Forum on the recent request for easy system and television signal transmission devices voices high, in order to meet the requirements of enthusiasts, the time to test this television transmitters. The circuit design guiding ideology is not any RF equipment will be debugging, performance professional equipment Although it is impossible to achieve the target, but its basic properties should be able to meet lovers experiment, audio-visual transmission effect should not be inferior to the universal-type machines level.
The transmitter from the RF modulator and amplifier two major components, all circuits are used by the most common and the most classic circuit. RF circuit structure of the circuit often do not have any particular place, the key lies in the work of different frequency matching circuit parameters. For example, the chart is one of the most commonly used total emitter amplifier.
Measured by this circuit in the 12-channel time gain of more than 10 dB, however, and following the 10 MHz signal amplification ability of 50 dB. 10 MHz in the low frequency end of 50 dB stability of the amplifier can still work. But when you need to access 12 channels 40-50 dB of gain, is certainly a need to enlarge three, we take a look to the amplifier of the three strings together what will happen.
Channel 12 is the release to meet the requirements of large, but not more than 50 db. However, the three 10 MHz amplifier for the release of up to 150 db lot more than it is in any case not work. An amplifier can not work, you can obtain the necessary gain? Therefore, the multi-level RF amplifier to try to curb its release to the lower frequency of large, relatively simple method is to press the circuit parameters set plans.
The TV Transmitter-coupling with the most simple way, as the following diagram.
Map shows the structure of the circuit only, no standard coupling capacitor value, as this map of coupling capacitor numerical performance of the complete machine comprehensive great, it did not leave producers superscript numerical experiments. Practice has proved that the filter after several coupling capacitors are used to stabilize the 10 P to work in the 6-12 channel (except for a pre-filter).
RF power amplifier transmitting antenna and the connection is amateur compared headache. If the antenna is a real pure resistance, of course, can be relatively simple, connected on the line. But the antenna impedance not so simple, in theory, the time could be perfectly matched antenna will be launched by resistance to pure analysis, but in practice Yulian transmitting antenna is equivalent to pure resistance it is very difficult, under the conditions in the amateur simply impossible. Therefore, the practice also used in the formation of a three resistors 3 dB attenuator to mandatory matching, the effect is very good, very stable, of course, it would consume half of output power. In fact, the channel 1-5 consumed half had no power, 2 SC2053 in this band RF power output is not large, there are several hundred milliwatts, there are also half of the loss Yierbai milliwatts, enough of. Moreover, if not for the matching network than 3 dB attenuator? As for the channel just 6-12, matching relatively easy, and sometimes through the DC-blocking capacitors feeder Direct Connect 2053 the collector can also stability. Work well when used Resistance isolation will not be late.
PCB as the following picture.
Complete the installation diagram can be inhibited in the second harmonic wave 40-50 dB lower than the level, but overall debugging difficult not small.
If some would like to modify plans can be produced by pressing, welding on the correct line, made a few arrangements for beginners, it is easy to success.
Remove the filter circuit, the second harmonic wave can only achieve 20 dB below the level, but work is stable and reliable.
Practice Chapter
Experimenters are not voluntary and not the whole experience in the production of electronic technology-loving, we will of this board divided into several steps to do.
The first step
Modulation of the installation, the following components in the circuit welding good.
1. TA7673 11 feet measured voltage is 3.8 V should be about.２．TV will be the input RF (antenna port) between the RF modulator output, TV should be able to receive RF signals, the screen showed very pure white grating. Note: I should be separated from direct antenna function, and if not should be connected in series capacitor at straight.
３．AV input signal, adjustable device for the audio and video with the best.
This step will determine the future of basic audio and video quality, it is necessary to repeatedly adjusted to achieve the satisfaction of audio-visual effects, should also be explored in this step to screen image transfer system on the impact of Audio-level, carrier frequency interference on the screen and so on. accumulate the necessary experience.
Step 2
Gain complete board-level welding installation, as the following picture.
This work is relatively simple step, very important, the highest gain complete machine is a link. SC3355 is usually as long as the Collector 2 in the 6-8 V voltage, circuit will be able to work very well.
This one should use better quality pipe.
The third step
As the two-level modulation after enlarge its output level may reach 100 dBuV more, in some small automatic gain adjustment of dynamic range and input circuit tv shortage of instability may arise simultaneously, it should be 2 SC2053 between the output and TV Insert an attenuator can be produced as follows Fig.
Figure attenuator can be produced in a wide frequency range, about 75 ohm impedance.
Do a good job of the attenuator, this will be on board filled the following circuit.
The main task of this step is to adjust the level amplifier biasing resistor on the emitter voltage of 1 V, if you know some transistor circuit, this step is usually not encountered any difficulties.
According to experience, if you do a low channels, such as 1 - channels or two channels, it is basically the end of this debugger, and a 2 SC3358 promote the launch of the total gain sufficient because of the 2053 launch of the channel in the 1-2 high gain, made in about 30 db, 3358 nearly 40 dB, in this band gain is not a problem and difficult to prevent self-excited. Self-defense is the primary means of signal attenuation, lower gain.
In the 6-10 TV channel, coupled with a need to enlarge it, as the following diagram.
increase in the level of magnification can be 6-10 channel, but the channel following the six suspected high gain, with the dashed line in Figure resistance to signal attenuation, 3-5 in the single-channel may also use this resistance can not effectively check the gain (at the same time must consider factors such as matching), the need for a more rational allocation to the attenuator, the chart is an example (3 channels).
