Description.
Circuit shown here can be used to jam FM radios in its vicinity. The circuit is nothing but a classic single transistor oscillator operating in the VHF region. Working principle of the circuit is very simple and straight forward. Powerful VHF oscillations from the circuit will interfere with the FM signals to nullify it. Jammer circuits like this are illegal in many countries and you must assemble this circuit on your own responsibility. This circuit is intended only for fun and i request you not to misuse it.
Circuit diagram.

Notes.

• For L1 make 6 turns of 16AWG enamelled copper wire on a 9mm plastic former.
• The circuit can be powered using a 9V PP3 battery.
• For extended range, use an antenna.
• A 30cm long wire connected anywhere on the coil will do for the antenna.
• For better performance, assemble the circuit on a good PCB.

A sweep frequency generator is a special type of signal generator which generates a sinusoidal output whose frequency is automatically varied or swept between two selected frequencies. One complete cycle of the frequency variation is called a sweep. The rate at which the fre­quency is varied can be either linear or logarithmic, depending upon the design of a par­ticular instrument. However the amplitude of the signal output is designed to remain constant over the entire frequency range of the sweep.

Sweep-frequency generators are primarily employed for measurement of responses of amplifiers, filters, and electrical components over various frequency bands. The frequency range of a sweep-frequency generator usually extends over three bands, 0.001 Hz – 100 kHz (low frequency to audio), 100 kHz – 1,500 MHz (RF range), and 1-200 GHz (microwave range). Performance of measurement of bandwidth over a wide frequency range with a manually tuned oscillator is a time-consuming task. With the use of a sweep-frequency generator, a sinusoidal signal that is automatically swept between two chosen frequencies can be applied to the circuit under test and its response against frequency can be displayed on an oscil­loscope or X-Y recorder.

Thus the measurement time and effort is considerably reduced. Sweep generators may also be employed for checking and repairing ox amplifiers used in TV and radar receivers.

The block diagram of an electronically tuned sweep frequency generator is shown in the figure below.

The main component of a sweep-frequency generator is a master oscillator, usually an RF type, with several op­erating ranges which are selected by a range switch. The frequency of the output signal of the signal genera­tor may be varied either me­chanically or electronically.

In the mechanically var­ied models, the frequency of the output signal of the master oscillator is varied (tuned) by a motor driven capacitor.

In the electronically tuned models, the frequency of the master oscillator is kept fixed and a varying frequency signal is produced in another oscillator, called the voltage control­led oscillator (VCO). The VCO contains an element whose capacitance depends upon the voltage applied across it. This element is employed for varying the frequency of the sinusoidal output of the VCO. The output of the VCO is then combined with the output of the master oscillator in a special electronic device, called the mixer. The output of the mixer is sinusoidal, whose frequency depends on the difference of frequencies of the output signals of the master oscillator and VCO. For example, if the master oscillator frequency is fixed at 10.00 MHz and the variable frequency is varied between 10.01 MHz to 35 MHz, the mixer will give sinusoidal output whose frequency is swept from 10 KHz to 25 MHz.

The sweep rates of sweep frequency generators can be adjusted to vary from 100 to 0.01 seconds per sweep. A voltage varying linearly or logarithmically according to sweep rate can be used for driving the X-axis of an oscilloscope or X-Y recorder synchronously. In the elec­tronically tuned sweep generators, the same voltage which drives the VCO serves as this voltage.

The frequency of various points along the frequency-response curve can be interpolated from the values of the end frequencies if it is known how does the frequency vary (i.e., lin­early or logarithmically). For more accuracy markers* can be employed.

The signal generator, like an oscillator, is a source of sinusoidal signals but the signal generator is also capable of modulating its sinusoidal output signal with other signals. This is the main difference between the two instruments (signal generator and oscillator). When the signal generators are employed for producing an unmodulated sinusoidal output they are said to be producing CW (continuous height wave) signal. When the produced output signal is modulated, the modulating waveforms may be either externally applied sine-waves, square waves, triangular waves, pulses or more complex signals, as well as internally generated sine-waves. Amplitude modulation (AM) or frequency modulation (FM) may be used. Nor­mally amplitude (AM) modulation is employed. Principles of amplitude modulation (AM) and frequency modulation (FM) are illustrated in the figure shown below.

Signal Modulaton

Signal generators are primarily employed for providing appropriate signals for calibra­tion, testing and troubleshooting of the amplifier circuits used in communication, electron­ics such as radio and television amplifiers. They are also employed for measurement of char­acteristics of antennas and transmission lines.

Block diagram of a signal generator is shown in the figure below.

AM sigal generator -Block Diagram

An RF oscillator is employed for generating a carrier waveform whose frequency can be adjusted typically from about 100 kHz to 30 MHz. Carrier wave frequency can be varied and indicated with the help of a range selector switch and a vernier dial setting. Range is selected by employing frequency dividers. Frequency stability of oscillator is kept very high at all frequency ranges.

Following measures are taken in order to achieve stable frequency output.

