[Photo] 45-925MHz FM receiver

(1) 45-925MHz FM receiver

Recently, I saw an article in the "Electronic News" that introduced a secondary frequency conversion FM receiver with a high-frequency head as the main component of an ordinary TV set. It is also possible to read the article. I bought a set. The components are intact and the components are welded. The installation is acceptable, the high-frequency head is installed vertically, and the tuning uses a 100K potentiometer with its own band switch. So I ca n’t wait to connect the power supply and the speaker test machine. It was only found during the adjustment that the 1K in the original picture The trimmer potentiometer is not installed in the component. The position of an LED was originally reserved at the lower left of the board, which was analyzed as a tuning indicator. After welding an LED, it was found that it had a certain effect on the received signal and was removed. Initial use A 15V switching power supply component is used as a power supply, and it is found that the interference is very large. If you read the instructions carefully, you can also find that this component cannot use a switching power supply. Use a dual 15V power transformer with about 30 watts as the power supply and take one of the direct input components. Connected to a speaker for listening, the overall feeling is that the frequency adjustment is not very fine; the reason is because the handle of the tuning potentiometer is too thin, and it feels much better after being put on the handle of an old radio. It was also found in the adjustment No more Less problems: For example, the power supply voltage regulator integrated blocks 7805 and 7812 generate more heat. In order to ensure that the components work properly, a heat sink is specially installed.The heat sink is only slightly hot when working.

The components work basically normally, but finding a suitable shell for the components is really a bit of a brain. I didn't find a suitable one when I found it. Just when the outer packaging of the mail order has not been thrown away, just use it! Install the transformer Into the packaging box, the components are simply fixed on the outside of the box with screws, and the switch and power indicator are provided. The speaker is directly installed on the casing. In order to enhance heat dissipation, a small fan on the computer CPU is used to blow the circuit board directly from the side. The power supply is supplied by a diode in another set of winding strings of the transformer. A Q9 socket is installed on the casing, and the 1.2-meter whip antenna or the Yagi antenna described above can be directly installed.

The final appearance (click to enlarge)

Overall evaluation: The frequency receiving range of the receiving head is basically the same as the article introduction, that is, there are individual stations that cannot be tuned for FM radio (88-108Mhz). Frequency adjustment is not very convenient; there is no frequency indicating device, and a table introduced in the article is used. Head and indicate the approximate frequency on the circuit, and find that there is a certain interference to the reception. Individual components generate more heat, it is recommended to install a heat dissipation device. For receiving 900M analog mobile phone, the use of a whip antenna is better than my Yagi antenna, and the antenna The length is critical for reception. The frequency stability of the component is acceptable. This component has certain practical significance as a part of learning the principle of TV audio reception or debugging the transmission circuit.

(2) I use 45 ～ 925MHz double frequency conversion FM receiver

(Xiaoxiao)

During the summer, an FM enthusiast gave me a component of a full-range receiver. Maybe he didn't know how to install it before sending it to me. It happened that there was a similar article in the newsletter (below), which was confirmed after several considerations. After installation, the effect is of course good, but there is no frequency indication. The received frequency is only speculated. The amateur frequency bands of 144-148MHZ, 220-225MHZ, 430-440MHZ, and 902-925MHZ cannot be played. Secondly, I suggest that enthusiasts use battery power to maximize their advantages in the development of this product. I think it is still very valuable, but my time is limited. Wish you can develop new features!

The following is a description of this product:

