2712 Bandpass Filter
Small footprint, up tp 15kW with blowers
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Shively Labs bandpass filters are designed as a one-purchase solution to any station’s filtering needs. Unlike a notch or single-reject filter, a bandpass filter isolates a transmitter from any interfering signal, ensuring that spurious emissions are completely eliminated, not only at the time the filter is installed, but even if new stations are added to the site in the future. If a station moves to a new site, the bandpass filter can be moved along with it, providing the same protection at the new site without needing to be retuned, regardless of the frequencies present.
Shively bandpass filters are the most transparent on the market, ensuring less signal distortion than any other type of filter. They are an ideal choice for installations where premium sound quality is important. Each Shively bandpass filter is custom-tuned to ensure that all critical electrical performance characteristics are in balance. No characteristic is optimized to the detriment of the others. This results in performance that is unsurpassed in the industry.
How Many Poles?
The number of cavities or resonators for a particular bandpass filter installation is a trade-off. Adding cavities or resonators increases isolation in close spaced frequencies. However, it also increases group delay, size, and cost of the filter. The graphs below compare isolation and group delay for typical bandpass filters.
In cases where additional isolation is needed for a specific frequency spaced within 1.8 MHz of the primary frequency, feedback loops can generally be used in place of additional cavities/resonators. For most applications, 3 cavity/resonator bandpass filters, with or without the addition of feedback loops, will provide the optimum performance vs cost as well as ensure that the filter is still appropriate if additional stations are added to the site.
Feedback loops are available for 3- and 4- cavity filters to supplement isolation of the bandpass filter at a specific frequency. They are most effective when used within 1.8 MHz of the primary frequency for resonant cavity filters and 1.6 MHz for interdigitals. Feedback loops increase isolation without narrowing the passband and adversely affecting the group delay. Feedback loops replace supplemental notch filters that were used on older systems.
2500- and 2914-Series bandpass filters use resonant cavity technology. Shively Labs pioneered the use of square resonant cavity technology in the 1980s and it remains the optimum configuration for filter applications where minimum insertion loss is required.
2600-Series bandpass filters use interdigital technology. While interdigital technology is commonly used at higher RF frequencies, Shively Labs is the only company offering it on standard production filters in the FM band.
Interdigital filters are an ideal choice for broadcasters with space restrictions, offering virtually the same performance as resonant cavity designs, but at a fraction of the size. In general, an interdigital filter has a smaller footprint but slightly higher insertion loss than the equivalent resonant cavity filter.
All Shively Labs bandpass filters are fabricated using modern temperature-compensation materials and techniques. These filters are designed to operate at surface temperatures exceeding 160° F (71° C). Shively Labs filters are an ideal choice for sites designed to operate from cold starts, even at marginally heated sites, as retuning is not required as temperatures rise. Careful attention should be paid to cold sites and units placed near ventilation systems to ensure that the filters remain free of moisture at all times.
Forced-air cooling can be used to keep the units operating at typical transmitter room temperatures, but is not necessary for electrical purposes. In the unlikely event that the forced-air cooling on a Shively filter should fail, the filter is designed to continue operating at rated power. A slight increase in VSWR may be experienced on some closely-tuned filters, but this increase should not generally cause any foldback in the transmitter.
Shively Labs bandpass filters should be maintained and operated at 60% or less relative humidity.
All Shively Labs bandpass filters are completely compatible with IBOC.
Last updated: 9/01/17