• Frequency of output voltage changes with the change in supply voltage so regulated power supply is used.
• Buffer amplifiers are used to isolate the oscillator circuit from output circuit so that any change in the circuit connected to the output does not affect the frequency and amplitude of the oscillator output.
• Temperature also causes change in oscillator frequency, so temperature compen­sating devices are used.
• Q-factor of L-C circuit should be very high, say above 20,000. This can be achieved by employing quartz crystal oscillator in place of L-C oscillator.

An audio-frequency modulating signal is generated in another very stable oscillator, called the modulation oscillator. Provision is made in the modulation oscillator for changing the frequency and the amplitude of the signal being generated.

In this oscillator provision is also made to get various types of waveforms such as the square, triangular waves or pulses. The radio-frequency and the modulation-frequency sig­nals are fed to a wide-band amplifier, called the output amplifier. Percentage of modulation ₍ can also be adjusted and it is indicated by the meter.

Modulation level can be adjusted upto 95% by a control device. The output of the am­plifier is then fed to an attenuator and finally the signal goes to output of signal generator. Output meter is provided to read the final output signal.

The accuracy to which the frequency of the RF oscillator is known is an important specification of the signal generator performance. Most laboratory type models are usually calibrated to be within 0.5 – 1.0% of the dial setting. This accuracy is usually sufficient for most measurements. For greater accuracy, if needed, a crystal oscillator, whose frequency is known to be within 0.01% or better, may be used as an internal RF calibration source.

Another key specification of signal generators is their amplitude stability. It is very important that the amplitude of the output signal remains constant as the RF frequency is varied.

While selecting an oscillator for a particular application, the following particulars are to be considered.

1. Frequency Range. The oscillator selected for a particular application should be ca­pable of supplying an output signal that’s upper and lower frequency limits exceed those required by the application.
2. Power and/or Voltage. The oscillator selected for a particular application should be capable of generating the pertinent quantity with a magnitude large enough to meet the requirement.
3. Accuracy and Dial Resolution. The accuracy of an oscillator specifies how closely the output frequency corresponds to the frequency indicated on the dial of the in­strument. Dial resolution indicates to what percentage of the output frequency value the dial setting can be read.
4. Amplitude and Frequency Stability. The amplitude stability is a measure of an oscillator’s ability of maintaining constant voltage amplitude with variations in the output signal frequency. Frequency stability determines how closely the oscillator maintains a constant fre­quency over a given time period. Sometimes the frequency stability is included in the accuracy specifications of the oscillator.
5. Waveform Distortion. This quantity is a measure of how closely the output wave­form of the oscillator resembles a pure sinusoidal signal. Sometimes the oscillator is employed as a source in a test used for measuring the tendency of a circuit to distort a sinusoidal signal. In such tests, the distortion caused by the oscillator should be much less than the anticipated distortion because of the circuit under test.
6. Output Impedance. The output impedance of an oscillator specifies the impedance value of the load which must be connected to it for maximum power transfer. It is very important that the output impedance of the oscillator be equal to the char­acteristic impedance of the system to which it is to be connected.

Negative resistance oscillators make use of negative resistance elements such as tetrodes, tunnel diodes, uni junction transistors etc. There are two types of negative resistance oscillators, which are commonly used for high frequency generation. These are dynatron and tunnel diode oscil­lators.

Dynatron operates in the negative resistance region of the characteristics of a tetrode, which is coupled to an L-C tank circuit.

Tunnel diode oscillator makes use of a tunnel diode for producing oscillations.

Tunnel Diode Oscillator Characteristics

Now consider a circuit shown in the figure (a) shown below where D is a tunnel diode, R is an external resistance, V is the battery voltage, V_D is voltage drop across diode and V_R is voltage drop across resistance R. The value of R is so selected as to bias the diode D in the negative resistance region AB. The working or quiescent point Q is almost at the centre of the characteristic curve AB.

When the switch S is closed, the current immediately rises to a value determined by R and the diode resistance which are in series. The applied voltage V divides across D and R according to the ratio of their resistances. However, as diode voltage V_D exceeds V_p (point A), diode is driven into the negative resistance region and its resistance starts increasing. So V_D increases further till it becomes equal to valley voltage V_v (point B). At this point further increase in V_D drives the diode into the positive resistance re­gion BC [figure (b)]. Now increase in current causes increase in V_R and de­crease in V_D, thereby bringing the diode back into the negative resistance region. This reduction in diode voltage V_D causes an increase in circuit current till point A is reached when V_D equals V_p. Thus the circuit will continue to oscillate back and forth through the negative resistance region, that is between points A and B on its characteristic. Its output across external resistance R is sinusoidal. A practical circuit of a tunnel diode oscillator in two slightly dif­ferent ways is shown in the figure shown below. Here R₂ sets the proper bias level for the diode whereas R_x in parallel with the L-C tank circuit sets proper current level for it. The capacitor C_c is the coupling capacitor. As the switch is closed, the tunnel diode is set into oscillations whose frequency is equal to the resonant frequency of the tank circuit.

Tunnel Diode Oscillator Circuit

The tunnel diode operates very fast and it is possible to make tunnel diode oscillators which operate in the microwave frequency region.