Compact DSP-based radio modules have made it incredibly easy to breathe new life into vintage equipment or build custom receivers from scratch. For a very modest price, you can get a ready-made AM/FM unit that already includes most of the tricky RF circuitry and requires only minor additions to become a highly capable radio. One such interesting module, built around the KT0936MB9 chip, turned out to be a great platform for experimentation and upgrades (https://s.click.aliexpress.com/e/_c4sIzae3). In this article I’ll show how it can be modified to improve reception, especially on the long and medium wave bands.
I bought this ready-made AM/FM receiver module on AliExpress for a small amount of money. It is assembled on the KT0936MB9 DSP chip and comes equipped with a built-in ferrite antenna as well as a tuning potentiometer. Modules like this, often sold without an enclosure, are commonly purchased by enthusiasts who want to retrofit old radios and add FM reception to them. Because the unit is based on the KT0936MB9 IC, additional bands can also be enabled or expanded if desired.
For my purposes, the AM part of the receiver—particularly longwave and medium wave—is far more important, while FM is of less interest. The chip itself offers very good sensitivity: about 1.6 µV in the FM band, 16 µV on medium wave, and 13 µV on shortwave. In practice, the reception quality is indeed quite impressive.
The schematic of this KT0936MB9 AM/FM radio module is shown below.
The receiver module operates from 2.1 to 3.6 volts, so powering it from two standard 1.5 V batteries is perfectly suitable.
Although the receiver already has decent sensitivity, it can be improved further. Instead of the small magnetic antenna made as a short ferrite rod with a coil, I replaced it with a longer ferrite assembly taken from an old radio. My ferrite rod has two coils—one for longwave and one for medium wave. I connected the medium-wave coil to the module.
I also replaced the batteries with a 3.6 V rechargeable cell. The original on-board potentiometer was removed, and a 100 kΩ variable resistor was wired off-board instead. To make tuning more convenient and precise, I added a vernier mechanism from an old radio receiver.
By default, the antenna for the FM band is the headphone cable. In my case it goes to the audio amplifier, so I soldered a separate piece of wire to the point on the PCB located between the 27 pF capacitor and the L1 inductor.
One feature I particularly liked is the dual-color tuning LED. While scanning across the band, it glows green, and once the receiver is accurately tuned to a station, it switches to orange. As a result, this AM/FM module based on the KT0936MB9 has turned into a very sensitive and easy-to-tune receiver. All that remains is to add an audio amplifier and mount everything into a proper enclosure.
