Wireless transmission of integrated TV signals for amateur productions

The transmission method of this family integrated TV signal is to mix the signals of satellite TV receivers, DVD (VCD), video recorders and other audio and video equipment with the signals of cable TV, amplify them through a wide-band amplifier, and then transmit them.

1. Working principle of wireless transmitter

The electrical principle of the wireless transmitter is shown in the figure below. It mainly consists of two parts: a broadband amplifier and a transmitting antenna.

(1) Determination of the transmission frequency range

The cable TV user distribution network is a radio frequency signal distribution network. The cable TV system is equipped with: 450MHz, 550MHz and 750MHz systems, and even a full-band system of 860MHz. At present, most areas use the 550MHz system, and its highest channel is channel 22 (UHF band). As long as the commercially available satellite receivers, DVDs (VCDs) and video recorders and other audio and video equipment are produced by regular manufacturers, their radio frequency signal output frequencies are all in the UHF band, generally marked as adjustable between channels 20 and 35, that is, adjustable between 526 and 654MHz. Since the lowest frequency of the signal transmitted by the cable TV system is 48.5MHz, the transmission frequency range of the home integrated TV signal is determined to be 45 to 650MHZ.

(2) Transmitter working principle

As can be seen from the figure, the integrated TV signal mixed and output by the "front end" is sent to the high-pass filter composed of C1, C2, L1, etc. through a 75Q coaxial RF cable. According to the design requirements, its cut-off frequency is 45MHz, which can filter out signals below 45MHz, and then amplified by the preamplifier composed of transistor V1, etc., coupled to IC1 (μPC1651G) ② pin by capacitor C3 for amplification, and the amplified signal is output from ④ pin and transmitted through the antenna.

IC2 (LM7805), LED, R3, etc. form a +5V voltage regulator circuit and a power indicator circuit. D1 and D2 are a pair of reverse-connected protection diodes to prevent the input voltage from being too high and burning out the integrated circuit μPCI651G.

2. Component selection and debugging

(1) Component selection and production

IC1 uses the high-gain high-frequency amplifier integrated circuit μPCI651G produced by NEC for TV antenna signal amplifiers. Its main technical parameters are: voltage 4.5-5.5V, current 15-25mA, power gain 16-19dB, bandwidth 1200MHz, noise factor 5.5dB; V1 uses low-noise, ultra-high frequency transistors 2SC3358 or 9018 (FR=700MHz), etc.; capacitors are all high-frequency ceramic capacitors; resistors can be 1/8W ordinary carbon film resistors; L1 is made of φ0.2mm enameled wire wound 16 times on a round rod with a diameter of 3mm, and then cast; D1 and D2 use 1N4148; the antenna can use the radio pull-rod antenna.

(2) Simple debugging and precautions

Because the transmitter works in the ultra-high frequency band, the cable should be as short as possible and fully shielded. When designing the printed circuit board and welding, the configuration of the components and the length of the leads must meet the requirements of the high-frequency circuit design. In order to reduce signal leakage, the entire circuit board must be placed in a metal container (such as a can) or shielded with aluminum foil. Only the antenna is left outside.

The working current of the whole machine is about 25mA, and the working current of transistor V1 is about 5mA. If the integrated TV signal is low, the transmission power can be increased by properly adjusting the resistance value of R1. The gain of the whole machine is designed to be 30dB, and the transmission radius is about 14m.

The antenna of the TV receiver in each room is solved by a 1m pull-rod antenna or a 1m copper core wire.