rf mod

Description: If the television receiver does not have a video input and the owner is unwilling to modify it, a TV modulator serves as an effective solution. This simple circuit processes video signals to allow direct connection to the television's aerial input. Essentially, a TV modulator functions as a small transmitter. It operates as a basic oscillator that generates a frequency within the VHF or UHF range. The oscillator is modulated with the video signal, producing a modulated carrier wave that is transmitted into the TV's aerial input via a cable. The television can then be tuned to the appropriate frequency. However, the design must meet specific requirements, such as frequency stability and display quality. A crystal oscillator is employed to ensure frequency stability, while careful component selection enhances display quality, allowing for a resolution of 80 characters per line, which is often necessary.

A significant consideration in the circuit design is the transmission frequency. If a single channel is used, it can lead to practical issues, as different users may prefer different channels, making the carrier wave challenging to locate. If the frequency is not precisely set, no signal will be received. A more effective approach is to include a wide range of frequencies in the high-frequency signal, facilitating tuning for various users. The block diagram illustrates this concept, with the TV modulator consisting of a modulatable crystal oscillator and a harmonics generator. The oscillator operates at 27 MHz, a low frequency that allows for the use of inexpensive crystals. The harmonics generator transforms the oscillator signal into a frequency spectrum containing multiples of 27 MHz, extending up to approximately 1800 MHz. The output signal of the TV modulator consists of numerous peaks, each representing a complete transmitter signal, ensuring at least one signal falls within VHF channels 2 to 4, one in VHF channels 5 to 12, and several in UHF channels 21 to 69.

The circuit, as depicted, is straightforward. The crystal oscillator utilizes a fast high-frequency transistor (Tl, BFR91) for amplitude modulation. It is crucial to select the correct component values surrounding Tl for optimal performance in this high-frequency application. The harmonics generator comprises two Schottky diodes (D1 and D2), which must switch rapidly in sync with the 27 MHz signal to produce robust harmonics extending into the gigahertz range.

The overall design of the TV modulator ensures compatibility with a variety of television sets while maintaining high-quality signal transmission, thereby enhancing the viewing experience without the need for physical alterations to the TV receiver.If the TV receiver in question does not have a video input and its owner is reluctant to vandalise it in order to fit one then this sort of modulator is the obvious solution. It is a simple circuit that processesvideo signals to enable them to be fed straight into the TV set`s aerial input.

A `TV modulator` is really no more than a transmitter. It is a very small transmitter, admittedly, but none the less that is what it is. What does a modulator actually do In general -and this design is no exception to the rule - it is a simple oscillator that generates a frequency somewhere in the VHF or UHF region. The oscillator is modulated with the video signal and the modulated carrier wave thus generated is fed into the TV set`s aerial input via a cable.

Then all that remains to do is tune the TV to the correct frequency. A `TV modulator` is really no more than a transmitter. It is a very small transmitter, admittedly, but none the less that is what it is. What does a modulator actually do In general -and this design is no exception to the rule - it is a simple oscillator that generates a frequency somewhere in the VHF or UHF region. The oscillator is modulated with the video signal and the modulated carrier wave thus generated is fed into the TV set`s aerial input via a cable.

Then all that remains to do is tune the TV to the correct frequency The whole business is not quite as simple as we have just suggested, of course, as the mini transmitter must meet certain requirements. The frequency stability must be very good as, indeed, must the quality of the display. The required frequency stability is achieved by the use of a crystal oscillator. A well thought out choice of component values takes care of the display quality: the modulator allows a resolution of 80 characters per line, as this is a value that is often needed.

A very important feature of the circuit that must be decided is the transmission frequency. If this is only a single channel, as suggested above, h gives rise to some practical problems. Different users will want different channels, the carrier wave can become somewhat difficult to locate, and unless the frequency is exactly spot on no signal will be received. A much better idea is to ensure that the HF signal contains a large number of different frequencies. This makes it much easier to tune the TV set to one of the frequencies as there will surely be one to suit every user.

The block diagram of figure I shows how this is achieved. The TV modulator is made up of two parts, namely a modulatable crystal oscillator and a harmonics generator. The oscillator operates at a frequency of 27 MHz, which is quite low so inexpensive crystals are readily available.

The harmonics generator converts the oscillator signal into a sort of frequency spectrum containing all the multiples of 27 MHz up to about 1800 MHz. The TV modulator`s output signal is made up of a large number of little peaks, each of which is a complete transmitter signal.

At least one of these will always be in band I (VHF channels 2. . . 4), one in band III (VHF channels S. . . 12) and many of them will be in bands IV and V (UHF channels 21. . 69). Like the block diagram, the circuit (shown in figure 2) is very straightforward. The crystal oscillator is based on a very fast HF transistor, Tl (BFR91), which performs the amplitude modulation. Apart from this there is little to be said about the oscillator except, perhaps, that it is essential to use the correct values for the components surrounding Tl.

This is, of course, simply common sense in this sort of HF circuit. The harmonics generator is formed by two Schottky diodes, Dl and D2. These diodes must switch very quickly in time with the 27 MHz signal so they provide strong harmonics up into the gigahertz range. Th