Roofing Filters; What Is A Roofing Filter; Main Band Roofing Filters - Kenwood TS-990S In-Depth Manual

03 RECEPTION

Roofing Filters

What is a Roofing Filter?

This section describes the roofing filter, which is the most important in the main band receiver. The roofing filter is a filter
placed at the ceiling, or the "roof" of the intermediate frequency (IF) circuit of the receiver. In other words, this represents
a narrowband filter where the signal converted to the intermediate frequency first passes through. A received signal
includes a number of strong adjacent signals other than the desired signal, thus attenuating the undesired signals in
an early stage in a preceding circuit enables the signal to not be distorted in the subsequent amplifying circuit. The
high intercept point obtained by the high‑performance first mixer gives full play to its ability in a frequency domain
slightly distant from the desired signal attenuated by the roofing filter. If the level of adjacent signals inevitably passing
through the roofing filter is high, it causes distortion to occur in the subsequent IF amplifier, mixer, and other circuits,
and the dynamic range deteriorates on the adjacent frequencies. This is why the roofing filters must be narrowed to their
minimum and attenuated to the steepest direction.
For the main band, 8.248 MHz is selected for first IF frequency, and for the sub band with the down‑conversion setting,
11.374 MHz is selected for the first IF frequency; these first IF frequency settings enable filters with good attenuation
characteristics in a narrowband and fewer characteristic changes by temperature.
Note:
◆ For example, narrowband filters with a high frequency, e.g., 73 MHz, may be implemented, but such filters have large
characteristic changes by temperature and cause problems such as increased attenuation, thus do not serve for practical
operation.

Main Band Roofing Filters

Five roofing filters are used as narrowband filters of high IP characteristics implemented by the full down‑conversion
method. The "Photograph of Roofing Filter in the Main Band Receiver" shows the roofing filters. In the photograph, the
15 kHz, 6 kHz, 2.7 kHz, 500 Hz, and 270 Hz bandwidth filters are mounted from the lower right side. In that order. The
15 kHz and 6 kHz bandwidth filters are 4‑pole monolithic crystal filters (MCF). The 2.7 kHz bandwidth filter is a 6‑pole
monolithic crystal filter. The 500 Hz and 270 Hz bandwidth filters are structured as the ladder type to suppress in‑band
insertion losses and steeply shape the adjacent out‑of‑band attenuation characteristics. The "Frequency response of
the 500 Hz Bandwidth" and "Frequency response of the 270 Hz Bandwidth" figures show the ladder filter frequency
response.
The roofing filters for the main band receiver can be selected regardless of operation mode (excluding FM mode).
Pressing and holding the [FIL/SEL] (M) key opens the RX Filter screen on the main display. The screen allows you to
select the roofing filter to be used.
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Fig. 13 Photograph of Roofing Filter in the Main Band Receiver