Key performance indicators required by the project
1. Performance requirements
(1) Input frequency: 11712.75GHz
(2) Noise figure: NF≤0.8dB
(3) Gain: G = 55 ± 5 dB
(4) Image suppression: ≥30dB
(5) Nominal local oscillator frequency: 10.75GHz - 1MHz
(6) Local oscillator stability: f±2MHz
(7) Carrier intermodulation ratio: >140dB
(8) Gain stability: ≤0.5dB
(@/36MHz)
(9) Output frequency: 950~2000MHz
(10) Local oscillator leakage level: ≤-50dBmW
(11) Local oscillator phase noise:
≥60dB (@/1KHz)
≥85dB (@/10KHz)
≥100dB (@/100KHz)
(12) Operating voltage range: 10-20V
2. Structural Requirements
The sample chamber is made of die-cast aluminum, and the surface of the chamber is treated with powder coating.
3. Environmental Adaptation Requirements
(1) Temperature requirements
The operating temperature range of the high-frequency head is -40℃ to +60℃.
(2) Humidity requirements
The LNB should be able to function normally in environmental conditions with relative humidity ranging from 5% to 100%.
(3) Atmospheric pressure requirements
The LNB should function normally under the following atmospheric pressure conditions: 86-106 kPa
Basic working principle and system block diagram of Ku-band dual-feed LNB
1. Basic Working Principle
Ku-band LNBs, also known as Ku-band downconverters , amplify and focus the downlink signal transmitted from satellites via an antenna before sending it to the LNB feed waveguide input. The signal is then fed to a high-frequency amplifier via a coupling probe (or coupled microstrip transmission line), amplified by a low-noise amplifier, and then filtered by a bandpass image frequency rejection filter to select the desired frequency band's power frequency (PF) signal. This PF signal is then mixed with the local oscillator signal to produce an intermediate frequency (IF) signal. The output signal is then sent via cable to a satellite receiver for QPSK demodulation and MPEG-2 decoding, providing the signal to the television in A/V mode. Users can select the signal from either of the two desired satellites using control signals.
Ku-band dual-feed LNB main unit circuit design
1. Field-Effect Transistor Low-Noise Amplifier (FETLNA)
When designing an amplifier circuit, many characteristics need to be considered, but the most important are stability, power gain, noise figure, output power, input and output voltage standing wave ratio (VSWR), dynamic range, and in-band power gain flatness. This product's low-noise amplifier uses the NEC NE4210S01 field-effect transistor (FET) low-noise amplifier. The function of the FET low-noise amplifier is to amplify weak signals fed from the cavity feed source.
(1) Design of the first-stage F-type LNA
This circuit is at the very front of the active circuitry of the high-frequency head, therefore optimal noise matching is employed. Typically, any noisy two-port network can be represented by a noise voltage source and a noise current source connected to the input of a noiseless two-port network. If the circuit is dominated by voltage noise, then using a high source impedance will minimize the transmitted noise signal; if it is dominated by current noise, then connecting a low source impedance will minimize the transmitted noise signal. When both noise sources are present simultaneously, a specific source admittance (or source impedance) will be derived from the circuit's minimum noise figure, called the optimal source admittance. The Smith chart can provide the circle of equal noise figures on the input admittance or impedance plane. We use the following relationship to describe how the noise figure deviates from its minimum value and increases:
F2Fmin+lKn/Gs】Ys-YoI
In the formula, F = noise figure, Fmin = minimum noise figure, Rn = equivalent noise resistance, Yo = optimal source admittance with minimum noise figure = Go + jBo, Ys = source admittance.
The noise figure of a multi-stage cascaded amplifier is calculated using the following formula:
NF=NFl+(NF2-1), Gl+((NF3-1)/GlG2+….
As can be seen from the above formula, the noise figure of the first-stage amplifier plays a decisive role in the overall noise figure of the tuner product.
The microwave circuit board we used is Rogers high-performance circuit board from Rogers Corporation, with a dielectric constant of 3.38 ± 0.005, a thickness of 0.5 mm, and a loss tangent of 0.0027. We used the advanced microwave circuit simulation software ADS for optimization simulation, and after careful debugging and several trials, we finally achieved our design requirements: the overall noise of the high-frequency head is less than 0.8 dB, as shown in the figure below.
2. Design of Microstrip Bandpass Filters
(1) Main technical specifications of bandpass filters:
① Passband boundary frequency and passband attenuation and fluctuation
② Stopband boundary frequency and stopband attenuation
(2) Design steps of microstrip bandpass filter
We used ADS software to design this filter. The design steps included:
① Schematic diagram drawing
② Optimization and simulation of circuit parameters
③ Simulation of the map, etc.
Based on the results of the software simulation design, we drew the circuit layout and fabricated it into a circuit board. We debugged the fabricated circuit, simulated it again using software, and debugged it again. After several rounds of simulation and testing, we ensured that it met our design requirements.
The main performance indicators achieved by our designed bandpass filter are as follows:
① Passband frequency: 11.7-12.75 GHz
② Passband attenuation less than 3dB
③ The ripple within the passband is less than 1dB.
④ Stopband attenuation greater than 30dB
