SW Converter for Digital AM Car Radio

This circuit is purposely offered with many bathroomse ends (not actually, of course) to stimulate experimenting with RF circuitry at a small outlay. Looking at the circuit diagram you could additionally recognize a modified model of the SW Converter for AM Radios described in different situations in that issue. The changes had been necessary to make the circuit compatible with a digital quite than analogue AM car radio. The main distinction between digital AM radios and their all-analogue predecessors is that tuning is in 9 kHz (some-times 4.5 kHz steps) in compliance with the international frequency allocation for the band. Obviously, that exact step dimension, fascinating as it can be on MW, is a stumbling block if you want to use a digital AM receiver together with a frequency step-up converter for SW, where chaos reigns and there isn't any fastened step dimension. The first try was once to make the crystal oscillator variable through about 5 kHz each and every approach.

 

Circuit diagram :

SW Converter for Digital AM Car Radio Circuit Diagram

 

Unfortunately, regardless of serious efforts, the crystal could not be pulled more than 1 or 2 kHz so some different resolution had to be found. After finding out the NE/SA602/612 informationsheet, it was once found that a variable LC based totally oscillator was once one of the best different. The circuit labored after winding a resonant LC circuit and including a zero.1 µF sequence capacitor to dam the DC element on pin 6 of the NE602 (612). When the tuning used to be found to be a bit sharp with the original capacitor, a simple bandspread (or advantageous tuning) characteristic used to be added via shunting the LC resonant circuit with a lightly loaded 365 pF tuning capacitor (C10) which, like the principle tuning counterpart, C8, was once ratted from an outdated transistor radio. The tuning coil, L1, consists of 8 to 10 turns of 0.6-0.8mm dia. enamelled copper wire (ECW) on a 6-8 mm dia. former with out a core. With this coil, frequency protection shall be from about 4 MHz to 12 MHz or so. Details on Tr1 may be discovered in the referring article.

 

Note that no tuning capacitor is used on the secondary — the input stray capacitance of the NE602 (612) does the trick. A BFO (beat frequency oscillator) was once delivered to let SSB (single sideband) signals to be received. The BFO built round T1 is inconspicuous, has a heap of output and that is steady enough to automobilery an SSB signal for a few minutes with out adjustment. The BFO frequency is tuned with C3. Tr2 is a ready-made four55 kHz IF transformer whose inner capacitor was once first crushed after which eliminated with pliers. When S2 is closed the BFO output sign is solely superimposed on the NE602 (612) IF output to the MW radio. The converter will have to be built into a steel box for shielding. If you in finding that the BFO provides too much output, disconnect it as steered in the circuit diagram and let stray coupling do the work. Sensitivity, even on a 1-metre size of automotive radio aerial, is quite superb. Bearing in mind that many of the main international SW broadcasting stations like Radio NHK Japan, Moscow, BBC and many others.) generate enough power to ensure that you are going to hear them, it's nonetheless kind of thrilling to pay attention to such signals for the first time in your automobile radio. 

http://www.ecircuitslab.com/2012/02/sw-converter-for-digital-am-car-radio.html