Note: FTA-100 and MD-109 are products of this century, so they are not listed.
Solve the intermodulation and false response starting from the high-frequency head
In the FM tuner, there is a high-frequency circuit shielded by iron sheet called a tuner, which contains high-amplification, mixing, oscillation and tuning circuits. The tuner is at the forefront of signal processing, and its quality directly determines the receiver's sensitivity, intermodulation false response and other indicators. In the 1960s, because there were not many FM radio stations in a region, the tuner design was very simple, and the double tuning could be well received. In 70 years, large cities have densely tuned frequency channels. In order to increase selectivity, the tuner is designed to be multi-connected tuning, up to 13 channels at most. After adopting the multi-connected structure, the selectivity is indeed improved, but the tracking error is also increased, the group delay characteristic is deteriorated, and the sound quality is deteriorated. At that time, because there was no high-quality sound source, people did not notice the change in sound quality. In the 1980s, tuners entered the ranks of high-fidelity equipment, and sound quality became the first important indicator. People realized that to improve sound quality, we must first eliminate the false response caused by intermodulation, and the tuner is obliged to take this responsibility. The number of false responses is related to the number of radio stations. If the number of radio stations is n, the number of false responses is (n-1)n. At present, my country's coastal and eastern cities can generally receive more than 30 FM stations, so there are as many as 870 false responses, which shows how serious the problem is. Therefore, when a city sets the frequency of an FM radio station, it will carefully calculate it to minimize the number of false responses that fall into the receiving frequency band. The false response on the surface causes the receivable stations to increase, but when tuning to the false response frequency, it will be accompanied by hiss, hiss, hiss, and chirp, chirp, and chirp sounds.
Since mixing is achieved by the non-linear characteristics of the device, and non-linearity is the source of intermodulation, in principle, the false response of the superheterodyne receiver cannot be completely eliminated, so the device with excellent linearity and large dynamic range Become a weapon to improve the performance of the tuner. In terms of intermodulation and cross-modulation indicators, bipolar transistors are the worst, junction FETs are slightly better, MOS FETs are better, and potassium arsenide FETs are the best. Because potassium arsenide single crystal is extremely easy to break, it is difficult to manufacture, and the price is expensive. The depletion type double-gate silicon MOS tube is equivalent to a cascode amplifier. It has a large dynamic range, a small Miller capacitance, and good stability. Its linearity is better than a six-tube balanced analog multiplier, and it is a high-amplifier and mixer. The ideal device.
How many connections do the tuner use? From the consideration of selectivity alone, the more the number of connections, the better; but from the linearization of the group delay characteristics and the improvement of sound quality, the fewer the number of connections, the better. In order to take into account the sound quality and selectivity, it is better to choose 4 to 5 connections. The next question is whether to use an air variable capacitor or a varactor diode for the tuning device? Tuners before the mid-1970s all used variable air capacitors. Since the first frequency synthesis tuner ST-910 came out in 1974, various manufacturers have imitated them one after another. Japan is the country that produces the most tuners in the world. In 1983, Alps discontinued production of the last Air Duolian, which has since brought an end to the variable capacitor tuners. In terms of insertion loss and capacitance-frequency characteristics, air variable capacitors are significantly better than varactor diodes. In order to improve the Q value of the varactor diode, the two varactors can be made into a back-to-back twin tube form, and the performance is close to that of an air variable capacitor. The performance of the 5-pair varactor diode tuning system is equivalent to that of the air 4-connector. The biggest advantage of using a varactor is that it can achieve digital tuning and multi-point tuning, and get rid of the trouble of manual tuning.
Multipath signal is the culprit of pop interference
When we receive FM broadcasts, in addition to receiving direct waves from the transmitting antenna directly to the receiver, we also receive reflected waves from mountains, buildings, and the ground. The harm of reflected waves can be directly felt from the ghost image on the TV when using the indoor antenna. However, when receiving FM broadcasts, the ghost images appear in the form of pops, pops, hiss, and hiss. When you move the position of the radio and the direction of the antenna, the intermittent broadcast sound is mixed with pops, pops, hiss, and hiss. This is the effect of multipath interference. Multipath interference is the most harmful to FM reception, and it is difficult to eliminate. Because this system does not have the ability to resist multipath. Due to the serious harm, FM broadcasting is not suitable for mobile reception.
