FM radio transmission technology discussion STL

An overview:
FM radio for good signal to noise ratio, anti-jamming, distortion, frequency bandwidth, large dynamic range, the advantages of rapid development in China, from the central station to the county radio stations were set up FM radio station. Because the FM wave propagation is linear, so, in order to expand the effective service radius, the FM broadcast antenna mounted on the launch tower, the top of high buildings or mountains, to reduce the high frequency transmission loss, also bear Transmitter Room relying on antenna construction, the launch of this place, away from the broadcast control center has a very far distance, so the program source transmitter, FM radio system has become an indispensable part. If the link quality is not good, it will make the entire FM radio broadcast system of lower quality, but also very difficult to compensate in other areas or to save. Therefore, we must attach great importance to program delivery issues. Broadcast Control Center in the choice of FM transmitter to program delivery means, the need to distance the two places, the number of passes Number of Programs, investment size to deliver appropriate programs to choose means to achieve not only deliver quality assurance programs, and can invest in small, set up quickly, and achieve economic and social benefits.
The present situation, from the broadcast center to the FM transmitter control program delivery methods, there is cable transmission, cable transmission, STL method, microwave transmission and satellite broadcast delivery methods and other means. Of course, the wireless transmission of broadcasts, can also be a low-power FM radio transmitters to take, how to choose work in the 150MHz FM machine, but it passed Number of Programs less. Number of Programs such as the need to send more, you can use microwave equipment, then not only can send multiple sets of radio programs, but can also transmit television programs. However, more complex equipment, investment is large, is also more difficult to maintain. So, not too many Number of Programs in the transmission when using STL equipment, should be a priority program of choice, especially for broadcasters to municipal use.
STL radio transmission equipment, the main control center for radio broadcasting and FM broadcasting programs transmitted between the Transmitter Room. Not only in the broadcast control center to the launch pad that sent between two fixed points, radio programs, can also be used from the broadcast site (such as venue, theater, sports, etc.) to broadcast radio signals sent back to the control center. It is a wireless transmission line, working frequency band 940-960MHz, now the production equipment in four frequency bands: L band :1000-1700MHz; S-band :2000-2500MHz; X-band: 7/8GHz; Ku-band: 11 / 12GHz.
STL's classification is according to different terms are used in single-channel transmission of program, or sets of programs need to pass both stereo program another program to distinguish between mono. Programs used in the single-channel transmission of STL, it sent the first program of bandwidth optional 15kHz, can also be combined with vice set Frequency to send the second program or send remote control signals. When sending the program within the stereo program, can be synthesized using a set of STL, it has sufficient bandwidth for transmission generated by the synthesis of stereo encoder signals also allow the additional number of sub-carrier frequency, to send a number of other program or signal. With a synthetic STL, easy to guarantee the quality of stereo (left and right channel phase and gain easy access to ensure consistency). STL bandwidth limit at this time within the 220kHz, so not only can guarantee the quality of signal transmission program or also to prevent interference with other adjacent STL to work properly.

Second, the composition of the STL system
STL system components shown in Figure 1.

  
Figure 1 STL system components

STL used in the selection of the required transmitter, you must select the transmitter output power can be sufficient to overcome the losses caused by transmission path. In calculating the path loss, the need to STL transmitter power, converted to gain (in dB) that the transmitter (power) gain of 1 mW (dBm) as the reference standard. For example, the STL provides 6W transmitter power gain 37.78dBm, can provide 14W of the STL transmitter power gain of 41.5dBm.

3 STL works:
1 STL transmitter
STL transmitter block diagram, Figure 2


  
 

Figure 2 STL transmitter block diagram

As can be seen from the block diagram, STL transmitter carrier frequency is working in UHF band VCO generated. The UHF VCO is a high-stability quartz crystal source (standard oscillator) frequency synthesizer as a reference, so IF VCO frequency is very stable, so STL transmitter carrier frequency is also very stable.
Containing the stereo and other programs (or signal) of the synthesized signal directly to the IF (60-80MHz) voltage-controlled oscillator modulation, this approach is conducive to a good signal to noise ratio and stereo separation.
2 transmission line and antenna selection
In the STL system map (see Figure 1), shown between the transmitter and the transmitting antenna, receiver and receiving antenna transmission line between the use of the high-frequency coaxial cable. Generally speaking, the other conditions being equal, the larger the diameter of UHF cable, the lower the loss. Now most of UHF coaxial cable used foam and air dielectric medium.
STL transmitter and receiver for antenna use, there are many types to choose from, such as parabolic or cut-off parabolic antenna, Yagi antenna, parabolic antenna can be divided into planar, linear, or mesh, usually ordinary cut-off parabolic antenna made linear. Different antenna, its gain, operating frequency range, polarization mode (sub-horizontal polarization and vertical polarization), beam width, front to back ratio, structural strength, wind load, etc. is not the same. So, use the antenna must be from the STL distance transmission systems, setting up conditions, the transmission path gain comprehensive consideration of all aspects of demand.

