Wiring & diagram Info: vhf

Thursday, November 13, 2014

VHF UHF TV Modulator

The electronic circuit is a TV modulator that 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 sets aerial input via a cable. Then all that remains to do is tune the TV to the correct frequency.

Circuit Schematic
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. The modulation depth can be set with Pl, while the oscillators d.c. value can be varied by means of P2. The combination of these two presets enables either positive or negative amplitude modulation to be selected.

This is essential as the harmonics produced vary in this respect. We will discuss the calibration of Pl and P2 later in this article. The power for the circuit can be provided by either an unstabilized 8...30 V or a stabilized 5 V. The latter could be taken from a computers power supply and in this case ICI is not needed.

The printed circuit board for the modulator is only single-sided. The largecopper surface acts as a ground plain.

Parts list
Resistors:
R1, R2 = 4k7
R3, R4 = 56ohm
P1 = 100 ohm preset
P2 = 500 ohm preset
Capacitors:
C1 = 4mf7/16 V
C2= 10p
C3 = 220p
C4 = 47p
C5 = 47n, ceramic
C6 = 100n*
C7 = 330n*
Inductors:
L1, L2 = 3.5 turns of 0.2 mm (SWG 35 or 36) CuL on a ferrite bead of about 3.5 x 3.5 mm
L3 = 1 microH
L4 = 1 turn of 0.8. . .1 mm (SWG 19...21) CuL, air wound with a diameter of 8 mm
Semiconductors:
D1, D2 = 1N6263 (Ambit/Cirkit)
D3 = lN4148
T1 = BFR91 (Ambit/Cirkit)
IC1 = 7805*
Miscellaneous:
X1 = crystal, 27 MHzd(3rd overtone) or other 3rd overtone crystal between 25 and 30 MHz

*= not needed if the circuit is powered from a stabilised 5 V supply

Wednesday, November 12, 2014

VHF RF Single Chip Preamplifier circuit with explanation

Here is a high performance RF amplifier for the entire VHF broadcast and PMR band (100-175 MHz) which can be successfully built without any special test equipment. The grounded-gate configuration is inherently stable without any neutralization if appropriate PCB layout techniques are employed. The performance of the amplifier is quite good.
The noise figure is below 2 dB and the gain is over 13 dB. The low noise figure and good gain will help car radios or home stereo receivers pick up the lower-power local or campus radio stations, or distant amateur VHF stations in the 2-metres band. Due to the so-called threshold effect, FM receivers loose signals abruptly so if your favourite station fades in and out as you drive, this amplifier can cause a dramatic improvement.
The MAX2633 is a low-voltage, low-noise amplifier for use from VHF to SHF frequencies. Operating from a single +2.7 V to +5.5V supply, it has a virtually flat gain response to 900 MHz. Its low noise figure and low supply current makes it ideal for RF receive, buffer and transmit applications. The MAX2633 is biased internally and has a user-selectable supply current, which can be adjusted by adding a single external resistor (here, R1). This circuit draws only 3 mA current.

VHF RF preamplifier circuit diagram

Besides a single bias resistor, the only external components needed for the MAX2630 family of RF amplifiers are input and output blocking capacitors, C1 and C3, and a VCC bypass capacitor, C2. The coupling capacitors must be large enough to contribute negligible reactance in a 50-Ω system at the lowest operating frequency. Use the following equation to calculate their minimum value:
Cc = 53000/ flow [pF]

Thursday, October 16, 2014

Simple 30 Watt VHF Amplifier by using 2SC1946A

The 30 watt amplifier schematic shown below provides an appropriate power boost with an input of 4 watt up to 6 watts. The circuit is designed to cover 88-108MHz FM Broadcast Band. However, the circuit is very stable at my place and provides a clean-output through seven (7) element Butter-worth low-pass filter.

