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AM Radio Kit image Zoom

AM Radio Kit

SKU: QK63
AM Radio is all on one PCB! Uses a tuned radio frequency front-end. Uses AM Radio IC and two amplification stages!

Availability: Out of stock

$12.95

SKU:QK63

Assembly Required!
Product Description

    Details

    Operates on 9vDC.

    This projectᅠpresents the building blocks of modern day mini-sized AM radio receivers as found in key-rings, watches & palm sized radios. They are:

    - the Tuned Radio Frequency (TRF) front end

    - a single chip AM radio IC, and

    - amplification of the audio signal into a speaker

    All these components are presented on a single printed circuit board so you can build and experiment with your radio. We have built the Kit using standard passive components. ᅠCommercial products usually use surface mount components which results in very small sized radios.

    AM radio broadcasts consist of a radio frequency (RF) signal generated at a specific frequency allocated to a particular station.ᅠ On this RF signal is superimposed an audio frequency signal.ᅠ The audio frequency is said to amplitude modulate the radio frequency carrier.

    AM RF signals of all frequencies are present all around us.ᅠ Our radio must be able to 'find' the station we want from all the thousands of signals present. It has to be able to tune into the desired radio station and exclude all other signals.ᅠ And it must be able to tune into weak signals just as easily as tune into strong signals. ᅠLet us introduce two technical terms used to describe these requirements.

    SENSITIVITY

    The ability to pick up weak signals while keeping background noise to a minimum.ᅠ

    SELECTIVITY

    The ability of a radio to tune into a particular station and reject all other stations. ᅠ For mini-radio users selectivity is usually the more important.ᅠ Most often the listener wants to pick up all the local stations without any interference even though adjacent stations may be very close on the dial.ᅠ Usually they are not much concerned with being able to detect distant AM signals say over 30 miles away.

    All the AM signals reaching the radio are very weak.ᅠ Only that signal which matches the TRF frequency is magnified by resonance so that it stands out at a very much higher level of signal strength.

    This Kit uses a standard 60/160 AM Tuning Capacitor.ᅠ It contains two film capacitors.ᅠ Their capacitance changes as you turn the knob.ᅠ We only use the 0-160pF capacitor for our radioᅠ(the other 0 - 60pF capacitor is used in better quality AM radios that use regeneration on the aerial coil). The center pin is the common connection.ᅠ The two screws on the back of the package are the trimmer capacitors associated with each capacitor.ᅠ If you are unfamiliar with this item connect it to a capacitance meter and play with the main adjustment knob and the trimmer capacitors so you understand what is happening.ᅠ In this radio the position of the trimmers does not matter.

    The second component of the TRF is the coil & ferrite bar.ᅠ We supply the coil prewound with 90 turns of 3 strand Litz wire.ᅠ We got these coils from a commercial manufacturer of these coils who supplies exactly the same item to AM radio manufacturers. ᅠLitz wire consists of many strands of fine enameled wire twisted together with cotton to add strength.ᅠ Litz wire has a significantly lower resistance to RF than a single wire and is used in virtually all commercial coils.

    The ferrite bar increases the inductance of the coil.ᅠ The two components connected in parallel form a LC network.ᅠ The Litz wire of the coil and the ferrite bar give the network a high Q, or Quality factor.ᅠ This is critically important for the selectivity of the radio, the ability to tune into one radio station only and not be able to hear several others at the same time.

    The coil winding & the ferrite bar acts as an efficient wire antenna.ᅠ No additional external antenna is needed.ᅠ The only disadvantage is that the tuned circuit is directional.ᅠ Signal strength depends on the orientation of the bar with respect to the origin of the signal.

    It is important to realise that the radio IC does not create any sounds by itself. ᅠ It can only take the RF signal provided from the TRF circuit, amplify it, separate the audio signal from the RF (called detection) and pass the audio signal on to be amplified.ᅠ It has no selective or rejection components contained in it.ᅠ (This is in contrast to superhetrodyne receivers).ᅠ The MK484 we use is a Japanese copy of the original ZN414.ᅠ It contains an RF amplifier, active detector and automatic gain control (AGC to improve sensitivity) all in a 3-pin package.ᅠ The input impedence is typically 4Mohm.ᅠ It operates over a range of 150 kHz to 3MHz.ᅠ DC supply of 1.1V to 1.8V & 0.3mA current drain makes it ideal for battery operation.ᅠ The output is typically 40 - 60 mV of audio signal. Optimal AGC is provided by R3 and C2. ᅠ R3 (the AGC resistor) should be in the range 100R to 1.5K.ᅠ A bandwidth of about 4kHz is achieved in this circuit.

    The audio signal output from the MK484 is too weak to drive a speaker directly. In our kit the signal is fed into two stages of amplification and then into a speaker. ᅠ These are standard designs.ᅠ The first is a transistor Class A amplifier. ᅠ The second is a Class AB amplifier.

    The two forward biased diodes D1 & D2 appear to short circuit the power supply to the MK484, but this IC only requires about 1.5V to operate and the combined forward drop of the 2 diodes is exactly right.

    The tuning capacitor has been securely mounted on the PCB by 2 screws.ᅠ We have provided an extension rod to fit onto the capacitor tuner so that a knob can be attached. ᅠ The volume potentiometer is also mounted on the PCB.ᅠ Mounting holes to tie down the aerial coil have also been provided.ᅠ

    We aimed to provide the complete radio, excluding speaker, on a single PCB so that it was easy to experiment with.ᅠ It can be modified to fit into a box.

    You can experiment with changing the number of turns on the coil to shift the TRF range.
Additional Information

    Additional Information

    SKU QK63
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