Under fixed receiving conditions, the time delay and amplitude of the multipath signal are fixed, and the position and rotation direction of the antenna can be moved to always find the point with small multipath interference. But to eliminate the multipath interference caused by cars and air planes, only a strong pointing antenna can be effective. The experience of foreign FM enthusiasts is to set up four 5-unit Yagi antenna arrays, which can obtain a main lobe of about 18 degrees, and set up a multi-path signal absorption wall inlaid with ferrite in the direction of the antenna side lobe to weaken the intensity of the reflected wave. It can significantly reduce the non-linear distortion caused by multipath and obtain excellent sound quality. But this kind of antenna is expensive and the cost of listening to the radio is too high, and only a very small number of audiophiles will do it.
The adaptive transversal filter is an effective weapon to eliminate multipath interference from the circuit. The experimental results in the past laboratory are exciting. In a city with high-rise buildings, only a whip antenna can be used to obtain excellent sound quality, and even under mobile conditions as long as the vehicle speed does not exceed 60 kilometers, a good reception effect can be obtained. Due to the complex structure of this filter, a high-speed processor is required to determine the amplitude and time delay of the multipath signal in real time during the tuning process, and automatically switch to the best cancellation node. The cost is relatively expensive, and it has not been implemented in consumer electronics. Regrets for generations of broadcast enthusiasts. Nowadays, software radio provides a simple and cheap method to solve this chronic problem. The improved anti-multipath transversal filter described by C code can detect the reflected signal in microseconds in real time in the DSP with a clock frequency of 700MHz, and automatically select the appropriate delay. The number of nodes and the attenuation coefficient completely cancel the multipath signal. It is a pity that this late technology missed the opportunity to be applied to the FM tuner. If you find a good tuner, setting up an outdoor directional antenna is the simplest and most effective way to resist multipath interference and improve sound quality.
IF amplifier is the source of distortion
The intermediate amplifier is the core of the FM receiver. Indexes such as sensitivity, signal-to-noise ratio, capture ratio, distortion and selectivity are directly related to the performance of the intermediate amplifier. The FM intermediate amplifier is the most concentrated place for the application of new devices and technologies. Tuning The technologies used on the device are as follows:
1) Super-linear solid-state filter: Four types of filters, including LC midrange, quartz crystal, multi-mode ceramic and surface acoustic wave, have been used in the mid-amplifier. LC Zhongzhou is the oldest and classic device. The combination of 4-6 loops can design the amplitude-frequency characteristics into Butterworth or Gaussian type. In the early years, in order to improve the selectivity, the Butterworth type was used. Due to the poor group delay characteristics, the Gaussian type with good group delay characteristics was popular in machines that value sound quality. The crystal filter has the best rectangular coefficient, but the group delay characteristic is poor. Ceramic filters are small in size and low in price, and the early product groups have poor delay characteristics. Later products have been greatly improved and have become the mainstream of intermediate frequency filters. The disadvantage is that the center frequency is highly variable, and it is necessary to select a pairing. The amplitude-frequency characteristics and phase-frequency characteristics of the surface acoustic wave filter can be designed separately, and the group delay characteristics can be done very well, but there is a sidelobe response. In order to take into account selectivity and distortion, a combination of multiple filters is generally used in the tuner. For example, the narrowband state uses crystal and ceramic filters to ensure the selectivity, the normal state uses the ceramic filter and the surface acoustic wave filter to balance the sound quality and selectivity, and the wideband state uses the LC filter to ensure the sound quality and capture ratio.
2) Frequency negative feedback and variable parameter amplification: The idea of frequency negative feedback is to reduce the frequency deviation to narrow the distribution width of the FM wave sideband. If the frequency band is narrow, the group delay characteristic at the center frequency of the ceramic filter can be used to be the flattest. A straight section of curve minimizes distortion. And it can make 100% of the sidebands pass through the filter to achieve full spectrum transmission. After reducing the frequency offset, the high-frequency signal-to-noise ratio will decrease, so the frequency positive feedback is used after the filter to restore the frequency offset to 75KHz. This technology first appeared on Onkyo’s T-727 tuner. It only uses 6 decibels of negative feedback and the distortion reaches 0.1%. After that, Kenwood Company invented a spectrum-free technology on this basis, which compressed the frequency offset to almost zero. This technology was applied to the famous L-02T tuner in history, and the distortion of the machine was reduced to 0.003%. Frequency negative feedback is to improve the linearity by changing the frequency offset parameters, and you can also use the method of changing the frequency offset to improve the signal-to-noise ratio. Because the signal-to-noise ratio of the FM wave is proportional to the frequency deviation, a simple frequency multiplier can double the frequency deviation. Each time the frequency offset is doubled, the signal-to-noise ratio increases by 6 decibels. If 5 times the frequency is used, the frequency deviation can be increased to 375 KHz, and the signal-to-noise ratio can be increased by 30 decibels. Suppose the signal-to-noise ratio at 75 KHz frequency offset is 65 decibels, after 5 times the frequency is 95 decibels, which is the same index as CD. After the frequency offset is increased, the linear range of the frequency discriminator is also increased, so the frequency multiplier should not exceed 5 times. Another variable that can be changed is the relative frequency offset, which can be changed to improve the frequency discrimination sensitivity. It is realized by double frequency conversion, and the relative frequency deviation is increased by reducing the intermediate frequency. The sensitivity of a wide linear discriminator is often low, and this method can increase the output amplitude of the discriminator.
