Many thanks to Dave G7UVW for the excellent revision of the following text
30 m Band Direct Conversion Receiver
The direct conversion receiver described consists of only a bandpass filter, mixer IC, VFO and audio filter. With only 22 parts, this simple circuit should take a few hours at the weekend to construct. For extra simplicity, eliminate L1 & C1 and connect C2 to the slider of P1.
Fig. 1: Schematic of the receiver
The oscillator section of the NE612 is configured as either a series-tuned Colpitts- or Clapp oscillator. Depending on the parts the receive range is from 10050 kHz to 10250 kHz. Due to component tolerances the number of turns and/or the capacity of L3 and C7 are adjusted to tune the required portion of the band.
Mixer and AF amplifier
The mixer stage is followed by a selective audio amplifier. Balanced DC voltage levels at the mixer outputs and amplifier inputs allow a component saving direct coupling. After suppressing the unwanted RF products by C9 and C10 the remaining AF is passed through to the non-inverting inputs of TL072 dual op amp. A series resonant circuit, built out of Dr1 and C1, aids some AF selectivity. The AF stage has a gain of 46 dB at its 660 Hz resonant frequency. Audio volume is controlled by the attenuator P1 at the antenna terminal. The TL072 outputs aren't of low impedance. So, when connecting 60 ohm headphones one should arrange a 470 ohm resistor in series.
The receiver operates in a voltage range from Ub=8 ... 15 V. The AF stage is supplied directly by Ub and the mixer gets a +6 V constant voltage from IC2. The receivers current consumption is approx. 6 mA with no input signal. You can save 1 ... 2 mA when using a 6 V regulator type with lower quiescent current.
|P1||1 kOhm potentiometer|
|R1, R2||10 kOhm|
|C1, 3||5 .. 90 pF trimmer capacitor|
|C4||2 x 20 pF
VHFbroadcast variable capacitor
|C8||0,1 u F|
|C9, 10||47 nF|
|C11||2,2 u F|
|C12||100 u F, 25 V|
|L1, 2||T50-2, 30 turns prim. / 3 turns sec.|
|L3||T50-2, 30 turns|
|IC3||TL072 DIP e.g.|
|KH||Headphone Ri > 60 Ohm|
Alignment For the receiver alignment use of an existing receiver is recommended. Check the VFO frequency first. Receiver tuning should be possible between 10050 kHz (C4 max.) and 10250 kHz (C4 min.) by means of the variable capacitor. For this, listen to the VFO signal on the other receiver. Finaly adjust C1 and C3 to 10100 kHz, maximum audio power of an operating amateur radio st