HIGH_PERFORMANCE_SHORTWAVE_CONVERTER

Published:2009/6/17 3:48:00 Author:May | From:SeekIC

The NE602 chip, U1, contains oscillator and mixer stages. The mixer combines the oscillator sig-nal with the input RE signal to produce signals whose frequencies are the sum and difference of the input frequencies. For example, an 8.5-MHz oscillator and a 10-MHz incoming signal will give output signals at 18.5 MHz (10 + 8.5) and 1.5 MHz (10 - 8.5). Recall that 1.5 MHz is 1500 kHz and an ordinary AM radio will tune to it.The choice of crystal depends on what shortwave band you want to hear. The 9.5- to 10-MHz band is less crowded and includes the time-signal station WWW. For that band, you'll need a crystal of 8.5 to 8.9 MHz. There is no standard microprocessor crystal in that range, but you can use an am-ateur radio crystal, have a crystal custom-made, or use a CB crystal.Transformer T1 rejects signals that are outside the band you are interested in. Transformer T1 should pass signals from 9 to 11 MHz and attenuate all others. The transformer, T1, used in the circuit is a 10.7-MHz IF transformer salvaged from an FM radio.They are fairly easy to obtain new from parts stores and mail-order houses. Most 10.7-MHz IF trans-formers will tune across the 9.5- to 10-MHz band without modification; all you need to do is turn its tuning slug. To receive the 6.0- to 6.5-MHz shortwave band, you'll have to add a 150-pF capacitor.



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