Vhf-tone-transmitter
The range of this transmitter is approximately 50 feet with a short whip antenna. The tone generator consists of a unijunction transistor (Q1) along with resistors R1, R2, R3, and capacitor C2. Transistor Q1 operates by pulsing on and off at a rate determined by the time constant of resistors R1 and R2, in conjunction with the capacitance of C2 and the base-emitter junction of Q1. The trimmer potentiometer R2 adjusts the frequency of the generated tone, allowing a range from approximately 100 Hz to over 5 kHz. Transistor Q2 functions as the RF oscillator, with its frequency set by tuned circuits composed of inductor L1, capacitors C5 and C6, and the interelectrode capacitance of Q2. The specified values provide a tuning range of about 55 to 108 MHz. Capacitor C6 facilitates positive feedback from the emitter to the collector of Q2, enabling oscillation. The audio tone generated by Q1 is fed to the base of Q2, causing the collector current to vary at the frequency of the tone, resulting in an amplitude-modulated (AM) signal. This modulation, in turn, alters Q2's collector-to-emitter capacitance, which is part of the tuned circuit, thereby causing the output frequency to vary and producing a frequency-modulated (FM) signal as well. The RF signal is coupled to the antenna through capacitor C7. Coil L1 is constructed with five turns of #18 bare wire, closely wound around a piece of 1/4-inch wooden dowel, with a winding length of about 1 inch. One end of capacitor C7 is soldered to the coil, one turn away from the 9-V supply end, while the other end is connected to the antenna. To adjust the vertical height and linearity of a TV set, the tone transmitter should be positioned near the set, and resistor R2 can be used to select the number of horizontal bars displayed. Once the picture stabilizes and the bars appear sharp, the vertical controls of the TV set should be adjusted to ensure all bars are of the same height and evenly spaced. The dual generation of AM and FM signals enables this circuit to be utilized for testing nearly any receiver within the transmitter’s frequency range. A TV set’s sound section (discriminator) will filter out the AM portion of the signal, while its video section will respond to it. Likewise, the TV sound section and FM receivers will react to the FM signal produced.
The transmitter circuit is designed to operate within specific parameters to achieve both tone generation and RF modulation. The unijunction transistor Q1 serves a crucial role in generating audio tones by rapidly switching on and off, influenced by the RC time constants set by R1, R2, and C2. The adjustment of R2 not only alters the frequency of the output tone but also allows for fine-tuning of the circuit's response in practical applications, such as aligning TV sets.
Transistor Q2, as an RF oscillator, utilizes the tuned circuit formed by L1, C5, and C6 to produce a stable RF output. The positive feedback provided by C6 is essential for sustaining oscillation, while the interaction between the audio signal from Q1 and the RF signal from Q2 creates a complex modulation scheme. This dual modulation allows for versatile testing capabilities across different types of receivers, particularly in distinguishing between AM and FM responses.
The construction of coil L1 is critical for achieving the desired inductance and tuning characteristics. The choice of wire gauge and the number of turns directly impact the coil's performance, influencing both the RF output and the overall efficiency of the transmitter. The physical layout, including the connection of C7 to the antenna, ensures effective coupling of the RF signal for transmission.
In practical applications, the circuit can be employed not only for testing television sets but also for other RF devices within the designated frequency range. The ability to generate both AM and FM signals makes this transmitter a valuable tool for engineers and technicians in the field of electronics, facilitating the alignment and troubleshooting of various audio and video equipment.The range of this transmitter is about 50 feet with a short whip antenna. The tone generator, is made up of a unijunction transistor, Q1, and R1, R2, R3, and C2. Transistor Q1 pulses on and off at a rate determined by the time constant of R1 and R2, together with the capacitance of C2 and the B1-emitter junction of QL Trimmer potentiometer R2 determines the frequency of the tone generated and allows a range of approximately 100 Hz to over 5 kHz. Transistor Q2 is the rf oscillator. Its frequency is set by tuned circuits consisting of Ll, C5, C6, and the interelectrode capacitance of Q2. The values shown will give a tuning range of about 55 to 108 MHz. Capacitor C6 provides positive feedback from the emitter to the collector of Q2, for oscillation. The· audio tone generated by Q1 is applied to the base of Q2, causing the collector current to vary at the frequency of the tone, yielding an amplitude-modulated (AM) signaL This, in turn, varies Q2"s collector-to-emitter capacitance, which makes up part of the tuned circuit, and causes the output frequency to vary similarly, producing a frequency-modulated (FM) signal, as well.
