Tuning Frequency: approximately 531-1602kHz
Output power: ~300mW into 4Ω
Operating current: typically 27mA
Want a simple radio that you, your children or grand-children can easily build? This one uses a small PCB with two ICs and not a great deal more. It’s not a superheterodyne so the alignment is very simple and you don’t need any special equipment.
The pocket-sized version is housed in a remote-control case incorporating a 9V battery compartment. If you want, there is the option to power it from a 9-12VDC external supply (eg, a plugpack) and to drive an external loudspeaker. It is tuned using a rotary thumbwheel dial and has a volume control, battery condition indicator and power switch.
The retro-style desktop version is designed to look a little like the old AM radios of a bygone era that took pride of place on top of the fireplace mantel. It incorporates a loudspeaker and a hand-span tuning dial. It is housed in a small timber box with an aluminium front panel and this carries the volume control, battery condition indicator and power switch. The sound from the loudspeaker is not overly loud but is quite sufficient for personal listening.
AM radio IC
The circuit for the AM radio is based on a single IC that includes RF (radio frequency) amplification, a detector and AGC (automatic gain control). A similar device was originally available in 1984 from Ferranti Semiconductors and was known as the ZN414Z but is now obsolete. The MK484 replaces this and although out of production, there are remaining stocks. Additionally, the TA7642 is also now available with similar performance to the MK484. These AM radio ICs will work from 150kHz to 3MHz.
Add a tuning coil, a variable capacitor plus some capacitors and resistors and the IC becomes a fully functional AM receiver. For our circuit, the receiver operates over the standard AM radio band of 531-1602kHz. The signal output from the IC is amplified to drive a pair of headphones or a loudspeaker.
We tested both the TA7642 and MK484 in our circuit and found that the TA7642 has greater sensitivity than the MK484. However, its selectivity is wider, ie, it’s not as good. This means that the TA7642 will exhibit greater crosstalk (or interference) between stations that have adjacent frequencies. We did not test a ZN414Z as we didn’t have one available.
Note that while the performance of this AM Radio Receiver is acceptable, it does not have the selectivity and sound quality that’s available from a superheterodyne receiver.