3) Signal conversion: After changing the frequency offset, the FM intermediate frequency becomes a sparse pulse after frequency reduction and amplitude limiting. It can be converted into a pulse width modulation (PWM) signal with a simple digital circuit, which is the same as CD and The one-bit quantized signal in the digital power amplifier is the same, but the modulated signal is not audio, but MPX signal. If a digital discriminator is used, the intermediate frequency signal must undergo this transformation. In software radio, the 10.7MHz intermediate frequency directly enters the ADC for sampling and then is processed by DSP. In the past, new technologies in intermediate amplifier, frequency discrimination and decoding can all be implemented by software algorithms.
High-frequency head Mid-amp Discriminator Decoder
Design Key Points Intermodulation and False Response Selectivity and Distortion Bandwidth and Linearity Resolution
Distortion distribution (%) 5 80 10 5
The key to the discriminator is linearity and bandwidth
The frequency discriminator is the second largest source of distortion in the FM receiver. It can be seen from Table 2 that the effect of the frequency discriminator on the sound quality is higher than that of the tuner and the decoder. In a tuner, the intermediate amplifier and the frequency discriminator jointly determine its performance, so it is paid special attention by the manufacturer. In order to compete in the market, 11 types of frequency discriminators have been used in history. They are proportional frequency discriminator, phase frequency discriminator, phase shift product frequency discriminator, PLL frequency discriminator, phase tracking frequency discriminator, Pulse count frequency discriminator, delay line frequency discriminator, differential frequency discriminator, PWM frequency discriminator, digital parameter frequency discriminator and DSP frequency discriminator. Manufacturers and designers lavishly exaggerate the advantages of their own frequency discriminators, and some circuits have been touted as such. In order to evaluate the performance of these discriminators, NHK used 12KHz audio frequency with a frequency offset of 5-10Hz, and scanned the passband of the discriminators to check their linearity. It turned out that there was no such discriminator. Better, because no matter what kind of circuit discriminator, as long as the linearity and bandwidth meet the requirements, and the differential gain is a horizontal line, good sound quality can be obtained.
How wide frequency band and good linearity can meet the requirements of high fidelity? In order to prevent the detuning of the intermediate frequency caused by temperature and offset errors, the linear bandwidth of the discriminator should be higher than the intermediate frequency bandwidth of 100KHz in ordinary radios, and it should be higher than 200KHz in the tuner. If there is a bandwidth option in the tuner, the wideband is generally 400 KHz, and the narrowband is generally 200 KHz, so the linear bandwidth of the frequency discriminator needs to reach 600KHz. In an analog circuit discriminator with a center frequency of 10.7MHz, the ratio and phase discriminator must use a double-tuned loop to meet the requirements. Tracking technology can also be used to generate linear bandwidth. For example, the phase tracking discriminator turns the frequency modulated wave into a phase modulated wave, demodulates the MPX signal in the phase discriminator, and regenerates the reference signal of the phase discriminator with a phase-locked loop. Due to the complexity of the circuit, Hitachi made it into an integrated circuit HA11211. JVC company favors this kind of circuit, and can often see it on their mid-to-high-end tuners, such as T7070, JT-V77, etc.
In the variable parameter mid-amp circuit, the situation is more complicated, and the changed parameters must be treated specifically. When the frequency offset is changed, the bandwidth of the discriminator will change accordingly. If the frequency offset is changed to 150 KHz, 225 KHz, 300 KHz, 375 KHz, the corresponding linear bandwidth is 800KHz, 1.2 MHz, 1.6 MHz, 2MHz. It is difficult for the analog frequency discriminator circuit to achieve a linear bandwidth above 600 KHz, so it is implemented in a digital way. The simplest digital method is to convert the sine wave frequency modulation signal into a width modulated pulse, and use a low-pass filter to restore the MPX signal, such as pulse count discriminator and PWM discriminator. This kind of frequency discriminator appeared on the AJ510 tuner of Heathkit Company in the United States in the early 1970s. After Trio Company learned it in 1976, it was used on all its high-end tuners. Another digital processing method is to down-convert the FM intermediate frequency into pulses below 2MHz, pass two CMOS gates with different delay times, and use an exclusive OR gate to demodulate the MPX signal. The digital frequency discrimination algorithm is very simple to implement in the DSP, which can be completed with a quadrature signal multiplier, and there is no linearity and bandwidth problem. Now on DAB/FM tuners, frequency discrimination and decoding are implemented in DSP using software algorithms.
