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5.3. NE612 5.3.1. Synchrodyne AM Receiver If the author remembers well an article that he read in a professional
magazine many years ago, the synchrodyne receiver is the ancestor of the
superheterodyne receiver. Sometime at the beginning of the 20th century this
device was called the Heterodyne receiver, and it was first constructed by
Levvy. Armstrong improved it and gave new name to the new device, by adding
the prefix SUPER to the old name.
They are being suppressed by the LF filter that comprises the R* resistor and C*
capacitor. The device we were testing did not, however, contain R*. It is to
be installed if some disturbances occur (whistling or similar), and its
optimum value is to be found experimentally. If necessary, greater
capacitance of C* is also to be tried out. * As mentioned earlier, it is very important for the supply voltage of the
NE612 to be stable. This values even more for the synchrodyne then the
superheterodyne receiver. The voltage control is done by the stabilizer,
made with 78L06 IC. It is being placed in the low-power transistor package,
either metal (as for BC107) or plastic (as for BC547), and its maximum
current is about 100 mA (pic.5.7-b). A simpler stabilizer, made with the
Zener diode, can be used instead, as on pic.5.9. * Instead of factory-made coil LO, the self-made one can also be used. The
simplest solution is to use the one from pic.3.6, in which case the mid leg
is not used. Over this coil, the feedback coil should be winded, acc. to
pic.5.7-c (its ends are marked with 4 and 1). When connecting with capacitor
C and pins 1 and 7 of NE612, care should be taken to join properly: coil
ends 1 and 3 with ground, 2 with capacitors C and 560 pF, and 4 with 1 nF
capacitor. It is, of course, possible to use smaller coil, wound on a
smaller body, with more quirks of thinner wire. Its inductance should be
about 350 mH, and the number of quirks required is to be found by testing.
The feedback coil (4-1) has app. 3x fewer quirks than the oscillatory
circuit coil (2-3). * On the pin 5 of the NE612 the LF signal is also obtained. It is the same as the one on pin 4, but has a 180° phase shift compared to it (in simple words, while one signal increases, the other one decreases, and vice versa). That gives us the opportunity to use the dual audio amplifier in the LF part, that has two amplifiers, with inverting and non-inverting inputs. As shown on pic.5.8, the counter-phase LF signals from NE612 are led onto the same inputs. The output signal has 2x greater amplitude, therefore making the output power 4x greater than when only one input is used (as on pic.5.79). 5.3.2. AM Receiver with Synchro Detector In previous project, the NE612 was in fact used as the AM signal
detector. The LF signal exiting the mixer is product of the simultaneous
(synchronous) action of the station signal and voltage from the local
oscillator upon it. That is how the term “Synchro Detector” emerged. There’s
also a possibility to use a station carrier instead of local oscillator’s
voltage, so that the station signal gets beaten by itself, however strange
this may sound. Electronic diagram of one such device is given on pic.5.9. 5.3.3. Input Circuits for Receivers with the NE612 IC All the receivers with NE612 that are described here work better, especially considering suppressing the noise in case of the symmetrical station, if the proper input circuitry is added to them. Pic.5.10 shows two examples of the MW receivers that use the ferrite antenna. In both cases, the antenna taken from an old commercial radio is being used. 5.4. The Universal Audio Amplifier We already spoke about the universal amplifier in the text connected with
pic.3.22. Pic.5.11 contains the diagram of another such device, where the
transistor amplifier with BC547 is used as the pre-amplifier, instead of
that with TLO71 IC. It can be used for practical check of all the earlier
mentioned radio receivers. The LF signal is being taken from the detector in
the HF part of the receiver to the hubs marked as In and Gnd (if the links
aren’t too long the ordinary wires are used, otherwise - the microphone
cable). On the third hub the DC voltage is outputted, which is used in some
HF circuits for their operation (such as e.g. those on pics.3.24, 3.25, 3.29
etc.). 5.5. Additional Circuitry 5.5.1. Fine Tuning During the tuning of the receiver to some station at the SW band with the variable capacitor, a problem occurs. In simple terms, the station frequencies are too close to each other, so the capacitor’s shaft should be turned for an extremely small angle in order to change station, which is practically impossible. It would certainly be useful if we could somehow stretch (a popular term for this) the part of the band near the frequency to which the receiver is tuned at. For the direct type (TRF) receivers that were described in the previous chapters, this can be accomplished if, acc. to pic.5.12, another variable capacitor (CR) is added in parallel to the variable capacitor at the input
circuit. Its capacitance should vary at substantially smaller scale than
that of C, meaning from a few pF til about 20 pF. The tuning is accomplished
by setting the receiver, by means of C, approx. at the middle of the band we
are interested in, then tuning by means of CR to some station in that area.
E.g. if the stations we want to receive are located in the part of the SW
band from 6.1 MHz till 6.2 MHz (it’s a well-known 49-metre band), first we
tune ourselves with C to approx. 6.15 MHz, and then we pick with CR some of
the stations located in that area. The same applies for the famous Magic
Band (at about 50 MHz).
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