Search for Varicaps


Posted on Feb 7, 2014

A 80m (3. 5 - 3. 9 MHz) SSB transceiver using a Collins 455 kHz mechanical filter in the IF chain. Such designs need some good front-end receiver selectivity in the form of at least a couple of selective tuned circuits. I`m also planning to build a simple transistor VFO for the transceiver. This design approach requires three tracking tuned circuits - two in the preselector filter ahead


Search for Varicaps
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of the mixer, and one in the oscillator. The simple approach calls for a three gang variable capacitor. Thirty years ago, you could visit a local parts supplier and purchase a suitable part, probably along with a nice 10:1 reduction vernier drive. More recently, say ten years ago, I would have simply bought a set of suitable varicap diodes. A common choice for HF band designs was the excellent Philips BB212. Three such high capacitance varicaps would be ideal for my transceiver. Guess what component has been on my rare and endangered` components list for more than three years Varicap (i. e. variable capacitance) diodes operate using a reverse bias voltage applied to the P-N junction of the diode. As this reverse bias voltage increases, the depletion layer at the P-N diode junction widens. With normal operation of a silicon diode, it will take about 0. 6V to force electrons across this depletion layer, the reason for the this voltage drop across a conducting diode. In the case of a reverse biased varicap, since minimal current can flow in a good reverse biased diode, the result is an insulating layer between two conducting silicon layers. This layer widens with increasing reverse bias voltage. The result is a device which can be used as a voltage variable capacitor. Varicap diodes are also produced with a hyperabrupt` junction in which the P-N junction is doped to enhance this effect. Capacitance can readily range across a...




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