Circuit Diagram:

Notes:
The heart of the circuit is 2SC1946A VHF RF power transistor. The transistor is specifically designed for operation in frequencies up to 175 MHz, with very good results. As you can see, the power line is well decoupled. The amplifier current can be over 5 amps. All the coils are made from 16gauge laminated wire (or Silver copper wire can do best) and the RFC can be of HF toroid core (as shown in the picture) or 6 holes ferrite bead.C3 and R1 forms snubber circuit while R2 and C6 prevent the amplifier from self-oscillation at VHF, sometimes you need to add 180 ohms in parallel with L7.That will cause the amplifier to dissipate UNDESIRABLE VHF thereby reducing spurious level.

The photo below is 60Watts VHF power amplifier using the above circuit. Two of 2SC1946A transistors are arranged at 90 degrees to each other and their outputs are combined using "Power Combiner Network”. It is quite difficult to combine powers at VHF and UHF bands.

However, I recommend that hobbies should stick to single power design due to its complicity and large rate of INTERFERENCE. (in attempt to go for double transistors which involves power combiner network). Since the two amplifiers are operating in different phase (out of phase).

Tuning:
Tuning of the amplifier is not hard at all. You just have to connect the output to a good antenna with a transmission line (RG214) of 50 ohms. First match the output network, and then do the same to the input network for a maximum power output. By way of adjustment, you can increase the output at its operating frequency.

Saturday, September 13, 2014

Simple TV Transmitter circuit diagram VHF

Most of people ask TV transmitters.So Today Im going to give you a very useful schema diagram.By using this schema you can send your signals 75m to 100m.This schema diagram is not my own schema one of my friends gave me this.I suppose you guys also can send your own schema diagrams for us.Then we can publish them through our website.Here They have used common transistor BC 108 If you are unable to find this transistor you can use equal transistors like Bc337 2n2222 Bc 546

Note

# To make L1 wound 6 turns of #24 enameled wire on a 10mm air former for frequency 60 - 80 MHz For 150 - 180 MHz wound 4 turns and for 180 - 200MHz wound 2 turns.

# This schema operates with 9V power supply

# When you build this schema you have to build this on a little space.

# The power of this schema is 80mW

# use a pcb to build this schema diagram.

# This is only for educational purpose.We can bear any responsibility of this If you misused this.

# This schema transmit only video signals .If you want to send sound signals you can use an audio transmitter

Thursday, August 21, 2014

Build a VHF UHF Low Noise Amplifiers using MAX2664 MAX2665

A very simple VHF UHF Low-Noise amplifiers schema can be designed using the MAX2664 and MAX2665 ultra-compact LNAs for VHF UHF applications.These devices incorporate a broadband LNA with an integrated bypass switch. The MAX2664 covers the UHF frequency range from 470MHz to 860MHz, and the MAX2665 covers the VHF frequency range from 75MHz to 230MHz. Each device has a zero-power bypass mode for improved high-signal-level handling conditions.

VHF UHF Low-Noise Amplifiers using MAX2664 MAX2665

Both ICs has a high gain around 15dB and require a single power supply , that can provide an output voltage between 2.4 to 3.5 volts .VHF UHF Low-Noise amplifiers has a very low current consumption of 3.3 mA and can be used in applications like : Smartphones/Handsets , MP3 Players , Home Audio/Video and other portable navigation devices .

Thursday, August 7, 2014

TV Transmitter circuit diagram VHF

Most of people ask TV transmitters.So Today Im going to give you a very useful circuit diagram.By using this circuit you can send your signals 75m to 100m.This circuit diagram is not my own circuit one of my friends gave me this.I suppose you guys also can send your own circuit diagrams for us.Then we can publish them through our website.Here They have used common transistor BC 108 If you are unable to find this transistor you can use equal transistors like Bc337 2n2222 Bc 546

Note

# To make L1 wound 6 turns of #24 enameled wire on a 10mm air former for frequency 60 - 80 MHz For 150 - 180 MHz wound 4 turns and for 180 - 200MHz wound 2 turns.

# This circuit operates with 9V power supply

# When you build this circuit you have to build this on a little space.

# The power of this circuit is 80mW

# use a pcb to build this circuit diagram.

# This is only for educational purpose.We can bear any responsibility of this If you misused this.

# This circuit transmit only video signals .If you want to send sound signals you can use an audio transmitter