Highpass filters are simple in function—passing signals without affecting them beyond a certain (cutoff) frequency and sharply attenuating signals below that frequency—but often difficult to realize in practice. By leveraging their high-performance Blue Cell™ low-temperature-cofired-ceramic (LTCC) technology, however, the engineers at Mini-Circuits (Brooklyn, NY) have created the HFCN line of seven-section highpass filters with cutoff frequencies from 600 to 3000 MHz at a fraction of the cost of traditional highpass filters. The compact components (supplied in a standard 1206 EIA package) are well suited for subharmonic attenuation, breakdown-voltage protection, passband matching, and as DC blocks.

Traditional highpass filters incorporate discrete components to create desired resonances. Unfortunately, such components are subject to variations in electrical characteristics and additional factors, such as inconsistent placement on a printed-circuit-board (PCB) layout, can cause further performance variations. Due to their monolithic-like LTCC process, the HFCN filters provide extremely repeatable performance and in fact offer guaranteed performance levels to 9 GHz, with good temperature stability and power-handling capability of 7 W. The LTCC filters are also insensitive to electrostatic discharge (ESD).

Examples of the HFCN highpass filter line include the models HFCN-650 (Fig. 1), HFCN-740, HFCN-2100, and HFCN-2700 (Fig. 2), with the cutoff frequency (in MHz) of each unit denoted in the model number. At the cutoff frequency, an input signal is attenuated by 3 dB, with attenuation decreasing beyond that frequency. The HFCN-650, the lowest-frequency model in the line, for example, has a cutoff frequency of 650 MHz and is nominally designed for a passband of 850 to 2490 MHz where signal attenuation is minimal. The highpass filter provides 20-dB rejection of signals at 480 MHz increasing to 40-dB rejection at 390 MHz. Above the cutoff frequency, the passband insertion loss is low, at less than 1.3 dB from 850 to 2000 MHz and less than 2 dB from 710 to 2490 MHz. The model HFCN-740 has a cutoff frequency of 740 MHz and nominal passband of 900 to 2800 MHz. Rejection is at least 40 dB at 430 MHz and insertion loss is less than 1.3 dB from 900 to 2200 MHz.

At the high-frequency end, the model HFCN-2700 offers a cutoff frequency of 2700 MHz with nominal passband of 3150 to 7550 MHz. It exhibits better than 40 dB rejection at 1600 MHz and less than 1.3 dB insertion loss from 3150 to 6500 MHz. Insertion loss rises to just under 2 dB at frequencies to 7550 MHz. The model HFCN-2100 offers a cutoff frequency of 2100 MHz with nominal passband of 2500 to 6000 MHz. It rejects signals at 1530 MHz by 20 dB and at 1050 MHz by 40 dB. Insertion loss is less than 1.3 dB from 2500 to 5000 MHz and less than 2 dB from 2200 to 6000 MHz. All HFCN filters are designed for operating temperatures from −55 to +100°C. P&A: $0.99 (HFCN filters, 1000 qty.) and $79.95 (K1-HFCN kit of 40 filters); stock. Mini-Circuits, P.O. Box 350166, Brooklyn, NY 11235-0003; (718) 934-4500, FAX: (718) 332-4661, e-mail: sales@minicircuits.com, Internet: www.minicircuits.com.