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A REPORT

ON

DESIGN OF AM RECEIVER

BY

Ashay Vedasan 2007a3ps149

Ravi Ranjan 2006B4A3728

Mitesh Kumar 2006B5A3650

Shubham Rai 2006B5A3539

PREPARED IN FULFILLMENT OF THE COURSE

EEE C383: COMMUNICATION SYSTEM

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE

PILANI, RAJASTHAN

23rd

APRIL 2010

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Description

This is a compact three transistor, regenerative receiver with fixed feedback.. The design is

simple and sensitivity and selectivity of the receiver are good. Regenerative circuits wereemployed in early radio receivers and are still built by electronics experimenters and students. A

regenerative circuit is often an AM detector, converting the RF signal on the antenna to an audiowaveform. Their use ofpositive feedbackgreatly increases both the selectivity and sensitivity of

a simple receiver. Positive feedback builds up the input signal to very high levels.

Circuit NotesAll general purpose transistors should work in this circuit. The tuned circuit is designed formedium wave, but the circuit will work up to much higher frequencies if a different tuning coil

and capacitor are used. Here a ferrite rod with loops around it and a gang capacitor can be used.Q1 and Q2 form a compound transistor pair featuring high gain and very high input impedance.

This is necessary so as not to unduly load the tank circuit. Q1 operates in emitter follower, Q2common emitter, self stabilizing bias is via the 120k resistor and the tuning coil. As Q2 operates

in common emitter its base voltage will be a Vbe drop higher than ground or about 0.71V. Thevoltage at Q1 base will be this Vbe drop plus the voltage drop across the 1k resistor and Q1's

own Vbe drop, from base to ground. For audio amplifiers, Q2 collector would be biased near halfsupply voltage, however the input signal levels at RF are tiny, appearing across the coil being

amplified by Q2 and being about 5mV RF across the 2k2 load resistor.

http://en.wikipedia.org/wiki/Amplitude_modulationhttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Antenna_%28radio%29http://en.wikipedia.org/wiki/Sound_recording_and_reproductionhttp://en.wikipedia.org/wiki/Positive_feedbackhttp://en.wikipedia.org/wiki/Electronic_selectivityhttp://en.wikipedia.org/wiki/Electronic_selectivityhttp://en.wikipedia.org/wiki/Positive_feedbackhttp://en.wikipedia.org/wiki/Sound_recording_and_reproductionhttp://en.wikipedia.org/wiki/Antenna_%28radio%29http://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Amplitude_modulation

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The 120k resistor provides regenerative feedback,between Q2 output and the tank circuit inputand its value affects the overall performance of the whole circuit. . Regenerative circuits wereemployed in early radio receivers and are still built by electronics experimenters and

students. A regenerative circuit is often an AM detector, converting the RF signal on the

antenna to an audio waveform. Their use of positive feedback greatly increases both the

selectivity and sensitivity of a simple receiver. Positive feedback builds up the input signal tovery high levels Too much feedback and the circuit will become unstable producing a "howlingsound". Insufficient feedback and the receiver becomes "deaf". If there is a lack of sensitivity,

then try increasing R1 to around 150k. R1 could also be replaced by a fixed resistor say 33k anda preset resistor of 100k. This will give adjustment of sensitivity and selectivity of the receiver.

Transistor Q3 has a dual purpose; it performs demodulation of the RF carrier whilst at the same

time, amplifying the audio signal. Audio level varies on the strength of the received station but Ihad typically 10-40 mV, this is audio voltage, not RF signal level. This will directly drive high

impedance headphones or can be fed into a suitable amplifier.

Antenna and Tuning Coil

The tuning coil, can be created by winding copper coils of around 50-60 turns around the ferrite

rod. This will create a tuning inductor. AM stations are directional so rotating the rod (or wholereceiver) should allow nulling of some signals whilst boosting others.

If you are in an area of weak reception then an external antenna may be required. Use severalfeet of flexible wire as an antenna.

Tuning Range

The frequency coverage or tuning range is controlled by L1 and VC1. If VC1 is fully meshed

(closed) then its capacitance will be about 500pF. The resonant frequency is given by:

where F is frequency in hertz, C capacitance in Farads and L the inductance in Henry's. With ameshed 500pF variable capacitor and 200uH coil the lowest frequency works out to be:

http://en.wikipedia.org/wiki/Amplitude_modulationhttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Antenna_%28radio%29http://en.wikipedia.org/wiki/Sound_recording_and_reproductionhttp://en.wikipedia.org/wiki/Positive_feedbackhttp://en.wikipedia.org/wiki/Electronic_selectivityhttp://en.wikipedia.org/wiki/Electronic_selectivityhttp://en.wikipedia.org/wiki/Positive_feedbackhttp://en.wikipedia.org/wiki/Sound_recording_and_reproductionhttp://en.wikipedia.org/wiki/Antenna_%28radio%29http://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Amplitude_modulation

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When the vanes are open a small capacitance is still present (about 40pF). The coil connections

add a slight amount of stray capacitance which may be 7 or 8pF. With 48pF capacitance and a200uH coil, the highest frequency will be about 1624kHz. Some variable capacitors, have built

in trimmers to adjust the highest frequency. For any coil and capacitor that tunes too high, a

50pF trimmer may be added in parallel with VC1 to control the highest tuneable frequency.

Tuning Coil for 500 to 1700KHz.

The MW coil described above results in an inductance of approximately 200uH. If coupled witha 500pF capacitor (full mesh) will tune to about 500kHz and open mesh (about 43pF) tunes to

1700KHz.

Construction

The tuning capacitor has fixed and moving plates. The moving plates should be connected to the

"cold" end of the tank circuit, this is the base of Q1, and the fixed plates to the "hot end" of thecoil, the junction of R1 and C1. If connections on the capacitor are reversed, then moving yourhand near the capacitor will cause unwanted stability and oscillation.

All measurements made with a fresh 9volt battery and three transistors with respect to the batterynegative terminal.Smaller rods will probably work better with the more compact plastic boxes or

cases that constructers tend to use.