Commercially available radios can generally only receive 88-108MHz FM broadcasts, with narrow frequency bands and low sensitivity. The receiving head introduced in this article can receive FM signals in the range of 45-925MHz, and can be used for listening to radio, TV sound, monitoring cordless phones, walkie-talkies, etc. It has a wide range of uses and has certain practicality and fun. The circuit is designed according to the communication receiver and radio circuit, and has the advantages of high sensitivity (1μV) and high stability, and is particularly suitable for remote reception.
1. Circuit principle The whole machine uses one high-frequency head and four integrated circuits to form a typical double-frequency superheterodyne receiver head. The first intermediate frequency can be selected between 31.5 ~ 38MHz, this machine is selected as 31 ~ 5MHz, the second intermediate frequency is the standard 10 ~ 7MHz. The audio output is about 0 ~ 2W, and the quiescent current is 100mA. See the attached circuit for the circuit.
After the radio signal is amplified and converted by high frequency, the output 31? 5MHz first intermediate frequency signal is sent to IC1 through the high frequency transformer B1, and is mixed with the 42? 2MHz second local oscillator to produce a 10? 7MHz second intermediate frequency signal The signal is sent to IC2 through a 10-7MHz three-terminal filter. IC2 has the functions of middle amplifier, audio demodulation, tuning indication and so on. The demodulated audio signal is amplified by IC3 and drives an 8Ω / 0? 5W speaker or headphones. The voltage of the 8-pin voltage output terminal of the IC2 frequency control AFC terminal of the high frequency head can achieve the purpose of automatic frequency fine-tuning.
The power supply of this machine is composed of 7805, 7812 voltage-stabilized integrated circuits and peripheral components. The DC-DC converter provides 5V, 12V, 30V working voltage to each part of the circuit. The 7812 requires a voltage difference above 3V, so the input DC voltage should be above 15V.
Second, component selection 1? The high frequency head should use the full supplemental channel high frequency head with high sensitivity, and use the frequency sweeper, oscilloscope and frequency meter to adjust the high frequency head. Under the condition that the overall performance remains unchanged, the The upper limit of the U-segment frequency is increased to 960MHz, while the actual upper frequency limit is 925MHz. The measured frequency range is 45-155MHz for L segment, 150-465MHz for H segment, and 440-960MHz for U segment.
2? The second local oscillator / mixing uses high-performance communication dedicated circuit NE602. The integrated circuit uses a double-balanced mixer, which has a good signal-to-noise ratio and three-stage intermodulation index. In the absence of external high-frequency amplifier, the receiver has a sensitivity of 0? 2μV, so the first can be omitted Put, simplify the circuit.
3? The second middle amplifier and audio demodulation use high signal-to-noise ratio FM / AM radio integrated circuit LA1260. The integrated circuit contains two parts, AM / FM. The FM part of this machine is taken. The AM part is vacant. The LA1260 has a high signal-to-noise ratio. The FM part is 81dB, which is 11dB higher than commonly used integrated circuits (CXA1019, CXA1238, TTA8127). The advantages of low distortion and strong anti-interference ability.
4? 30V DC-DC converter uses NE555 dual time base circuit, NE555 and peripheral components form a self-excited multivibrator, output 400kHz square wave on its 3 feet and send it to the voltage doubler rectifier circuit. When the input voltage is 12V, the output voltage is 31 to 5V.
5? The tuning component uses a 100kΩ potentiometer with a band switch for coarse frequency adjustment, and a 1kΩ single-connect potentiometer for fine frequency adjustment. The tuning component has no frequency indication. A 30V DC voltage meter can be used between the VT end of the high-frequency head and ground, and the receiving frequency can be roughly understood according to the indicated voltage. The frequency band switching sequence is L-H-U. When tuning clockwise, the frequency increases and counterclockwise decreases.
6? High frequency transformer B1, oscillating coil B2 use finished TV mid-period TRF1445, the original resonant frequency is 38MHz, B1 does not need to be changed, and the resonant capacitor can be increased, B2 is removed 1 turn, the resonant frequency is about 41? 5MHz.
7? The antenna is a key part of the machine. If necessary, a dedicated antenna can be selected according to the receiving frequency band, or a 1 to 5 meter rod antenna can be selected. When receiving U-segment signals, the optimal length of the antenna is 15 to 30 cm according to the actual situation. Choice. The antenna and the high-frequency head are connected with thick single-strand copper wire, and the length of the wire is not more than 3 cm.
3. Debugging and using This machine has only one tuning point except the high-frequency head, that is, the second local oscillator frequency adjustment. When the requirement is higher, it can be adjusted with a frequency meter. Without frequency timing, you only need to turn the B2 core that has been changed. The outer (counterclockwise) rotation is 1/3 turn, the resonance frequency is about 42-2MHz. This machine is relatively simple to use. Put the trimmer potentiometer in the middle position during tuning, then slowly turn the tuning potentiometer, and then adjust the potentiometer after receiving the signal to make the sound quality the best.

