Main performance indicators required by the project
1. Performance indicator requirements(1) Input frequency: 11 7 12.75 GHz
(2) Noise figure: NF ≤ 0.8dB
(3) Gain: G=55±5dB
(4) Image rejection: ≥30dB
(5) Local oscillator nominal frequency: 10.75 GHz_1MHz
(6) Local oscillator stability: f±2MHz
(7) Carrier intermodulation ratio: >140 dB
(8) Gain stability: ≤0.5 dB
(@/36MHz)
(9) Output frequency: 950~2000MHz
(10) A local leakage level: ≤ -50dBmW
(11) Local oscillator phase noise:
≥60dB (@/1KHz)
≥85dB (@/10KHz)
≥100dB (@/100KHz)
(12) Operating voltage range: 10--20V
2. Structural requirementsThe sample cavity is made of aluminum die-casting, and the surface of the cavity is treated by a spray-molding process.
3. Environmental adaptation requirements(1) Temperature requirements
The operating temperature range of the tuner is 40 °C - +60 °C
(2) Humidity requirements
The tuner should work normally in an environment with a relative humidity of 5% to 100%:
(3) Atmospheric pressure requirements
The tuner should work normally in the following atmospheric conditions: 86-106KPa
Basic working principle and system block diagram of Ku-band dual-feed tuner
Basic working principleThe Ku-band high-frequency head is also called the Ku-band down converter. The downlink signal transmitted by the satellite is amplified by the antenna and transmitted to the LNB feed waveguide input, and then input to the high frequency through the coupling probe (or coupled microstrip transmission line). The amplifier is amplified by the low noise amplifier, and the PF signal of the desired frequency band is selected by the band pass image frequency suppression filter. The frequency difference is mixed with the local oscillator signal to output the intermediate frequency signal, and the output signal is sent to the satellite receiver through the cable to complete the QPSK solution. Harmonize MPEG-2 decoding work. It is provided to the TV in the mode of A/v signal. The user can select the signal on any of the two satellites required by the control signal.
Main unit circuit design of Ku-band dual-feed high frequency head
1. FET low noise amplifier (FET LNA)
There are many characteristics to consider when designing an amplifier circuit, but the most important are stability, power gain, noise figure, output power, input and output voltage standing wave ratio, dynamic range, and power gain in-band flatness. The low noise amplifier of this product uses NEC's NE4210S01 FET low noise amplifier. The role of the FET low noise amplifier is to amplify the weak signal fed from the cavity feed.
(1) The design of the first level F staring at the LNA
This circuit is at the forefront of the active circuit of the tuner, so the best noise matching is used. In general, any noisy two-port network can be represented by a noise voltage source and a noise current source connected to the noiseless two-port network input. The fake female Il circuit is dominated by voltage noise. A high source impedance will minimize the transmitted noise signal. If current noise is dominant, connecting a low source impedance will minimize the transmitted noise signal. When both noise sources are present, the minimum noise figure of the slave circuit will result in a particular source admittance (or source impedance), called the optimal source admittance. The angle of the Smith chart can give an equal noise figure circle on the input admittance or impedance plane. We use the following relationship to describe how the noise figure deviates from the minimum:
F2Fmin+lKn/Gs】Ys-YoI
Where F = noise figure, Fmin = minimum noise figure, Rn = equivalent noise resistance, Yo = the best source admittance giving the minimum noise figure = Go + jBo, Ys = source admittance.
The noise figure of the multistage cascade amplifier is calculated as follows:
NF=NFl+(NF2-1), Gl+((NF3-1)/Gl G2+....
It can be seen from the above formula that the noise figure of the first stage amplifier plays a decisive role in the noise figure of the entire high frequency head product.
The microwave circuit board we used is Rogers' high performance circuit Rogers sheet, which has a dielectric constant of 3.38-+0 05, a thickness of 0.5 mm, and a loss tangent of 0.0027. We use the advanced microwave circuit simulation software ADS to optimize the simulation, and then through careful debugging, after several trials and debugging. Finally, we met our design requirements, the overall machine noise of the tuner is less than 0.8dB, as shown below:
2. Design of microstrip bandpass filter(1) Main technical indicators of the bandpass filter:
1 passband boundary frequency and attenuation and fluctuation in passband
2 stopband boundary frequency and stopband attenuation
(2) Design steps of the microstrip bandpass filter
We use ADS software to design the filter. The design steps include:
1 schematic drawing
2 circuit parameter optimization, simulation
3 layout simulation, etc.
The circuit layout is drawn according to the results of the software simulation design and processed into a circuit board. We debug the processed circuit, then simulate it through software, and then debug it. After several iterations of simulation and translation, it meets our design requirements.
The main indicators that our designed bandpass filter achieves are as follows:
1 pass band frequency: 11.7 - l2.75GHz
2 passband attenuation is less than 3dB
3-way in-band undulation is less than ldB
4 stopband attenuation greater than 30dB
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