Frequency Modulation (FM) radio receiver in recent years, particularly last year, a hot topic, because FM radio than AM (AM) radio good sound quality, high fidelity, strong anti-jamming, rich program, the most critical is a low cost, although the number of sources now abound, such as CD, VCD, DVD, etc., but the cost is high, is an endless music software spending, and FM radio reception is free, so the income is not rich The music and want to enjoy the many enthusiasts, listening to FM stereo (FM ST.) radio show is the best choice. However, they never-ending pursuit of enthusiasts, Taiwan received more selective higher, but local radio stations is limited, and this is the origin of FM remote receiver.
I am also a member of FM enthusiasts, hobby FM receiver for more than ten years, summed up some experience. Previous years, "electronic newspaper" be released on a number of articles on aspects of FM reception. Especially in the United States, Dodge introduced the radio with digital tuning, and have evoked a nationwide FM receiver heat, then have to weather, terrain, and, at a time across the country more and more of the reasons FM radio, making many Shaoyou joined the ranks of the receiver. But things are not so simple as we think, there has been this or that problem, to date, but also phone ringing constantly, many letters to me and ask questions about the selection, reception, maintenance and other issues, the other by the editors of about, once again pick up a pen, the system introduces high-fidelity FM receiver, choice of material, the relevant knowledge, so that the majority of FM receiver enthusiasts can find it, circle FM fever dream.
A complete form FM radio receiver system shown in Figure 1, including outdoor and indoor two parts. Outdoor consists of antenna tower and antenna amplifiers; interior parts, including FM tuner, amplifier and speakers. FM receivers from the FM tuner and stereo decoder circuit in place. Described in detail below, respectively.
First, the antenna
Antenna is the gateway to the receiving system, is the first stop, no antenna, no matter how good the receiver can not receive a signal, not a good antenna, the remote receiver is impossible. In order to facilitate better understanding, we compare with the TV reception, TV reception because everyone is quite familiar, and they belong to FM transmission. FM antenna design, exactly the same as TV antenna, do not ask, then, can be directly used instead of Channel 5 TV outdoor antenna FM antenna. Antenna has the following three indicators:
1. Gain of the antenna gain is used to illustrate the ability of antenna to receive signals an important indicator, it turn, angle and direction of the antenna both before and after the radiation than the decision. The smaller the angle the direction of the antenna’s reception is stronger, the direction of interference by other smaller, which is why the direction of the antenna to be aligned in order to receive a clear picture and sound reasons. Before and after the radiation than the antenna, the antenna is the ratio of received signal strength before and after, the greater this value, indicating the better performance of the antenna. Basic unit of dipole antenna, there are the two biggest receiving surface, that is around 1:1, how to back ratio increases, the direction of more acute, the best way is to increase the director and the inverter, which is why Only when the signal strength with a single oscillator, while in weak signal using multi-element antennas reasons. In theory, the director more, the higher the gain, while the inverter can be only one, but the director too, will affect the bandwidth, poor mechanical strength, installation size large, so the FM antenna for general use segment three-unit antenna, a maximum of five units.
2. It is said that the antenna pattern in different directions with the reception of signals, it is different in meaning and direction angle. In principle, should make a half wave dipole antenna and radio transmitter signal direction perpendicular to, but in fact to receive a distant signal, the spread of change may be biased in the reception, the best performance should be based on the actual standard. Of course, we demand not only in one direction, but for the 360 ° omni-directional reception, which resulted in multi-direction and the entire direction of antenna. In fact, in front of a single oscillator is multi-directional antenna that if two oscillators vertical integration, the cross became omnidirectional antenna, this antenna is also introduced to the original author, but because of its low efficiency, gain and low, it is used in signal Weak areas not suitable. The only way is to use steering antenna, it has both a high gain, but also a good pattern, it can signal from any direction and vertical receive, and obtain maximum gain, it can improve the signal to noise ratio and eliminate adjacent channel interference or co-channel interference, the FM receiver is the best choice for enthusiasts.
3. Bandwidth, it is said to receive an indication of the frequency of FM signal. The wider the pass band, said the station received more of the provisions of the frequency range FM 87.5 ~ 108MHz, the total bandwidth of 20MHz, is still narrow-band antenna, which
Center frequency = v ?1? ?2 = 97.4MHz, generally take 98MHz, if the bias of a band, such as a low section (88 ~ 98MHz) or high frequency (98 ~ 108MHz), antenna design, a tendency to deliberately end flexibility.
In short, an outdoor antenna to receive FM radio on demand is higher gain, with the omni-directional reception (if the other direction without radio, can also be directed to receive), easy to use. General three-unit antenna gain is 7dB, five element antenna up 11dB. Last year I used FM9901A antenna is the first domestic FM radio receiver according to the three units dedicated antenna design, the use of power supply box, you can control the full range of rotating the antenna to receive good results. Antenna size due to design complexity, the following five elements are given only to the production of FM antenna size, shown in Figure 2, for the fans made reference.