(3) 45-925MHz FM stereo receiver

(1) The circuit introduced in "FM Receiver Head" has many shortcomings in design. The following receiver heads summarize and improve the previous two articles, and the receiver head has greatly improved in performance.
1. Problems with the original receiver 1. Poor selectivity. The second local oscillator / mixing circuit (NE602) uses a simpler single-tuning loop, so the selectivity and passband characteristics are poor, resulting in the phenomenon of mixing stations, strong stations and weak stations in the receiving head. To solve this problem, a double-tuned loop must be added before NE602 to reduce the bandwidth to about 1MHz.
2. Low sensitivity. Although NE602 has a high input sensitivity index, the effect is not obvious in actual use. The main reason is that the low-frequency intermediate-frequency signal voltage of the high-frequency head when receiving a weak signal is low, which reduces the mixing efficiency of NE602 and the mixing gain Of reduction. Therefore, it is necessary to add a level of intermediate frequency amplification between the tuner and NE602.
3. Poor tuning accuracy. Although the receiver head tuning component uses a precision 30-turn potentiometer, the actual tuning accuracy is still low (3 15MHz per turn) because the receiving frequency is too wide. You can use resistor divider and switch to increase the tuning accuracy by 36 times.
4. Poor signal-to-noise ratio. In order to simplify the power supply and facilitate the use of DC power, the tuning voltage of the receiving head is provided by a high-frequency DC-DC converter. Because the receiving head printed board uses a single panel, the high-frequency radiation interference generated by the converter is very obvious. Therefore, it is necessary to use double-sided large-area perforated printed boards and use large areas to screen all levels. Experiments have proved that the effect is excellent, the interference noise is reduced to an unheard level, and the signal-to-noise ratio has been greatly improved.
2. New receiving head 1. Circuit composition. The receiving head consists of high-frequency tuner, first intermediate amplifier, second local oscillator / mixing, second intermediate amplifier, FM demodulation, stereo decoding, automatic gain control, tuning components, power amplifier, 30V boost circuit, voltage regulator circuit Etc.
â‘  The high-frequency tuner is the most important part of the receiving head. Although the high-frequency tuner dedicated for the communication machine has good performance, the price is quite high, and it cannot be purchased from the market. Add a tuner. The circuit structure of the full supplementary high-frequency head is not much different from the high-frequency tuner of the communication machine. As long as it is not a special requirement, it is feasible to select a high-frequency head for TV. There are many varieties of fully supplemented high-frequency heads, different circuits, and the sensitivity of discrete components is low. It is not available. The high-frequency head with TDA5300T series as the main circuit should be selected. Take the "Chengdu Xuguang" TDQ-3B6 tuner used by the author as an example. The tuner consists of three independent sets of LC input loops, dual gate field effect high amplifier, dual tuning loops, and TDA5330T. The three-band circuits are independent and do not affect each other. The frequency covers 45-900MHz, which is particularly convenient for adjustment. The adjustment of the high-frequency head is relatively simple, and the input loop and local oscillator do not need to be adjusted. The focus of the adjustment is the double-tuned loop, that is, the original 8MHz intermediate frequency bandwidth is compressed to below 1MHz to improve the selectivity index. Conditional can be adjusted with a frequency sweeper. If there is no instrument, it can be used directly without adjustment, but the sensitivity and selectivity are slightly worse.
â‘¡ The 38MHz first intermediate frequency filter is composed of B1, B2, C3, C4, and C5. B1 and B2 use the modulation-free type. The size of C4 capacity determines the intermediate frequency bandwidth. The center frequency of the filter is 38MHz and the bandwidth is 600KHz.
â‘¢ The first intermediate amplifier and the second local oscillator / mixing circuit are composed of TA7358AP. TA7358AP was used in a large number of FM radios. The high amplifier gain is 23dB and the mixing gain is 31dB. The stability of the local oscillator is quite good. The circuit configuration at this level is basically the same as the FM radio tuner, but the operating frequency is different. B3, B4, B5 all use the adjustment-free type, no debugging is needed at this level.
â‘£, the second pre-amplifier consists of T1 and related components, gain 20dB.
⑤ The second intermediate amplifier, FM demodulation, and stereo decoding circuit are the same as the basic circuit of the "Electronic News" this year's eighth issue of "Hi-Fi FM Stereo Tuner". The difference is that the stereo decoding circuit is changed from LA3370 to LA3430. LA3430's voltage-controlled oscillator uses a 456KHz ceramic magnetic oscillator, which improves stability and reduces distortion. A high frequency head AGC control circuit has been added to prevent strong signal jams. In order to further improve the stereo separation, the stereo output filter is changed from RC double T filter to LC filter, and the stereo separation can reach 50dB.
â‘¥ The AGC control circuit is composed of T2 and related components. The output voltage of the â‘¥ foot of IC2 varies between 0 and 8V with the strength of the input signal, but this voltage cannot be directly used for the reverse AGC high frequency head Only then can it be used for AGC control.
⑦, 30V DC-DC converter provides tuner tuning voltage, input voltage 12V, output voltage 30V. The step-up circuit is used instead of the power transformer to tune the voltage winding, which simplifies the power circuit and can be used for both AC and DC.
⑧ The tuning component is composed of a tuning potentiometer-band switch and channel switch. The tuning potentiometer uses a 30-turn precision potentiometer. The band switch is combined with the tuning potentiometer. The function of R1 R4 is to combine One section is divided into three sections: high, middle and low, and the tuning accuracy is improved by three times through a 3 × 3 switch.
⑨ The power amplifier is composed of LM386. This level is only used for communication signal monitoring. It is best to connect an external active speaker when listening to stereo programs.
2. The quality of the antenna is directly related to the quality of listening and the indicators such as sensitivity. When the requirements are not high, a 1.5-meter whip antenna can be used, which is suitable for listening to local FM radio and communication signals. If you want to receive remote signals, you must choose an outdoor antenna. To listen to remote FM radio, it is best to choose an outdoor antenna dedicated to the FM radio frequency band. If you want to receive communication signals at the same time, you can also use a full-band TV antenna, but the sensitivity and selectivity are worse than the dedicated channel antenna.
3. The quality of the power supply is also closely related to the overall sensitivity and signal-to-noise ratio. Transformers, rectifiers, and pre-regulated voltage should be far away from the receiver to avoid interference. The transformer should use 25W high-quality products, and a first-level adjustable pre-regulation circuit is added before the receiver head voltage regulator circuit, which can reduce the power consumption of the receiver head, reduce the heat of the receiver head voltage regulator circuit, and improve the stability of the receiver head . The voltage regulator circuit uses a low-power, low-dropout voltage regulator integrated circuit with a minimum input voltage of 12.3V, an optimal input voltage of 12.6V, and a maximum input voltage of 15V.
4. Main performance indicators Sensitivity ≥0.5uV Selectivity ≥75db Stereo separation ≥50db Static current ≤100mA

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