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Thursday, 23 May 2019

Audio Frequency Oscillator Working

AF OSCILLATOR

The block diagram of an Audio Frequency Oscillator is shown in figure. All the blocks have been labelled
Block Diagram of an AF Oscillator

Description of the Block Diagram and Audio Frequency Oscillator Working  

The first block is that of the sine wave oscillator. These oscillators are of the Wien bridge or phase shift type. They produce pure wave form. However when the output voltage required is large, beat frequency oscillator is preferred to generate the audio signal. The design and use of the beat frequency oscillator is complex. Cost wise also the beat frequency oscillator is uneconomical. Most of the audio signal generators use Wien bridge oscillator. The oscillator will be provided with a frequency range selection switch for coarse frequency change. A fine frequency control is also provided. The output of the sine wave oscillator goes to the amplifier.

The amplifier amplifies the signal generated by the oscillator. The amplifier is provided with negative feedback to maintain stability of operation. A voltage follower circuit is used to provide a very high input impedance, very low input current, very low output impedance and unity gain for the amplifier. The voltage follower will be the final stage in the amplifier section.

The amplifier's output goes to the attenuator through a switch. The switch selects the sine wave output direct from the output of the amplifier. Setting the switch in to position will connect the output of the wave shaping network to the attenuator.  The wave shaping network is supplied from the output of the amplifier. The sine wave is shaped to wave a square wave in the output of the wave shaping network. 
The attenuator has a coarse adjustment and fine adjustment. The attenuator is calibrated to indicate the output voltage from the audio oscillator. 
The frequency counter and its display shown in the block diagram are optional. The present day signal generators use frequency counters in them for the correct display of frequency. The frequency counters frequency range switch and the oscillator's range switch are gauged to get the correct display of frequency. When frequency counter is used to display the frequency of the oscillator, the dial need not be very accurately calibrated. 

Specifications of an Audio Oscillator 

The following are the specifications of an AF oscillator

1. Frequency range : It specifies the range in which the instrument has to generate the signal. A range of 10 Hz to 50 kHz is generally specified.

2. The output power or voltage : The output voltage is specified. For general purpose application 5 V, is sufficient. For better work the range can be up to 20V.

3. The output impedance : Two standard impedances are specified, one is 60M and the other is 100 Ω. Both the outputs are to be provided over the output sockets. The 600Ω impedance is for matching, with general transmission lines. The 100Ω impedance is for the low impedance test work.

4. Dial resolution and accuracy: Precise calibration of dial is essential. For accurate setting of the frequency a vernier is to be provided. Provision of a frequency counter is the best solution for tuning problems. The accuracy of the dial is important to have accurate setting of frequency.

5. Frequency stability: The frequency of the oscillator must be stable for long periods. Hence the long term stability is important.

6. Amplitude stability: The amplitude of the signal must be maintained constant throughout its operation.

7. Distortion: Distortion in the output of the signal generator is to be avoided. When the distortion is present, presence of harmonics will lead to spurious results in signal analysis and measurements. 

 Typical Specifications of an Audio Oscillator 

Power Supply : 230V ±10 % 50 Hz, A.C. Mains.
Frequency : 15 Hz to 150 kHz in five ranges.
Accuracy : ± 3% ( +0.15 Hz)
Stability : i. Less than ± 0.05% short term drift after initial 1 minute.
                ii. Change of frequency with temperature is typically less than ± 0.1% per degree.
Output Impedance :  i. 600 f with attenuator ± 2%
Output Voltage : Continuously variable up to 2.5 V.

It has been stated that the range of an audio oscillator is from 10 Hz to about 50 kHz in above paragraphs. The actual range of their audio oscillator depends on the manufacturer. It should cover the audio frequency range. That is the main requirement. 

 Audio Frequency Oscillator - A.M. Standard Signal Generator a Comparison

1. The audio oscillator produces sinusoidal signals or if required square wave voltages, with in audio frequency range and may have its range from 10 Hz to around 100 kHz. Though the frequency range at the upper end is in the low R.F. range it is not a modulated signal.
The standard signal generator in turn is basically an R.F oscillator. It has its frequency range from 100 kHz to around 80 MHz. It produces sinusoidal voltages in this frequency range under continuous wave (carrier wave) mode. Normally its output is a modulated signal. The R.F. signal will be amplitude modulated by an audio signal of 400 Hz or 1 kHz.

2. An audio oscillator has no provision for modulation. Its use is limited to audio frequency applications. The standard signal generator has provision for modulation. Modulation can be effected using internal audio frequency generated by an audio frequency oscillator. There is also provision for external modulation using an audio oscillator externally.

3. The output voltage of an audio oscillator can he from around 2 V to as large as 100 V, in special types. The Standard signal generator has its modulated R.F. output voltage restricted to a maximum value of around 2 V.

4. In a standard signal generator a socket is provided to take out the single frequency (fixed frequency) audio signal with or without attenuation. Such a facility is useful in trouble shooting audio circuits when a separate audio oscillator is not available.

5. Provisions like dummy antenna, S meter, modulation switch are not necessary in audio oscillators. They are essential in standard signal generator.

6. The shielding requirements of R.F.signal generators are critical. Effective shielding is necessary to prevent interference of radio frequency signals with the neighboring circuitry. Audio oscillators have less rigid shielding requirements.

A table showing the comparison between an audio oscillator and Standard signal generator is given below

Parameter
Audio Oscillator
A.M Standard Signal Generator
Frequency Range
10 Hz to 50 kHz
100 kH to 80 MHz (Modulated)
Wave form
Sine and Square
Audio Modulated Sine
Provision for Modulation
No need
Yes by audio Fixed frequency or by external audio signal
Output Voltage
2 V to 100 V
2V
Output attenuator
Yes coarse and fine attenuators
Yes Coarse & fine attenuator for both audio & R.F. output
Output impedance :
600 Ω & 1000 Ω
600 Ω with dummy antenna
Dial facilities
Yes generally without vernier scale

Currently digital frequency counter is provided to show output   frequency
Yes with precision vernier

Currently digital frequency counter is provided to show the output frequency
Output indicator
Usually no
Panel meter with switch to indicate both the R.F. and A.F. outputs to set correct  % modulation required
External inputs
No external input
Provision for External A.F. input.
Additional features
No
Dummy antenna, Modulation switch, A.F. output socket
Provision for calibration
No
Yes using internal crystal oscillator
Shielding
Simple shielding
Critical Shielding is to he provided to prevent R.F. radiation
Power supply
230 V ± 10%
50 Hz 50 W
230 V ±10 %
50 Hz 50W

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