Materials: Use ? 10mm aluminum tubes, according to map making, brass better, the larger the diameter the better, but equivalent to the bending vibration in the first midnight filled with sand, and then bending, so difficult to break, bend, and smooth. Connect the middle of the same material available, but note that the director and must be the midpoint of the inverter. Insulating materials can also be connected, shall not affect the results.
Second, the antenna amplifier
FM remote receiver amplifier is super-essential components and did it, and ultra long-range reception is impossible. It is the role of the antenna to amplify weak signals to improve signal to noise ratio. In addition, the remote receiver’s antenna to set very high, the introduction of feeder to be a very long room, which has some loss, the amplifier transfer to another role is to make up for losses caused by feed line.
An important indicator of the antenna amplifier is gain. Generally speaking, higher gain, higher output level accordingly, but the noise also increased, especially in weak signal even more so, when the strong input signal, the output level is too high will make the receiver overload distortion produce obstruction, so the gain should be appropriate, generally around 30dB, you need to pay attention not to the amplifier as a universal arms, as if the amplifier, all problems are solved, where there are many complex reasons (will be described later) . FM bandwidth, because only a section of 20MHz, is a narrow-band amplifier, as long as the first class to join the corresponding band-pass filter can be.
The principle of selection of high-gain amplifier, low noise, preferably with AGC function. Early products are composed of two or three high-frequency transistor big noise; late product uses a low-noise transistors coupled with an integrated amplifier, the most common form is 2SC3358 + µPC1651 this form by many manufacturers are selection. In recent years, is the use of better performance and better AT415 + A08 circuit, I think this is the best combination, in line with low noise, high gain selection principle. I have introduced the QG836 amplifier is in this form. If conditions, the choice of two pairs of gate field effect transistor consisting of amplifiers, which in addition has lower noise, higher gain, there is strong resistance to overloads, should be integrated in addition, the discrete parts the best choice, dual-gate control can be used K73, BF964 models.
In fact, as long as the TV on the use of amplifiers can be used for FM section, only to be replaced before the class frequency band pass filter BPF-specific integration can be, its shape like a ceramic capacitor, but three pin, the input, the output, its role is to signal other than the 88 ~ 108MHz filtered out, while the FM signal is smooth, effectively improving the signal to noise ratio. If purchase less than the parts, available inductors, capacitors, combination, shown in Figure 3.
Tell you that I tested some time ago tuner for satellite reception antenna amplifier to do to obtain good results. Satellite tuner on the use of fully shielded metal rain structure and its internal mixer stage and do not remove the vibration level. There are three levels constitute the internal high-frequency FET amplifier circuit, which receives the frequency of 3.4 ~ 4.2GHz, a frequency segment several times, high gain, good results. Although the internal resonant circuit, but in terms of microstrip inductance on the FM signal is only the equivalent of straight wire, the first class to join BPF is a very good amplifier. Of course, I just did a test only, not long, its stability has yet to be tested.
Third, the connection and set up the outdoor part of the system
With a good antenna, amplifier, its connection is also very important.
1. Antenna amplifier matching frame and in general, the amplifier input and output impedance are 75O, while the equivalent impedance of oscillator 300O, the impedance matching must be 4:1 (300/75 available on the TV socket match control alternative) to match. If the direct connection, loss increase, the already very weak signal, and then there is a loss, how much more the signal is small, it should attach great importance to match the problem. Although the half-wave oscillator impedance of 75O, but their mechanical strength is poor, and also with the 75O coaxial 1:1 balun matching. Feeders and transmission lines do not flat because of the former with a round feeder loss is too great, so choose a smaller loss of 75O coaxial cable, usually with a 75-3 type, with a 75-5 better, of course, cost is also higher. Antenna amplifier antenna installed in the aircraft usually around 30cm below, there are also loaded into the integrated form, such as the FM9901 is this form of antenna, while addressing the problem of anti-rain and snow.
2. The connection from the amplifier to the receiver if the receiver is a single whip rod antenna, then only the middle of coaxial copper wire, outer shield net empty. If the tuner products, generally two sets of 75O and 300O antenna input terminal, which can be accessed is 75O terminal crimping method, only the correct match to get the maximum level input.
If you set up an outdoor antenna, maximizing antenna height, usually more than 6m above the ground, higher than 3m above the roof. But it is not as high as possible, but may be too much worse results, the decision should be set according to the actual reception of the height of the antenna if the surrounding objects are not high, but also to install a lightning rod, and make it a good ground to avoid lightning.