Working principle of satellite TV down converter (high frequency head)

1 The function of satellite TV downconverter (high frequency head)
Satellite TV low noise downconverter is also called high frequency head (also called outdoor unit of satellite TV). It is composed of microwave low noise amplifier, microwave mixer, first local oscillator and first intermediate frequency preamplifier. Its block diagram is shown in Figure 1.

Figure 1 Principle block diagram of high-frequency head

A general satellite TV reception system mainly includes: (1) antenna; (2) feed source; (3) low noise down converter, also known as high frequency head (a component integrating low noise amplifier and down converter), represented by LNB; (4) cable; (5) terminal connector; (6) satellite receiver; (7) TV receiver. The
block diagram of satellite TV reception system is shown in Figure 2.

Figure 2 Satellite TV receiving system block diagram

Since the satellite downlink microwave signal received by the ground antenna in the satellite TV receiving system is very weak after a long-distance transmission of about 40,000 km, the antenna feed source output carrier power is usually about -90dBmW ^[Note] . If the feed line loss is 0.5 dB, the carrier power at the input of the low noise amplifier is -90.5 dBmW. The loss of the first frequency converter and the bandpass filter is about 10 dB, and the gain of the first intermediate amplifier is about 30 dB. In this way, if the low noise amplifier gives a gain of (40 to 50) dB, the down converter output can output a signal of (-30 to -20) dBmW. Therefore, the role of the satellite TV down converter is to amplify and convert the received satellite downlink frequency signal with low noise while ensuring the original signal quality parameters.

2 Structure of satellite TV downconverter

The low noise amplifier in the satellite TV downconverter is generally a component that combines a waveguide coaxial converter and a low noise amplifier. If low noise temperature and high gain are to be achieved, it usually includes 3 to 4 stages of amplification. The first two stages are low noise amplifiers, which mainly use high electron mobility transistor HEMT devices, and the last two stages are high gain amplifiers, which mainly use gallium arsenide field effect transistors GaAsFET. The noise temperature of a typical LNA is about (20 to 40) °K in the C band. The gain is about (40 to 50) dB, and the output-input voltage standing wave ratio (VSMR) is less than 1.5. Figure 3 shows the electrical schematic of a low noise amplifier (LNA). When designing, the necessary parameters are usually given first, such as S parameters, circuit levels, matching circuit methods, noise parameters, output and input impedance, etc., and then computer CAD software is used to optimize the design and make a microstrip line circuit diagram.

Figure 3 Electrical schematic of a low noise amplifier

The first frequency converter and bandpass filter are composed of the first local oscillator, the first mixer and the bandpass filter. Its function is to convert the downlink microwave signal output by the low noise amplifier into an intermediate frequency signal. The bandwidth of the signal remains unchanged
before and after the frequency conversion. The first local oscillator usually uses a dielectric resonator oscillator as a resonant circuit, uses a coupled microstrip line to couple energy, and uses CaAs-FET as a basic amplification circuit to realize the oscillator. The dielectric constant of the dielectric resonator is very high, usually between 35 and 40. When resonating, due to the high dielectric constant, most of the electromagnetic field is concentrated inside the dielectric, similar to a metal resonant cavity. The advantages of the dielectric resonant cavity are good temperature stability, high quality factor Q value, small size, low price, and easy coupling with microstrip lines to make MMIC.
Figure 4 shows two actual electrical schematics of dielectric resonator oscillators.

Figure 4 Electrical schematic diagram of dielectric oscillator

In the actual dielectric resonator oscillator, not only the parameters and position of the dielectric resonator and the parameters of the microstrip line need to be considered, but also the impedance matching of the output and input of the field effect transistor and the setting of the DC bias circuit.
The first mixer consists of nonlinear elements, a mixing network of the input signal and the local oscillator signal, and some additional circuits, as shown in Figure 5.

Figure 5 Block diagram of the first mixer

The input signal is mixed with the local oscillator signal and then superimposed on the nonlinear element. The nonlinear element usually uses a crystal diode and a triode to make it work in the nonlinear area of ​​the volt-ampere characteristic curve. Due to its nonlinear effect, the output end generates a series of signals such as sum frequency, difference frequency, and frequency doubling. The required difference frequency signal can be selected by a filter to achieve the purpose of mixing. In actual circuits, a diode impedance mixer is often used. It has a simple structure, is easy to integrate, has stable operation, low noise coefficient, wide operating frequency band, and a large dynamic range. Although this mixer has no frequency conversion gain, only frequency conversion loss, this loss can be easily compensated by adding an amplifier. In practical applications, the isolation of the input signal and the local oscillator signal and the suppression of parasitic frequencies must also be considered. A double-balanced mixer is usually used, which is mainly composed of a diode bridge and a balanced and unbalanced converter. The electrical schematic diagram is shown in Figure 6 (the balun in the figure is a balanced and unbalanced line transformer).

