This system is composed of five parts, including signal input, amplifier, modulation, filtering, demodulation, so we will according to the function of the system can be divided into multiple parts, signal input is produced by signal generator, so design focused on amplification, modulation, filtering, demodulation circuit and the local oscillator signal source,

1. The low noise amplifier
According to the topic request, the input impedance of 50 Ω low noise filter, to meet the 250 MHZ to 300 MHZ bandwidth, in order to meet the requirements, we selected the LNA module rf signal amplifier, 0.005 4 GHZ bandwidth, 15 dB gain, noise is 0.7 dB, so that can reach the specified bandwidth,
2. The mixer
Solution a: choose AD831 chip, AD831 chip consists of mixer, limiting amplifier, low noise amplifier and output bias circuit and so on, the amplifier has low dc disturbance characteristics, thus it can be used to directly to baseband frequency conversion and quadrature amplitude demodulation direct coupling application, and module has shielded enclosure protection diode and the defence of the circuit is stable, low distortion, mixer adopts the working frequency of RF and LO input can be up to 500 MHz, can meet the topic request,
Scheme 2: choose the triode discrete component structures, mixing circuit, the mixing circuit mainly consists of three parts: local oscillator, the transistor inverter circuit and the intermediate frequency filter network, circuit is more complex, and the inverter circuit exists interference, circuit stability is poorer,
Game time is limited, and the precision meet the topic request, this design adopts the plan a,
The local oscillator signal source
Solution A: choose chips AD9854, AD9854 digital synthesizer is high integration devices, on chip integrates two high-speed, high-performance orthogonal D/A converter can be output by digital programming I, Q two synthetic signals, under the drive of high stability of clock, AD9854 will produce high and stable frequency, phase and amplitude of programmable sine and cosine signal, but the highest AD9854 allows the output signal frequency 150 MHZ, does not meet the design requirements,
Scheme 2: choose ADF4351 chip, ADF4351 combined with external loop filter and external reference frequency when using, it can realize the decimal or integer N points frequency and phase lock loop (PLL) frequency synthesizer, its fundamental wave output frequency range to 2200 MHZ to 4400 MHZ, using frequency division can produce the required frequency,
To sum up, option 2,