Varicap selection or the delta-C / delta-V issue

Published:2011/7/28 1:15:00 Author:Phyllis From:SeekIC

For some strange reason varicaps (or variable capacitance diodes) have always been rather elusive components. Try this: design, engineer and publish a design at instant T and you’ll find that the varicap you’ve specified after hours of careful researching has disappeared from the market at [T+1 day]. Here at Elektor we’re optimists by nature but because we anticipate supply problems with the BB911 varicap specified for this present receiver, we thought we’d give you a few clues to help you find equivalent types.

The component values in the circuit diagram guarantee a tuning range of 68-88 MHz, with trimmer C20 defining the edges of the tuning range and the capacitance ratio of the BB911 defining the width of about 20 MHz. In other words, trimmer C20 shifts the tuning range and D1 determines the width of the tuning range. The two parameters have some interaction, of course.

If you are only interested in, say, the 4-m amateur band (70.0-70.5 MHz) then a narrow tuning range is sufficient and you will have no trouble getting just about any old VHF varicap to work in the receiver, simply by adjusting C20 to a ’known-good’ signal in the band (ask for assistance from a local licensed radio amateur).

If, on the other hand, you want everything from low-band TV (68 MHz) to police, MoD and government PMR (in some cases just below 87 MHz) then some thought should be given to the selection of the varicap.

The Philips BB9II was chosen because of its relatively large capacitance range of 25 pF to 65 pF for a corresponding tuning voltage of 0.6 V to 5 V — see Figure A (courtesy Philips Semiconductors). That’s right, the capacitance presented by a varicap is inversely proportional to the voltage applied across the device! Mathematically, though, [delta-C / delta-V] is a dimensionless device constant which, in the case of the BB911, works out at about 9 for the linear part of its capacitance range.




If you can’t get the BB9II locally there’s no reason to abandon the project or send Blue Murder emails to the Editor because there are lots of alternatives. Do not be afraid to experiment. In many cases, unlabelled varicaps picked up at radio rallies or salvaged from an FM radio may be used, provided you know they are for VHF. Connecting a few varicaps in parallel (’slacking’) is perfectly legitimate in order to arrive at the desired ±V/AC value and hence the receiver’s tuning span.

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