Figure 6 Double balanced mixer schematic

Four mixer diodes with the same characteristics are connected in the same polarity sequence to form a ring bridge circuit. The input and local oscillator signals are coupled through a transformer, and the unbalanced input is converted into a balanced output and added to the two diagonals of the diode bridge, so that the total intermediate frequency current is equal to the sum of the intermediate frequency currents generated by the four diodes.
The main features of the double-balanced mixer are as follows:
(1) There is high isolation between the input signal and the local oscillator signal;
(2) The operating frequency band is wide;
(3) The dynamic range is large and the overload resistance is strong;
(4) It has a good suppression ability for parasitic frequencies;
(5) It can suppress the noise introduced by the local oscillator.
The first intermediate frequency amplifier is also called the pre-intermediate frequency amplifier, which is usually directly connected to the mixer. Its function is to amplify the weak intermediate frequency output by the mixer to compensate for the attenuation caused by the mixer, the bandpass filter, and the high-frequency cable connecting the outdoor and indoor units. The first intermediate frequency amplifier is usually directly used as an integrated circuit block.
Since the first intermediate frequency of the secondary conversion satellite receiving system is usually selected at around 1 GHz, this frequency is at the junction of the microwave amplifier and the high-frequency amplifier. Therefore, the circuit structure can be distributed parameters, lumped parameters, or a mixture of the two.
The lumped parameter circuit is basically the same as a general high-frequency amplifier. The circuit components use lumped parameter resistors, capacitors and inductors, see Figure 7.

Figure 7 Lumped parameter circuit

Since the IF amplifier is a wide-band circuit, a tuning loop cannot be used, the components are leadless, and the circuit size is compact. However, due to the discreteness of the R and C components, it is often difficult to obtain values ​​that strictly meet the design requirements, so the single-stage gain is low; but this can be solved by increasing the number of stages, generally consisting of 3 to 4 stages, with a gain of about 20 dB.
The IF amplifier circuit with distributed parameters can be implemented in microstrip form, as shown in Figure 8. The S parameters of the transistor can be measured first, and then the microstrip matching circuit can be designed. The advantage of the distributed parameter circuit is that the circuit consistency is better and it is easy to achieve the best performance of a single stage, so the amplifier is generally 2 to 3 stages.

Figure 8 Distributed parameter circuit

The hybrid circuit is composed of a part of microstrip line and a part of lumped parameter elements. When the S11 value of the first-stage tube _is appropriate, a shorter transmission line and a branch microstrip can be used to form the input circuit, which can obtain lower noise. The interstage and output circuits can use a combination of microstrip and lumped parameter elements. It is flexible in design and has the advantages of both distributed and lumped parameter circuits.
The DC power supply of the outdoor unit is provided by the 75 Ω high cable core connected to the outdoor unit. The DC power supply of the indoor unit is transmitted to the outdoor unit through a high-frequency choke, which has no effect on the (3.7~4.2) GHz microwave signal and the first intermediate frequency signal. Usually the voltage of 16 V~24 V is sent to the LNA on one way and to the voltage stabilization circuit of the outdoor unit on the other way. After voltage stabilization, it is used by other stages of the outdoor unit.

3 Main technical requirements for satellite TV downconverter

Since the satellite downlink microwave signal received by the antenna in the satellite TV receiving system is very weak, in order to ensure the quality of the signal, the received satellite downlink frequency signal is amplified and converted. The main technical requirements that the C-band satellite TV downconverter should meet are as follows:
(1) Good amplitude-frequency characteristics. The amplitude-frequency characteristics refer to the characteristics of the output level change when the input signal frequency changes under the condition of constant input level, mainly including passband, power gain, gain fluctuation and gain slope.
The passband requires that the input frequency band of the downconverter is consistent with the satellite downlink frequency band, the output frequency band is consistent with the input frequency band of the satellite receiver, and the bandwidth of the input and output frequency bands of the downconverter is consistent;
power gain refers to the ratio of output power to input power;
gain fluctuation refers to the difference between the maximum gain and the minimum gain within the intermediate frequency output frequency band;
gain slope refers to the rate of change of gain within the unit frequency band within the intermediate frequency output frequency band.
(2) Low noise factor. The noise factor refers to the equivalent input noise of the downconverter as a whole, that is, the thermal noise generated by the entire circuit is equivalent to a noise source at the input end, usually expressed as noise temperature.
(3) Good local oscillator frequency characteristics. It includes the nominal value of the first local oscillator frequency, the stability of the first local oscillator frequency, and the leakage of the first local oscillator frequency.
(4) The input and output voltage standing wave ratio and return loss are small. The output voltage standing wave ratio and return loss are measured in the intermediate frequency band, and the input voltage standing wave ratio and return loss are measured in the downlink microwave band.
(5) High power gain. It means that the output power of the intermediate frequency signal of the downconverter is large.
(6) Good gain stability. This means that the gain fluctuation over time in the intermediate frequency output frequency band is small.
(7) Small multi-carrier intermodulation ratio. This means that the mutual modulation product when multiple signals of different frequencies enter the downconverter is small.
(8) High input saturation level. This mainly means that when the input signal exceeds the rated range, the impact of causing the downconverter to enter the nonlinear working area is small.
(9) High image interference suppression ratio. This means that the downconverter has a good ability to suppress image frequency signals. When the downconverter works in the linear range, the level ratio of the in-band signal and the image frequency signal with equal input amplitude at the output end is the image interference suppression ratio.
(10) Good group delay characteristics. This means that the group delay caused by the downconverter is small.
(11) Few spurious signals. This means that there are few useless signals other than intermodulation products.
(12) Low residual modulation noise. This means that when a pure sinusoidal signal of nominal frequency and nominal level is added to the input end, the additional noise contained in the output signal is small.
Among these technical requirements, high local oscillator frequency stability, low noise temperature, and good amplitude-frequency characteristics are the most important.
The above main technical requirements for C-band high-frequency heads can be summarized as shown in Table 1. However, Table 1 is for high-frequency heads that receive C-band satellite analog TV signals. If you are receiving satellite digital signals, in addition to selecting a high-frequency head with low noise temperature, high local oscillator frequency stability, and large dynamic gain, you must also select a high-frequency head with low local oscillator phase noise, because when receiving satellite digital signals, the local oscillator phase noise and local oscillator frequency stability of the high-frequency head are crucial to the quality of the received signal (because it will affect the bit error rate of the digital signal). The high-frequency head used in the digital compression satellite receiving system requires the local oscillator phase noise to be less than -65 dBc/Hz (at 1 kHz); the local oscillator frequency stability to be less than ±500 kHz.

