Local Oscillator : Block Diagram, Circuit, Working & Its Applications

An oscillator is an electronic or mechanical device used to generate an oscillating or periodic electronic signal, frequently a sine wave, Generally, an oscillator converts DC from a power supply to an AC signal. So, these are applicable to a wide range of electronic devices which range from simple CLK generators to digital devices, complex computers, etc. There are different types of oscillators available that are used based on the requirement like Harmonic, Tuned Circuit, RC Crystal, etc. So this article discusses one of the types of oscillators like a local oscillator – working with applications.

What is a Local Oscillator?

A local oscillator is one type of oscillator which is used to modify the signal frequency with a mixer in a receiver. This signal frequency modification procedure also called heterodyning generates the sum & difference frequencies from the oscillator’s frequency & input signal’s frequency. In various receivers, this oscillator & mixer functions are combined within a single stage known as a converter which reduces the power consumption, cost & space. A local oscillator generates a sinusoidal signal including a frequency so that the receiver is capable of generating the accurate intermediate frequency or resulting frequency for further amplification as well as conversion into audio detection.

Local Oscillator Working

The local oscillator working with a mixer in a superheterodyne radio receiver is shown below. Generally, a superheterodyne radio receiver mixes the frequency of the received signal with the generated signal’s frequency through a local oscillator.

First, the receiver receives the signals from the antenna. After that, these signals are fed to the RF Amplifier. In this amplifier, the signals are tuned for removing unwanted signals from other frequencies.
From the RF amplifier, the tuned signals mix with the generated incoming local frequency signals from a local oscillator. This mixing procedure can be done within the mixer & it creates an IF (intermediate frequency).

The IF formed by the mixing is more suitable for processing than the original carrier frequency.
After that, the intermediate frequency is amplified & filtered. So this amplitude is simply maintained through a limiter. So throughout filtering, a particular channel’s signals can be selected. As compared to RF filtering, the IF filter can be tuned well than the RF filter because it is mainly designed for a fixed frequency.

After that, this signal is given to a demodulator which is also known as an FM detector. So this detector simply demodulates the output. So it is also achievable to switch between different demodulators to attain the preferred form of output.

After that, this demodulated signal is amplified with a loudspeaker where it changes to sound signals with audible frequency.

Thus, the superheterodyne FM receiver’s specialty is to mix the original incoming frequency from a source with generated frequency, consequently, this allows the receiver to filter & choose only the preferred RF signals.

Local Oscillator Circuit Diagram

Here, we are going to explain the local oscillator working in the superheterodyne receiver. The circuit diagram of a superheterodyne receiver using a local oscillator is shown below.

A heterodyne receiver is an electronic circuit that transmits a signal from one carrier signal to another carrier signal through a different frequency. It mixes the i/p signal with a generated wave through an oscillator to generate two new signals which are known as beats. Heterodyning is an easy procedure that is governed by the trigonometry laws, most heterodynes are very complex devices with several amplifiers & filters.

Here, a beat is a signal generated by two i/pt signals by different frequencies. Generally, a heterodyne receiver generates two beats, where one beat has a frequency that is the amount of the mixed frequencies, whereas the other beat has a frequency that is the variation between the mixed frequencies. So, for instance, an i/p signal including a 10MHz carrier wave is mixed by a 15MHz carrier signal to make two o/p beats. The higher beat has a 25MhHz frequency & the lower beat has a 5MHz frequency.

The superheterodyne receiver uses the principle of heterodyne to permit high-frequency signals to be identified through low-frequency receivers. Once a signal comes into a superheterodyne receiver, then it is simply amplified & mixed by the local oscillator signal before it is filtered to generate an IF (intermediate frequency). Usually, it is again amplified & filtered before reaching the output. The receiver is tuning by changing the oscillator wave frequency.

There are many local oscillators that are widely used in radio receivers are; the Hartley oscillator, the Tuned collector oscillator, and the crystal oscillator.

Please refer to this link to know more about the Hartley oscillator.
Please refer to this link to know more about the Tuned collector oscillator.
Please refer to this link to know more about the crystal oscillator.

Local Oscillator Frequency Formula

In the local oscillator, when the mixer generates both the sum & difference frequencies, it is achievable to produce the 455 kHz IF signal if the oscillator is either below or above the IF.

Case1:

When the local oscillator is above the IF, then it needs to tune from approximately 1 to 2 MHz. Normally, it is the capacitor within a tuned RLC circuit, which is changed to regulate the center frequency when the inductor is fixed.

Since fc = 1/2π√LC

By solving C = 1/L(2πfc)^2

Once the tuning frequency is highest, then the tuning capacitor is minimum. When we know the frequencies range to be created, we can deduce the required capacitance range.

Cmax/Cmin = L(2πfmax)^2/ L(2πfmin)^2

= L(2MHz)^2/ L(2πfmin)^2

= (2MHz/1MHz)^2 = 4

Case2:

When the local Oscillator is below the IF, then the oscillator needs to tune approximately from 45 kHz to 1145 kHz. So,

Cmax/Cmin = (1145kHz/45kHz)^2 = 648.

With this type of range, it is not practical to make a tunable capacitor. Thus, the oscillator in a normal AM receiver is over the radio band.

Why Local Oscillators are used?

These oscillators are used for changing a signal frequency with a mixer in a receiver.

Why Local Oscillator Frequency is Higher?

The oscillator frequency is higher always as compared to signal frequency because a higher frequency is normally preferred in a super heterodyning receiver to leave more distance in between the difference between otherwise intermediate frequency & other two frequencies so that the intermediate frequency signal is more simply passed throughout a filter & the original two signals will be attenuated.

Advantages

The advantages of a local oscillator include the following.

• The local oscillator in a radio communication system is the main phase noise source.
• In radio receivers, the functions of both the combined local oscillator & mixer within a single active device reduce the price, space & consumption of power consumption.
• This oscillator processes a signal at a fixed frequency to improve a radio receiver’s performance.

Applications

The applications of local oscillators include the following.

• Local oscillators are utilized in many communications circuits like cable television set-top boxes, modems, telemetry systems, microwave relay systems, frequency division multiplexing systems utilized in telephone trunklines, radio telescopes, atomic clocks, and military electronic countermeasure systems.
• These are used in superheterodyne receivers & radio communication systems.
• These oscillators are necessary whenever heterodyning is used in receiver architectures to change
• HF signals to an IF spectrum for easy processing.
• The microwave frequencies in satellite television reception are used from the satellite down to the receiving antenna to convert into lower frequencies through an oscillator & mixer by mounting at the antenna.

Thus, this is an overview of a local oscillator – working with applications. This oscillator plays a key role in the FM receiver. It is the most significant circuit within the whole receiver because any instability or drift within the oscillator will convert into drift &instability within the received signal. Here is a question for you, which type of oscillator is used as a local oscillator?