As circuit elements go, traditional trimmer capacitors can take up a generous portion of a printed-circuit board (PCB). With the continual shrinking of circuitry in handheld and mobile devices, however, engineers require ever smaller trimmer capacitors and Voltronics International has responded with the J Series of ceramic chip trimmer capacitors.
These surface-mount components, which are available in tape-and-reel format for use with automated manufacturing equipment, bring the frequency-tuning function to circuits in a fraction of the size of the usual industry-standard trimmer capacitors. These tiny trimmers offer capacitance tuning ranges as wide as 10 to 120 pF in a single unit, with self-resonant frequencies (SRFs) as high as 6 GHz for microwave applications.
Variable capacitors, of course, are useful in HF through microwave designs for tuning the frequency of an oscillator or shifting the centre frequency of a tuned filter. The J Series of variable capacitors includes a variety of package styles and sizes, all meant for surface-mount and/or PCB applications. The family of trimmer capacitors currently consists of seven different package styles covering wide capacitance tuning ranges (Table 1).
The available capacitance tuning range is a function of size for these trimmers: the tradeoff is a narrower capacitance tuning range in the smaller models. Still, for applications that only require a shift in capacitance of 1 or 2 pF, the smallest family of J Series trimmer capacitors, the JN Series, features units capable of adjustable capacitance from 1,5 to 4,0 pF and 3 to 8 pF. And, the size of the JN series trimmers is a mere 1,7 x 1,5 x 0,9 mm. Model JN040 provides the 1,5 to 4,0 pF tuning range. Unlike many trimmer capacitor specifications, where the self-resonant frequency (SRF) of components are rated at their minimum capacitance setting, Voltronics chooses to show worst-case results by specifying the J Series trimmer capacitors at their maximum capacitance setting. The SRF of the model JN040 at that maximum setting (4 pF) is 2,7 GHz. Model JN080, with the 3 to 8 pF capacitance tuning range, exhibits an SRF of 1,8 GHz at its maximum capacitance setting of 8 pF.
Both trimmers (and other members of the JN Series) are rated for a DC working voltage of 25 V and a DC withstanding voltage of 55 V. The minimum quality factor (Q) for the JN040 and JN080 models measured at 1 MHz is 300. The two trimmers exhibit minimal changes in capacitance as a function of changing temperature, as evidenced by temperature coefficient of 0 ±500 ppm/°C for the JN040 and -750 ±500 ppm/°C for the JN080 - this is across an operating temperature range of -25 to 85°C.
The other two members of the JN Series, the JN010 and JN015 trimmers, feature capacitance tuning ranges of 0,55 to 1,0 pF and 0,7 to 1,5 pF, respectively, with minimum Q of 500 and SRFs of 6,0 and 4,8 GHz, respectively. Both have temperature coefficients of 0 ±300 ppm/°C.
The largest of the J Series capacitors are the trimmers in the JB Series, which measure 6 x 6 x 4,8 mm. The larger size requires some additional PCB real estate, but brings with it the benefit of greater available capacitance tuning range. The two widest capacitance ranges belong to the models JB900 and JB1200, at 9 to 90 pF and 10 to 120 pF respectively. As expected, those high maximum capacitances result in relatively low SRFs of 200 and 170 MHz, respectively, although the SRFs would be in the 1 GHz range if specified at the minimum capacitance setting. Additional members of the JB Series include models JB023, with a capacitance tuning range of 1,25 to 2,3 pF and SRF of 1,4 GHz, and JB050, with a capacitance tuning range of 1,5 to 5,0 pF and SRF of 0,9 GHz.
The J Series ceramic chip trimmer capacitors incorporate rotary tuners to adjust capacitance by means of a special screwdriver or tuning tool. Tuning requires minimal torque, 0,099 to 0,990 mN.m for the JN Series and 1,977 to 14,829 mN.m for the larger JB Series. The trimmers are compatible with modern automated manufacturing processes - larger models are supplied in bulk packaging while smaller units are shipped on 8-mm tape-and-reel. The trimmers are designed for a maximum reflow solder temperature of 230°C for maximum soldering time of 20 s.
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