Complete translation thanks to Google translate.
The series started with the "Trenty" and the "Toucan" continues with a voice transceiver using the principle of DSB (double sideband) transmission and direct conversion reception (upper and lower SSB), all driven by a VXO (quartz controlled variable oscillator). This greatly simplifies its construction, makes it possible to mount it on a small circuit and reduces the cost by using a minimum of components, all standard and available. There are of course limits, direct frequency conversion reception is not as selective as that using the superheterodyne principle with a quartz filter, and the transmission, with a power of 1 watt HF, is done on both sidebands instead of just one as in SSB (single sideband). But if the construction is done properly, the results will be up to par. The reception, equipped with an AGC (automatic gain control) circuit is pleasant, and the frequency, thanks to the use of the VXO, is very stable. In transmission, your correspondent, if you do not specify that you are transmitting in DSB, will not see the difference with the SSB. Despite the reduced coverage of a portion of the 80 meter band, many QSOs were made almost daily with the prototype, most with the power of 1 watt, others with the addition of a small amplifier of about twenty watts. In the latter case, the QSOs were practically as pleasant as with a normal station. Description: At reception, there is a low-pass filter C1-C2, L1, C3-C4 (common to transmission). The relay RL1 (at rest) directs the signal to the double input band-pass filter. The first tuned filter L2-CA1 is a low-capacity series filter rejecting low frequencies well to avoid the direct entry of signals from powerful broadcast transmitters in the short wave range. Next comes a parallel tuned filter L3-C5. The good characteristics of these two filters avoid the use of an input attenuator. Diodes D1 and D2 protect the NE612 input from RF overvoltages in transmission. IC1 (NE612) is a gain oscillator-mixer circuit. In the direct conversion receiving system, the local oscillator frequency is very close to the receiving frequency, the difference being in the LF spectrum. There is no IF (intermediate frequency), and the low frequency is directly amplified by IC2 (LM386). The local oscillator frequency is determined by the set of X1-X2, L4-L5 and D3. The combination of these elements makes it possible to "pull" the frequency of the quartz quite significantly without too much harm to the frequency stability. Parallel connection of two quartz of identical frequencies further increases the frequency range. The side-by-side arrangement of the inductors L4 and L5 also increases this range. The smaller the gap, the greater the mutual induction and therefore the frequency range. However, there are limits that should not be exceeded. Indeed, by placing the inductors in contact with each other, we arrive at a range of almost 70 KHz, but it is reasonable not to exceed about twenty KHz, because beyond that the frequency becomes unstable. On the prototype, the available range goes from 3667 to 3684 KHZ, which is largely sufficient, because there are people on these frequencies and in particular quite a few QSOs in French. Beyond these values, the VXO loses its stability, and a slight frequency modulation may appear on transmission. The BF signal from pins 4 and 5 of IC1 is applied to the symmetrical input of IC2 (symmetrical connection). C10 reduces the amplitude of high-pitched signals. C11 and C12 are isolation capacitors for DC voltages. IC2 has its gain set to maximum (46 dB) by using capacitor C14 (maximum applicable value). This is necessary for a receiver of this type if you want to enjoy comfortable listening through a loudspeaker. R4 and C15 have the function of reducing the white noise generated by the high amplification of IC2. R5 and C17 prevent IC2 from oscillating if the impedance of the loudspeaker used is too low. C16 is an isolation capacitor for the DC voltage present at 5 of IC2. The BF, on the other hand, easily passes through this capacitor to reach the loudspeaker. The signals are powerful enough to power a loudspeaker, even a large one (which will provide excellent rendering). A simplified automatic gain control circuit has been added to this assembly, so that signals that are too strong do not saturate the receiver, which makes listening much more pleasant. This very simple system requires some explanation. Click to enlarge The DC voltage, normally present at pins 1 and 2 of the NE612 is about 1.4 volts. If we decrease it, the gain of IC1 also decreases (it's a bit orthodox but it works...). We insert a Led diode
Bruce W4BRU