Direct-conversion receiver for 27 MHz frequency on the TA7358P microcircuit

A direct-conversion receiver operating at 27 MHz.This class of radio receivers receives telegraph signals and signals with single-sideband modulation or dual-sideband modulation. It is not designed to receive AM and FM signals. This radio receiver mixes the input signal with an oscillator signal to extract the beat signal, or sound, which is then filtered and fed to the input of a highly sensitive audio amplifier. The high-frequency section of the receiver is implemented using the TA7358P microcircuit. This microcircuit contains an input high-frequency amplifier, a mixer, and an oscillator, which is exactly what is needed. This microcircuit has many similar designs in the same package, such as the LA1185 or TA7378P. The audio amplifier is based on the TDA7056B microcircuit, and the audio preamplifier is assembled using two C1815 transistors. C8, C9, and L4 act as a filter that extracts the audio signal and filters out other signals.

Coils L1 and L2 are used in the high-frequency amplifier. L1 has 8 turns of 6 mm diameter coil wound with 0.5 mm wire. L2 has 10 turns of 4 mm diameter coil wound with 0.25 mm wire. These are approximate values. L1 contains a ferrite rod, and L2 contains a copper rod that must be inserted into the coil when tuning the receiver. The 50 cm antenna is connected via a 50 ohm coaxial cable; do not connect the antenna directly to the receiver input. An oscillator is built around a quartz resonator. To slightly shift the frequency, a choke with an inductance of approximately 7 µH is placed in series with the quartz resonator. You should select this inductance based on the signal reception quality.

L4 has an inductance of approximately 20-30 mH. Wind 50 turns of 0.18 mm wire on a green ferrite ring; this will be sufficient. The green ring is located in the common-mode filter in consumer electronics. You can simply replace L4 with a 1 kOhm resistor, but the signal quality may be worse. Use film capacitors in the audio amplifier; use a TDA7056B microcircuit, not a TDA7056.

Place aluminum foil under the assembly and solder the ground to this foil. Wires  should be as short as possible.

To test the radio receiver, I assembled a simple radio transmitter that transmits a signal at a frequency of 27 MHz with various tones. The transmitter has no antenna and was placed approximately 10 meters from the receiver.

My radio receiver received the signal from the transmitter, but the xhdata d-808 radio receiver did not receive the signal, which means that the sensitivity of the homemade radio receiver is excellent.