3 STL Receiver
STL receiver block diagram shown in Figure 3.


  
  
Figure 3 STL receiver block diagram

Receiver sensitivity, signal to noise ratio, selectivity and passband width are the main factors determine the quality of the receiver. The high sensitivity of the receiver is an indispensable condition STL system. Sensitivity usually expressed as the requirement to achieve a certain SNR, the receiver needs the most input low. In the STL system for computational, often to the sensitivity of the receiver units - Micro-shaped (μν) is converted to dbm. For example, a receiver to get 55db of signal to noise ratio, the required input level for the 14μν, the 50Ω input impedance when it is equivalent to-84dBm. Another example of a receiver to get 60dB SNR, the required input level for the 40μν, the 50Ω input impedance when it is equivalent to-74.96dBm.
In order to facilitate the calculation of the transmitter power (W) and receiver sensitivity (μν), take into dBm, can be obtained according to the following formula.
dBm (decibel milliwatts) = 101g (power / 1mw) = 20lg [voltage (ν) / 0.224] = 20lg (μν/22400), the formula for the 1mW = V2/50.
The formula is in the transmitter's output impedance and receiver input impedance of 50Ω conditions are used.
Within the low-noise receiver front-end amplifier performance has a high frequency noise the size of the receiver sensitivity and signal to noise ratio is good or bad great influence. On the first level up, the use of a good performance surface acoustic wave (SAW) filters, available sharp selectivity, but also reduce phase distortion (compared with the lumped parameter filter), can be super-line of frequency response, good isolation and low distortion. Pulse counting discriminator can be accurately demodulated audio signal, while maintaining a small distortion. Low-pass filter with linear phase low-pass filter to obtain the best isolation.
4 STL transmission path selection
As the STL system is working in the ultra high frequency (UHF) band, so it is a form of wave propagation is a straight line, usually line of sight transmission distance. However, in actual communication, in this band when the radio propagation distance may be beyond the line of sight distance. This is because with the increase in height from the earth's surface, atmosphere (this frequency) of the refractive index gradually increased, the spread of radio is no longer a straight forward, but downward bending. Therefore, the actual use of STL systems, may simply use the transmission line of sight distance, on the identification of transmission paths. Transmission distance S after taking into account atmospheric refraction obtained the following formula:
  
  
The formula, H1, H2, respectively transmitter and receiver antenna height, in units of m. In the transmission path, not by some human or natural obstacles, or will affect the transmission quality, or even unreasonable. Thus, the better to make two points first, between the terrain (including buildings, including the) profile, and then verified by experiments, If not we should consider changing the route or the additional transfer stations.
Of course, to ensure the transmission quality, not only can not have blocked access objects, and requires access to block height can not enter inside the beam spread. Experience, the top obstacle distance h from the transmission beam should be more than 0.6 times the first Fresnel radius h> 0.6d.
  

  The formula: d - the first Fresnel zone radius (m)
S1 - the distance from the launch ended to obstacles (km)
S - take the whole transmission path (km)
S2 - S-S1 (km)
f - Frequency (GHz)
The plan was finalized, STL system can fulfill the normal transmission, the system must do the calculation. In the path calculation, there are many factors to consider and, most importantly, free space loss and tolerance is the decline of reserves, followed by other losses within the system, antenna gain etc. Finally, to Shi transmitter RF power level transmitted to the receiver after the input line receiver input requirements.
Free space loss Ls, can be calculated according to the following formula:
Ls = 92.4 20lgf 20lgS (dB)
The formula: f - frequency transmitter (GHz)
S - Transmission full (km)
STL Ls is the distance between the transmitter and receiver antenna function, that is, double the distance, signal level loss of 6dB. At frequencies below 1GHz, STL system, resulting in the decline is mainly due to the direction of signal transmission path of the atmospheric refractive index change at the same time, the process of reflection or refraction transmission signal and mixed signal transmission line, the receiver side signal level may increase or decrease, which resulted in the phenomenon of signal fluctuations (although the atmosphere in rain, fog, snow will be on the high-frequency energy absorption and scattering, but the frequencies below 1GHz, such effects can negligible).
Total system loss, including path loss, transmitter and receiver transmission line loss, system loss of the connection plug seat, and the possible use of the synthesizer, the insertion loss caused by the distributor so.
The total gain including the transmitter output power (be converted into dBm), the transmitter output power (be converted into dBm), the transmitter antenna gain and receiver antenna gains.
The total gain minus total losses, that is, to provide an effective signal to the receiver to use. STL this level and the minimum required receiver signal level compared to the difference for the decline of its reserves.
From the broadcast control center to the launch pad with the STL system transmission signal, and sometimes encountered such a situation: that the distance between the transmitter and receiver system can transmit more than a single STL distance, or between the existence of a large unable to get clear of obstacles (line of sight) path, which need to adopt active in the transmission system. A forwarding system includes a transmitter and a receiver. Forwarding system is a better way forward by IF. This approach is not in the forward point of signal to the received audio signal (or the stereo composite signal), but only converted to IF, and then sent to the forwarding of the transmitter, which avoids the signal degradation caused through the forwarding problem. (See Figure 4)
  
  
Figure 4 forward by way of STL IF transmission system

In Figure 4, the transmitter I set the control center in the broadcast, the receiver I and II hair dryer machine set up in the transit station. IF good performance when using the transponder, not only can the original baseband signal to be intact, and can relay several times, the performance remained the same. Can see from the box, in order to ensure high-quality high-frequency signals transmitted to the next stop, the receiver's intermediate frequency (63MHz) filter using surface acoustic wave (SAW) filters. This will not only meet the requirements of linearity and symmetry can be ± 20kHz IF bandwidth, transmit stereo quality to meet the requirements. The filtered output of the receiver IF signal, and sent forward transmitter IF input. Transmitter in the frequency of the VCO PLL lock, thus ensuring the original baseband signal without reducing quality. Transform into a high-frequency signal, the actuator, power amplifier amplified, sent to the transmitting antenna, the signal quality delivered to the next stop.

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