Direct Conversion Receiver for 40 and 80 meter bands
The direct conversion receiver described in Radio Magazine 1977, November, can be modified to work on 40 and 80 meter bands. It takes to make some changes to the original circuit diagram. The new circuit diagram is shown in Figure 1, its antenna and local oscillator networks are modified.
Fig. 1.
Direct Conversion Receiver (DCR) circuit diagram.
FLO=1/2Fsignal
C1, C3, C4 - 8..30 pF trimmer capacitors;
C2 - 270 pF, C5 - 360 pF;
C6 - 10...365 variable capacitor;
C7, C9 - 2.2 nF, C8 - 3.3 nF;
C10, C11 - 0.05 μF, C12 - 30 μF x 12 V;
R1 - 110k, R2 - 1.8k; http://zpostbox.ru
V1, V2 - KD503A (1N4148)
T1 - KT315G (silicon transistor, ft=250 MHz, Cc=7pF, HFE min=50);
L3 - 100 mH, L1..L5 - see the text.
The coil L2 and capacitor C2 are tuned to 80 meter band (3.5 MHz). To switch the receiver to the 40 meter band, the coil L1 must me connected in parallel with the coil L2. As a result, the summary inductance of L1 and L2 gets lower, and the resonant tank is tuned to the middle of 40 meters band (7.05 MHz).
The local oscillator network for 80 meter band is formed by the coil L5 and capacitors C5...C9. In contrast to the old circuit, the local oscillator network is connected to the base of the transistor, it simplifies commutation. The local oscillator signal feeds to the mixer through the coupling coil L4. To switch the receiver to the 40 meter band, capacitors C3 and C4 connected in series with the coil L5. Such configuration reduces the common capacity of the resonant tank and makes the frequency two time higher, from 1.75 MHz to 3.5 MHz (in this DCR the local oscillator frequency is two times less than the radio signal frequency). In the same time the local oscillator frequency coverage gets twice lower, that is very useful because the 40 meters band is narrower than 80 meters band (it's for the USSR amateur radio frequency allocation). For the capacitor values shown on the Figure 1, the tuning range is 300 kHz for the 80 meter band, and about 150 kHz for the 40 meter band. The tuning range can be increased by shortened the capacitor C5 or reduce its capacity.
Any suitable two-position switch can be used as B1. It is mounted on the front panel of the receiver, near antenna socket. Coils L1, L2, L4, L5 are wound on a plastic former (with ferrite slugs) of 6 mm in diameter, using silk enameled copper wire 0.15 mm (AWG #34 or #35). Number of turns: L1 - 14, L2, L5 - 14, L4 - 6. The coil L2 is tapped at 5th turn at its cold end. The coil L4 is wound on the coil L5.
The coil L3 has the inductance of 0.1 H, it is wound on a ferrite toroidal core with dimensions of 18x8x5 mm (relative permeability of the core is μ/μ0=2000).
To adjust the receiver, turn the switch B1 to the 80 meter band. Adjust the slug of L5 to tune to the proper frequency range, then adjust the slug of L2 to get maximum volume of radio stations. Next, switch B1 to the 40 meters band, use the trimmer C3 to tune to the corresponding frequency range. If it can't help, then match the value of C4 (220pF...330pF). Adjust the slug of L1 to get the maximum volume of radio stations. That's all!
"Radio" magazine, October, 1978.