High Power VHF Frequency-Hopping Filters With High Suppression of Second Harmonic

Abstract

A compact helix structure implementation and associated design formula of lumped element second-order bandpass filter circuit for high power frequency-hopping (FH) filter are proposed in this paper.The filter schematic provides one, two or three finite transmission zeros (Tzs), and these Tzs locate in the upper stopband to improve the rejection of stopband above the center frequency, especially the suppression of second harmonic with two Tzs. The filter is built on a common grounded helix coil of inductive coupled resonator tanks whose suspectance is tunable. Due to the parasitical capacitance of the helix coil, the filter has a feedback capacitor between input and output. Its working mechanism is revealed both mathematically and graphically. The measured results have a good agreement with the 3D full-wave electromagnetic simulation responses. The experimental filter has an insertion loss < 1.2 dB, a return loss > 15 dB, a 3-dB bandwidth of 5.8% ∼ 8.3% over entire operating range with the power handling capability greater than 49 dBm and the suppression of second harmonic better than 66 dB.

Tunable VHF Miniaturized Helical Filters

“…Furthermore, it shows a significantly reduced form factor when compared with filter geometries that are based on waveguide or helical resonators [20] while featuring a much higher figure-of-merit. Note that the tunable helical filter described in this paper features an average figure-ofmerit ( /volume) of 15 cm (calculated assuming an average factor of 321 at a center frequency around 300 MHz).…”

“…Note that the tunable helical filter described in this paper features an average figure-ofmerit ( /volume) of 15 cm (calculated assuming an average factor of 321 at a center frequency around 300 MHz). Since the filter in [20] is reported at a different frequency (50 MHz), we scale its expected at 300 MHz assuming that the filter occupies the same volume and its quality factor scales proportionally to the square root of frequency. This assumption yields Fig.…”

“…Several resonators and filters of this type were presented in the open literature with factors 800 [18], [19]. However, helical resonators with reconfigurable characteristics have not yet been reported with the exception of a single two-pole filter in [20] whose center frequency was tuned by connecting a p-i-n switchable capacitor array at the terminal ends of the helix. The filter showed wide 0018-9480 © 2014 IEEE.…”

“…The majority of them feature compact size, fast tuning speed, and relatively wide tuning range. However, their operation is limited by high insertion loss (quality factor: 20-50) and low RF power handling (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30). Representative filter topologies of the aforementioned tuning concepts can be summarized as follows.…”

“…However, this might come with the expense of large physical volume. Helical resonator filters have been extensively utilized in ground-based mobile communication systems due to their advantages toward compact physical size (compared with conventional approaches using quarter-or half-wavelength-long waveguide resonators) and high factor (compared with lumped or planar transmission-line resonators), [16]- [20]. Several resonators and filters of this type were presented in the open literature with factors 800 [18], [19].…”