Table 1 Electrical performance requirements for C-band tuners (outdoor units)
(cited from GB11442-95)

Due to the progress of science and technology and the intensification of international market competition, the production of high-frequency heads has become more and more sophisticated, the performance has become more and more excellent, the circuit has become more and more integrated, the size has become smaller and smaller, the reliability has become higher and higher, and the lightning protection capability has been increased. The following is a detailed introduction to the main characteristics of modern high-frequency heads and their development trends.
(1) Ultra-low noise characteristics
Due to the advent and widespread application of HEMT tubes, the noise temperature characteristics of the C band as low as 20°K and the power gain of about 40 dB, as well as the noise temperature characteristics of the Ku band of about 40°K can be obtained.
(2) Self-oscillating mixing circuit
The use of a self-oscillating mixing single-chip circuit greatly simplifies the inverter circuit. Using this single-chip circuit, the local oscillation, mixing and first intermediate frequency amplifier functions are completed. This single-chip circuit not only has no frequency conversion loss, but also obtains a frequency conversion gain of nearly 10 dB, simplifies the circuit and increases reliability. The most common single-chip circuits are MSA0886, MSF8885, etc.
(3) Monolithic IF amplifier circuit
In order to obtain 20 dB IF gain, 3 to 4 IF amplifier circuits are required. In the 1980s, two monolithic circuits were usually used internationally to obtain a gain of about 25 dB and a 1 dB compression point output power of about 10 dB. The monolithic IF amplifier integrated circuit obtained a 22 dB IF gain and a 1 dB compression point output power of 12.5 dB. The circuit is simplified. The commonly used monolithic circuits for this circuit are MSA0886, INA10386, etc.
(4) Surface mount technology and high-integration design
In the 1970s and 1980s, many high-frequency heads in the world mostly used resistors and capacitors with leads, which were large in size and heavy in weight. The IF amplifier circuits used also used multi-stage cascaded IF tubes, and the local oscillator mixers were all separate circuits.
Today's high-frequency heads (LNBs) use surface mount components and self-resonant mixer circuits. The monolithic IF amplifier circuit has achieved high integration, small size, light weight, and high reliability.
(5) Integrated moisture-proof design
In the past, some high-frequency heads sold on the market failed prematurely due to poor waterproof sealing design. To solve the waterproof sealing design, modern high-frequency heads tend to be integrated structural design. The waveguide and cavity parts of the high-frequency head are integrated by die-casting, and the covers of the RF and IF circuits are sealed with "O"-shaped rubber rings.
(6) Lightning protection circuit The
high-frequency head is an outdoor unit that works behind the antenna. In order to improve the equivalent noise temperature of the antenna, the antenna is often installed in open areas and high places. Whether it can protect against lightning is an important aspect of the reliable design of the high-frequency head. Most high-frequency heads in the 1970s and early 1980s did not have a lightning protection design, and some high-frequency heads on the market now do not have a lightning protection design. Most high-frequency heads with lightning protection designs have a lightning protection capacity of about 1,500 V, while the improved modern high-frequency heads have a lightning protection capacity of up to 3,000 V.
(7) New development of modern high-frequency heads
Modern high-frequency heads have been made into dual-polarization high-frequency heads and dual-band high-frequency heads. The former can simultaneously receive two polarization wave signals from satellite downlinks, and the latter can simultaneously receive satellite downlink signals in the C and Ku bands. This greatly simplifies the entire feed system and improves the use effect of the entire antenna feed system.
(8) In view of the widespread application of satellite digital television, modern high-frequency heads with high local oscillator frequency stability and low local oscillator phase noise have been developed. Because the digital signal they generate has extremely low bit errors, they are particularly suitable for good reception of satellite digital television signals.