Electrically tunable preselectors are key elements in communications, avionics, and radar receivers. Narrowband RF and microwave preselectors prevent large off-channel signals from overdriving a receiver front end. Microstrip combline and interdigital tunable filters have been described by several authors.^1-5 By combining a suspended-stripline bandpass filter (BPF), microstrip low-noise amplifier (LNA), and input/output matching networks, an electrically tunable L-band preselector can be assembled with typically 3-dB bandwidth from 18 to 24 MHz.

The tunable BPF is split to provide partial selectivity with minimum insertion loss prior to amplification for improved input noise figure. The first two-pole filter before LNA prevents undesirable signals from overdriving the LNA. The second three-pole filter after the LNA provides selectivity against receiver image and spurious frequencies. The three-pole BPF placed after the LNA has a negligible effect on the overall preselector input noise figure.

A conventional combline filter (Fig.1) consists of a set of parallel-grounded resonators loaded by the variable capacitors made of tuning screws. Combline filter can be realized on different transmission lines. Suspended stripline provides high quality factor (Q) of approximately 500, stability over a wide temperature range, and high impedance range.⁶ In the high-Q suspended stripline (Fig.1b), the parallel strips are printed on both sides of a substrate in a symmetrical configuration. Plated through holes (vias) provide electrical connection between top and bottom conductors. When dual-center conductors are located symmetrically with respect to each other, they are excited in phase, causing most of the electromagnetic field to propagate in the air dielectric. Therefore, substrate dielectric losses and dielectric constant variations have negligible effects on the attenuation and phase velocity of the transmission media.

Suspended stripline resonators are placed between two parallel ground planes. Adjacent suspended stripline resonators are coupled by the fringing fields. The typical length of the combline filter resonators is between Λ₀/16 and Λ₀/6, where Λ₀ is the center guide wavelength at the resonator. For this resonator length, magnetic coupling predominates.⁷ The minimum practical length of the resonators is limited by the Q. Practical Q values are dependent upon the ground-plane spacing (base), the frequency of operation, the finish of the ground surface, the plating material of the printed-circuit board (PCB), and the suspended-stripline structure.

Short resonators yield a compact structure with excellent stopband performance. With a resonator length, l, of l =Λ₀/8, the second passband will appear at better than four times the fundamental operating frequency, while at l = Λ₀/16, the second passband will occur at more than eight times the fundamental frequency. Combline filter trade-offs for various resonator lengths are described in ref. 4.

The bandwidth of a combline filter is a function of the ground-plane spacing, b, to wavelength ratio, b/Λ₀ and the spacing, S, between resonators. The bandwidth increases with higher S and b/Λ₀. For combline filters, bandwidths of 2 to 50 percent can be achieved.

The spacing (b) between two ground planes (cover and housing) defines resonator impedances and lengths and maximum power and Q. Resonator impedances range from 70 to 140 Ω at frequencies (f) of f < 1 GHz. Large bases (ground planes) lead to higher power handling and increased Q, but also to an increase in resonator length and housing height.

Spacing S (between resonators) is proportional to base b. For a distance between printed resonators and ground planes of b/2 = 200 mils, with a substrate thickness (h) of 10 mils, the base should be equal to b + h = 410 mils. For these conditions, the resonator impedance is approximately 100 Ω (an admittance of 0.01 Ω⁻¹.

The loading capacitance for each combline resonator is¹:

where:

Y_I = the admittance of the ith resonator when the (i − 1)th and (i + 1)th resonators are shorted and θ₀ = (2πl)/θ₀ = the electrical length at the center frequency.

For 1-GHz suspended-stripline resonators with the above dimensions, the guide wavelength is equal to θ₀ = 28.8 cm. According to Eq. 1, the total capacitance, C_TOT = 2.75 pF for a resonator length of θ₀ = 30 deg. (l = Λ/12).

Usually, capacitors are also used as tunable elements to compensate for production tolerances. The use of capacitors is especially critical for narrow bandwidth. At low frequencies, capacitor Q's are higher than the Q's of resonators. At microwave frequencies and for higher capacitance values, capacitor Q's can be lower than resonator Q's, dominating performance when filter losses are calculated.

Table 1 compares experimental results for tunable combline filters with Λ/12-long suspended-stripline resonators and with air-dielectric trimmer capacitors (Giga-Trim products from Johanson Corp., Boonton, NJ) with capacitance range of 0.4 to 2.5 pF.

Figure 2a shows an electrically tunable BPF consisting of suspended-stripline resonators grounded at one end, high-Q GaAs varactor diodes, and lumped-element loading capacitors between the ground plane and the other end of each resonator. The tunable BPF was realized with reverse-biased varactor diodes D₁, D₂, D₃, D₄, and D₅ which were used as tuning elements to adjust the combline passband over the full frequency range. Tuning is performed by altering the bias of the varactor diodes. A two-pole BPF fabricated with trimmers provided a 3-dB BW of 41 MHz with 1.2 dB insertion loss. A similar two-pole BPF filter with varactors yielded a 63.8-MHz BW with 1.85-dB insertion loss.

Preselector selectivity depends on filter Q. The total Q of the tunable BPF is taken as the combination of the resonator, loading capacitor, and varactor diode Q's. For a low-loss L-band combline filter, the varactor diode Q is an very important parameter. The Q of the best varactor diodes is lower than that of the suspended stripline resonators and loading capacitors, and is a dominating factor when calculating filter losses.