Intermediate amplifiers and frequency discriminators were once a DIY paradise for broadcasting enthusiasts. There are many classic circuits with ingenious ideas and excellent performance. Today, many enthusiasts still talk about it.
The most assured is the stereo decoder
Today, whether the factory produces or DIY FM radio, the most assured part of the stereo decoder. Even if a two-cell battery-powered portable machine, the decoder uses A TA7343 can easily get a stereo separation of 40 decibels. In the past, this was almost unthinkable.
In history, it took twenty years to improve the stereo separation of the receiver. In the pilot control system, the amplitude difference and phase difference between the sum signal and the difference signal; the phase difference between the regeneration subcarrier and the transmitting end subcarrier will affect the degree of separation. If there is an amplitude difference of 3 decibels or a phase difference of 20 degrees from the difference signal, the regenerated subcarrier phase and the original subcarrier have a phase difference of 20 degrees, and the stereo effect will disappear without a trace. In the decoder, the phase difference and the amplitude difference exist at the same time, and these parameters will also change with temperature and time. There are two types of stereo decoders, matrix type and switch type. The matrix type is simple in principle and easy to implement, but it has strict requirements on circuits and devices. This is destined to be born with a matrix decoder designed with tubes or transistors in the early years. The degree of separation. I have tested the historically expensive high-end stereo radios, and the resolution is generally around 12 decibels, which is far less than the handheld phones sold on today's stalls. The switch decoder can get a higher degree of separation in principle, but it is required to regenerate a switch signal that has no phase difference with the subcarrier of the transmitting end. The switching signal that does not need to be regenerated by the phase-locked loop cannot meet the phase requirements, so the switching decoder can not get a high degree of separation, the highest is about 20 decibels. Therefore, for more than two decades after the launch of FM stereo, the degree of separation has always been the weakness of FM receivers.
In the early years when Europeans and Japanese were having a headache for stereo separation, in 1972, American Motorola developed the world's first integrated phase-locked loop stereo decoder MC1310, and the stereo separation jumped from ten decibels to 40 decibels. , The distortion is reduced from 1% to 0.3%. after that. Japanese manufacturers have learned to imitate and produced a variety of decoders with better performance. For example, the TA7343 commonly used in cheap machines has a resolution of 45 decibels, a distortion of 0.08%, and a signal-to-noise ratio of 74 decibels. The stereo decoder specifically used in the tuner has a resolution of 65 decibels, a harmonic distortion of 0.006%, and a signal-to-noise ratio of 89dB. Since the appearance of this device, the stereo sound in FM radios has become worthy of the name. Moreover, it has broken the boundaries between advanced and popular, and people have to sigh the progress of technology and the changes of the times.
Can't ignore low frequency preamplifier
Although the low-frequency preamplifier is in an inconspicuous position in the tuner, its role cannot be ignored as a part of the receiver. In an excellent tuner, it should have the following functions:
1) De-emphasis circuit: In the mono receiver, the 50-microsecond de-emphasis circuit is connected after the discriminator. In the stereo receiver, in order to ensure that the pilot and difference signals are not attenuated, the de-emphasis circuit is connected to the stereo After the decoder.
2) Pilot frequency and sub-carrier frequency filter: The main purpose is to remove residual pilot frequency and sub-carrier frequency components in the audio to prevent intermodulation distortion in the low-frequency amplifier. They also produce bird calls when recording and AD conversion beat the bias frequency and sampling frequency.
3) Squelch circuit: The gain of the FM receiver is very high, and there will be a lot of noise when there is no signal input. In the past, the squelch was mainly used to avoid noise during tuning, and the location without a radio station was still very quiet. There is no tuning noise in digital memory tuning. Because many receivers retain the flywheel manual tuning, the squelch circuit is still necessary.
4) Equal loudness control: according to the human ear's equal loudness curve to compensate for the narrowing of the auditory frequency response at low volume, this function can obtain rich effects of high and low sounds when making family background music with low volume.
