Direct-conversion receiver
This simple direct-conversion receiver circuit was described in "Radio" magazine of 1977, 11, by V. Polyakov (RA3AAE).
A direct-conversion receiver, also known as DCR, is a radio receiver design that converts RF signals directly into audible frequency. A simple low-pass filter provides good selectivity, but the image frequency cannot be suppressed with the filter without using more complicated design. The DCR is used to receive SSB (LSB or USB) and CW signals. AM and FM signals can't be demodulated properly without using a phase locked loop system, because for AM there is a beating frequency that interferes the reception, but AM signal still can be received if the frequency of the local oscillator is very stable. FM signal will be very distorted to be read.
The circuit diagram of the receiver for 80 meters band is shown on the Figure 1. An RF signal from antenna goes through the capacitor C1 to the resonant tank L1C2C3C4, then it goes further to the mixer made of antiparallel diodes V1 and V2. The Π-shaped low-pass filter L3C10C11 with the cutoff frequency of 3 kHz is the load of the mixer. The local oscillator signal applied to the mixer through the capacitor C10, this is the first capacitor of the filter.
The local oscillator of the direct-conversion receiver is a common base Colpitts oscillator circuit with the capacitive feedback, it is based on the transistor V5. The resonant tank of the local oscillator is connected between the collector of the transistor and the ground. The local oscillator and the input resonant tank are turning simultaneously - they must be tuned in tandem by the two-section air variable capacitor C3, C6. Note, the local oscillator frequency (1.75...1.9 MHz) is twice lower than the frequency of the input resonant tank at the antenna input (3.5...3.8 MHz).
Figure 1. The circuit diagram of the direct-conversion receiver with antiparallel diodes.
V1, V2 - KD503A (si); V3 - P28 (ge); V4 - MP14 (ge); V5 - KT315G (si);
R1* - 110k; R2 - 4.3k; R3* - 75k; R4 - 10k; R5 - 1.3k;
C1 - 8..60 pF (a trimmer capacitor); C2, C5 - 220 pF;
C3, C6 - 10...365 pF (two-section air variable capacitor);
C4 - 620 pF; C7 - 750 pF; C8 - 4.7 nF; C9 - 33 nF;
C10, C11 - 50 nF; C12 - 0.1 μF; C13 - 5 μF x 12 V;
C14 - 6.8 nF; C15 - 30 μF x 12 V;
L1 - 14 turns of silk enameled copper wire of 0.15 mm in diameter (AWG=34);
L2 - 32 turns of enameled copper wire of 0.1 mm in diameter (AWG=38);
L1 and L2 are tapped at fourth turn from the ground,
both coils have ferrite slugs;
L3 - 100 mH. http://zpostbox.ru
Headphones should be high impedance type (kilohms), just like TA-4.
A modern low-impedance headphones also can be used, but it takes a resistor or potentiometer of several kilohms, connected to the to the collector of V4, and a LM386 - based audio amplifier.
The transistor V5 (KT315G) has these parameters:
Transition frequency (ft), MHz: 250
Collector capacitance (Cc), pF: 7
Forward current transfer ratio (hFE), min: 50
The transistor can be replaced with 2N3904 or a similar model.
Transistors V3, V4 can be replaced with modern silicon transistors.
Diodes KD503A can be replaced with 1N4148
The audio amplifier is based on transistors V3, V4 with the direct coupling between stages. The load of the amplifier is a high-impedance headphones with resistance of about 4 kohms, just like TA-4 (the USSR headphones model).
A power supply that provides a voltage of 12 Volts can be used with this receiver. The consumption current is typically about 4 mA.
The low-pass filter coil L3 has an inductance of 100 mH. The coil is wound on a ferrite toroidal core with dimensions of 18x8x5 mm (relative permeability of the core μ/μ0=2000). The coil L3 can be replaced with a resistor of 1...1.3 kohms, but it will reduce the selectivity and sensitivity of the receiver.
How to adjust the circuit. At first, check out the voltage at the collector of the transistor V3, it should be between 7..9 Volts. If it's not so, then match the value of the resistor R3 to get that voltage. Second, check out the voltage at the emitter of V5, it should be in rage of 6..8 Volts. Adjust it by matching the value of R1.
Next, check if the local oscillator works. Short the ends of the coil L2 together, it will reduce a noise level in headphones just a little. It happens because without the signal the noise of the mixer getting lower.
Connect the antenna to the receiver, tune it to a signal, then find a tap point on the coil L2 (within -2..+2 turns) to get a maximum volume in headphones. (See also RF mixer with automatic biasing for DCR)
Use the slug of the coil L2 to set the operating band of the receiver. Then adjust the slug of the coil L1. Be sure the slug is not in an utmost position, else change the amount of turns of the coil L1.
Use the trimmer capacitor C1 to adjust the coupling with the antenna, set the capacitor in a position where stations can be heard with an average volume, such arrangement spares a volume control potentiometer.
The well-adjusted receiver provides a common gain of 15000. This is the ratio between the voltage of the audio signal at the headphones and the RF voltage at the antenna input.
The selectivity of the receiver is determined by the low-pass filter L3C10C11, it is 35 dB at the detuning frequency of ± 10 kHz. Using a two-stage low-pass filter can provide a better selectivity.