The rf signal is coupled to the antenna through capacitor C 7. Coil L1 consists of five turns of #18 bare wire, dose-wound on a piece of "/.-inch wooden doweL The length of the winding is about "I. inch. One end of capacitor C7 is soldered to the coil, one tum away from the 9-V supply end, and the other capacitor end is connected to the antenna.
To adjust the vertical height and linearity of a TV set, place the tone transmitter near the set and use R2 to select the number of horizontal bars to be displayed. Once the picture is steady and the bars are sharp, adjust the set"s vertical controls, so that all the bars are of the same height and are evenly spaced.
The fact that both AM and FM signals are generated makes it possible to use this circuit to check almost any receiver within the transmitter"s frequency range. A TV set"s sound section (discriminator) will reject the AM portion of the signal, but its video section will respond to it.
Similarly, the TV sound section and FM receivers will respond to the FM signal produced.
The transmitter circuit is designed to operate within specific parameters to achieve both tone generation and RF modulation. The unijunction transistor Q1 serves a crucial role in generating audio tones by rapidly switching on and off, influenced by the RC time constants set by R1, R2, and C2. The adjustment of R2 not only alters the frequency of the output tone but also allows for fine-tuning of the circuit's response in practical applications, such as aligning TV sets.
Transistor Q2, as an RF oscillator, utilizes the tuned circuit formed by L1, C5, and C6 to produce a stable RF output. The positive feedback provided by C6 is essential for sustaining oscillation, while the interaction between the audio signal from Q1 and the RF signal from Q2 creates a complex modulation scheme. This dual modulation allows for versatile testing capabilities across different types of receivers, particularly in distinguishing between AM and FM responses.
The construction of coil L1 is critical for achieving the desired inductance and tuning characteristics. The choice of wire gauge and the number of turns directly impact the coil's performance, influencing both the RF output and the overall efficiency of the transmitter. The physical layout, including the connection of C7 to the antenna, ensures effective coupling of the RF signal for transmission.
In practical applications, the circuit can be employed not only for testing television sets but also for other RF devices within the designated frequency range. The ability to generate both AM and FM signals makes this transmitter a valuable tool for engineers and technicians in the field of electronics, facilitating the alignment and troubleshooting of various audio and video equipment.The range of this transmitter is about 50 feet with a short whip antenna. The tone generator, is made up of a unijunction transistor, Q1, and R1, R2, R3, and C2. Transistor Q1 pulses on and off at a rate determined by the time constant of R1 and R2, together with the capacitance of C2 and the B1-emitter junction of QL Trimmer potentiometer R2 determines the frequency of the tone generated and allows a range of approximately 100 Hz to over 5 kHz. Transistor Q2 is the rf oscillator. Its frequency is set by tuned circuits consisting of Ll, C5, C6, and the interelectrode capacitance of Q2. The values shown will give a tuning range of about 55 to 108 MHz. Capacitor C6 provides positive feedback from the emitter to the collector of Q2, for oscillation. The· audio tone generated by Q1 is applied to the base of Q2, causing the collector current to vary at the frequency of the tone, yielding an amplitude-modulated (AM) signaL This, in turn, varies Q2"s collector-to-emitter capacitance, which makes up part of the tuned circuit, and causes the output frequency to vary similarly, producing a frequency-modulated (FM) signal, as well.
The rf signal is coupled to the antenna through capacitor C 7. Coil L1 consists of five turns of #18 bare wire, dose-wound on a piece of "/.-inch wooden doweL The length of the winding is about "I. inch. One end of capacitor C7 is soldered to the coil, one tum away from the 9-V supply end, and the other capacitor end is connected to the antenna.
To adjust the vertical height and linearity of a TV set, place the tone transmitter near the set and use R2 to select the number of horizontal bars to be displayed. Once the picture is steady and the bars are sharp, adjust the set"s vertical controls, so that all the bars are of the same height and are evenly spaced.
The fact that both AM and FM signals are generated makes it possible to use this circuit to check almost any receiver within the transmitter"s frequency range. A TV set"s sound section (discriminator) will reject the AM portion of the signal, but its video section will respond to it.
Similarly, the TV sound section and FM receivers will respond to the FM signal produced.