5) Tone control: to compensate for the defects of the speakers and the listening environment
6) Passband control: Listening to FM broadcasts in marginal areas with weak signals, setting the bandwidth of the low-frequency circuit at 150-8000 Hz can significantly reduce high-frequency noise.
7) Intimacy control: Properly increasing the amplitude in the range of 2000-3000 Hz can make the human voice feel intimacy, and appropriately limiting the rate of the low-frequency amplifier can weaken the mouth feel and enhance the softness of the sound.
FM radio is not suitable for listening with Hi-Fi headphones
When listening to FM broadcasting with Hi-Fi headsets, I always feel that the sound is a bit rough. When the program interval and low level, you will hear hissing noise and crackling discharge sound. This is the inherent noise floor of FM broadcasting. Interference from external power, industrial and household appliances, 99.9% of which can be suppressed by the limiter of the intermediate amplifier and frequency discriminator, and the remaining parasitic frequency modulation, transistor thermal noise and flicker noise limiter There is nothing that can be done, they will be superimposed on the audio signal and become the noise floor.
So why can't the background noise be heard with speakers? There are two reasons. One is that the weight of the earphone voice coil and diaphragm is very light. If a pair of earphones and a speaker are also marked with a sensitivity of 90 decibels, the earphone means that the sound pressure of 90 decibels can be obtained by driving 1 milliwatt of electric power at one centimeter; the loudspeaker is driven by 1 watt of electric power at a distance of 1 cm from the speaker. The same sound pressure can be obtained at the meter, and obviously the sensitivity of headphones is thousands of times higher than that of speakers. Another reason is that in the direction of sound transmission, the sound energy per unit area is inversely proportional to the square of the distance, and the sound frequency rises from 1KHz to 10KHz, the frequency rises 10 times, but the air absorption loss rises 100 times. The energy of low-level interference and noise is distributed in the middle and high frequency bands of the audio frequency, and attenuates quickly in the air, almost attenuating to zero at one meter away. Distance and air play the role of a filter, making the human ear completely unable to feel the presence of background interference and noise. Listening with headphones is completely different. The eardrum is very close to the headphones, which is equivalent to bypassing the background noise filter. In addition, the sensitivity of the headphones is very high, so that music and noise are collected in the ears, making us feel the sound Rough.
In addition, the adaptability of speakers and headphones to music and the psychological feelings of listening are different. The speakers are suitable for appreciating spirit, such as symphonies, concerts and operas. In the show, the heart-shattering Bess, the soft and sweet midrange, and the gorgeous and bright high-pitched tone like a heart mark will induce listeners to feel the overall outline of the music without having time to take care of the minor details. Headphones are suitable for appreciating details, such as the weeping violin and erhu, and the fading triangle. The sweet vocals and slender treble will induce the listener to feel the melody and technique of the music, and capture the rich layers and relatively small differences. Therefore, the experience of the veteran is to listen to CDs with headphones and listen to the radio with speakers.
Welcome to the digital tomorrow
FM radio has been carrying music and happiness for 66 years. In the last century, my country’s radio enthusiasts were not lucky enough to enjoy music FM. After the reform and opening up, FM radio stations have sprung up everywhere. However, there are two regrets that bother the majority of fans: one is that the broadcast content is uneven, and the local and county-level FM stations are not only poor in equipment, but the content of the program is also not flattering. Another regret is that there is no good receiver to enjoy the broadcast. Since 2002, the Central People's Broadcasting Station put forward the reform slogan of "frequency specialization, management frequency", the sound of China, the sound of economy, and the sound of music have been launched one after another. Local radio stations have also followed up and reformed their broadcast programs. After careful comparison with Shanghai FM radio stations, the sound quality of 107.7MHz Music Sound is the best; 94.7MHz classic music has the best content, but unfortunately the transmission power is the smallest. You can find cheap and good-quality second-hand tuners at the Qiujiang Road Electronic Market and the Xiangyang Road Modern Electronic Mall.
Although FM broadcasting is facing the end of being replaced by digital broadcasting, the quality of broadcasting has reached its peak. In the future, both DAB and IBOC have undergone bit rate compression. Theoretically speaking, the compressed signal is redundant signal, but listening evaluation is a complicated matter. The conclusions drawn in theory and laboratory cannot cover the individual psychological and physiological differences of each person. DAB broadcasting in the UK has become very popular. European fans carefully compared the sound quality of DAB and FM, and questioned the propaganda of DAB's crystal sound. No matter what kind of digital broadcasting our country adopts in the future, the inherent nostalgia of mankind will definitely make many people miss the FM broadcasting that once brought us endless happiness. A pastor in the gospel church once enlightened me: "Treasure today, only today is the happiest one."
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