My latest creation, a 12 Transistor-5 IC dual-conversion superheterodyne receiver with continuous coverage of the SW bands from 5.7 to 22 MHz. The receiver has a DDS VFO
(Direct Digital Synthesis Variable Frequency Oscillator) and a back-lit LCD frequency display. The receiver copies AM broadcasts and SSB (Single Sideband) Radio Amateur signals and has the following features: a front-end hybrid-cascode RF amplifier, 2 Gilbert-cell balanced mixers, 2 IF amplifiers, AM and SSB mechanical filters, an AM ceramic filter, an FET infinite impedance AM detector, a high stability analog BFO, a diode-ring product detector and a signal LED for signal strength. The receiver runs on 12 volts DC and performance is similar to that of high quality commercial SW receivers.
Dimensions: 10"(L) X 5"(D) X 3"(H). Controls from left: Top row: Volume, LSB/USB, LCD frequency display, RF Peak and RF Gain. Bottom row: Phones jack, SSB, Signal LED, Memory Down, Memory Up, Memory Call, Tuning, HIFI, Narrow Filter, Slow AGC and Power On/Off. RF Peak tunes/peaks the front-end RF amplifier (I do this while monitoring the brightness of the signal LED). Memory push-buttons handle DDS VFO functions such as IF shift, Tuning step and calling of saved frequencies, to name a few. In SSB mode (Single Sideband), Slow AGC and Narrow IF filter are switched on, and LSB/USB BFO capacitor is adjusted to coppy HAM stations correctly. HIFI switch bypasses both IF mechanical filters and connects a wider ceramic filter for high fidelity reception of AM broadcasts.
Wire antenna (roughly 450 ohms) is matched to the input of the RF amplifier (2.2K) with a 9 T/20 T transformer. Some may frown at the inclusion of an RF amp. ahead of the NE602 mixer, but gain of this amp. is controlled manually from the front panel, and with AGC that applies 5 V at no signal and about 0.75 V at max. signal to the base of the 2N3904 transistor in the amp. so the NE602 will not overload. Received signals between 5.7 and 22 mHz are preselected with a single high-Q LC circuit ahead of the NE602, which is sufficient to attenuate the first image frequencies which are 21.4 mHz (10.7 mHz x 2) away from the received signals.
Two active NE602 mixers are used to convert all received signals to a first IF of 10.7 mHz and a second IF of 455 kHz. These mixers work at reduced supply voltage, and thus reduced gain, to keep most gain in the RF and IF amplifiers to achieve very good dynamic range. Output from a DDS VFO (16.4 to 32.7 mHz) feeds the first mixer and a crystal oscillator running at 11.155 mHz is part of the second mixer. These mixers will not overload as the amplifiers ahead of them all have AGC.
Two MC1350 IF amplifiers with AGC amplify both IF frequencies of 10.7 mHz and 455 kHz. AGC applies 5.45 V at no signal and about 5.75 V at max. signal. at pin 5 of both amplifiers. AGC from this voltage increase (0.3V) proved to be more than enough to handle the strongest SSB and AM signals received so far.
The first IF frequency is filtered with a 10.7 mHz monolithic crystal filter with a BW of 15 kHz. This filter attenuates the second image frequencies which are 910 kHz (455 kHz x 2) away from received signals. The second IF frequency is filtered with 3 user selectable 455 kHz filters. For SSB, a 2.5 kHz BW mechanical filter is selected. For AM, a 5.8 kHz BW mechanical filter or a 6 kHz BW ceramic filter are selected. The ceramic filter allows higher fidelity reception of AM broadcasts. A tail-end 455 kHz ceramic filter at the output of the second MC1350 cleans out any noise products generated by both amplifiers. The 10.7 mHz filter has I/O impedances of 1.8K, and all 455 kHz filters have I/O impedances of 2K except for the selectable ceramic filter which has I/O impedances of 1.5K and 2K. All filter impedances are well matched to the impedances of the NE602 and MC1350 integrated circuits.
Detectors and BFO
For AM reception of SW broadcasts, an infinite impedance FET detector is used. This detector is more sensitive and has less distortion than the usual germanium diode. For SSB reception of HAM stations, a diode-ring product detector is used. This detector has insertion loss but is much less prone to distortion from strong signals than an active product detector. Insertion loss is made up for with an AF preamplifier ahead of the AF amplifier. The analog BFO I am using runs around 455 kHz and stabilizes very quickly due to the low frequency it is working at. A buffer/amplifier isolates the BFO and matches it to the low impedance of the product detector.
Two IF-derived AGC circuits control the gain of the RF and IF amplifiers. Both circuits are fed from a single voltage doubling AGC detector. A reverse AGC circuit controls the gain of the RF amp. This circuit applies 5V at the base of the 2N3904 in the RF amp. at no signal, and about 0.75V at max. signal, lowering the gain of the RF amp. A second forward AGC circuit controls the gain of both IF amps. This circuit applies 5.45V at pin 5 of both MC1350 amps. at no signal, and about 5.75V at max. signal, lowering the gain of these amps. Running both NE602 mixers at reduced gain allows these AGC circuits to control most IF/RF gain to give very good strong signal handling and dynamic range.
A DDS VFO (Direct Digital Synthesis Variable Frequency Oscillator) built from a kit
supplies LO injection to the first NE602 mixer and tunes the receiver. The 250 mV output from this VFO is enough to drive the NE602 and runs 10.7 mHz higher than all received signals. So to tune 5.7 to 22 mHz, the VFO runs from 16.4 to 32.7 mHz. This frequency is shifted down by 10.7 mHz to display the correct tuned frequency. The VFO connects to a 16 x 1 Newhaven backlit LCD display to show frequency and other functions in the VFO, such as IF shift, saved frequencies and tuning step, to name a few. A good quality mechanical encoder is connected to the VFO for tuning with good results, but an optical encoder is preferred. All controls on the VFO board were remoted to the front pannel without any problems.