, the design of the microstrip low-pass filter circuit, the harmonic inhibition.

2.7 Power Design

Power din yun power supply; power source voltage supply is provided by the device. The other forms of energy into electrical energy is called power. 12 V power supply circuit used to enter into non-regulated use of the various components of the transmitter 5 V and 3.3 V voltage. One 5 V interface circuit and amplifier circuit for the power supply, power consumption due to the larger, and the choice of DC / DC converter circuit, in order to achieve high conversion efficiency; 3.3 V for the PLL local oscillator circuit, using a linear regulator circuit 5 V step-down to get through in order to reduce ripple. Were used in the design of the LTs LT3431 and LT1962. PCB layout in order to reduce power when the ripple of power supply wiring carried out a detailed optimization.

3 design results

This project in a 75 mm of the completed circuit board design, assembly after the completion of the inter-stage matching adjustment. Eventually reach the target is: local oscillator phase noise -95dBc/Hz @ 1 kHz; LO spurious rejection of 72 dBc; output power of 29 dBm; output signal EVM of 5.5%.

4 Conclusion

The transmitter has been developed to complete, by high temperature, shock, vibration and other environmental testing, participated in a live firing test low-level telemetry system. Throughout the process, the transmitter is stable and reliable, can meet the design goals.

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