A roofing filter is the first filter the signal reaches that is similar
bandwidth to the signal in the receiver. The closer it is to the antenna
the least effort the following stages have in handling strong adjacent
frequency signals. Ideally that roofing filter can be the same bandwidth
as the final filter, but that can reduce the receiver versatility, e.g.
with a CW bandwidth roofing filter, copying SSB becomes impossible. And
if the noise blanker pulse detection takes off after the roofing filter,
it won't work with that narrow a roofing filter. So there are trade
offs in the selection of the roofing filter.
In some receiver designs there are three mixers (always the most
limiting stages for noise generation and maximum signal handling) before
significant selectivity. Modifying those designs with a 15 KHz bandwidth
roofing filter (so FM will pass) up front, providing some of the fine
tuning isn't done with a later oscillator makes the receiver handle
stronger unwanted signals better.
For decades, TenTec receivers have had the main selectivity filtration
right after the first mixer, the normal location for a roofing filter.
That's not the case in the DSP based receivers so they add a roofing
filter. Many other brands put the main selectivity two or three mixers
later than the first mixer and need some sort of a roofing filter up
front, but its hard to get tight selectivity with the first IF at 45 MHz
so those receivers have much better intermod performance beyond 20 KHz
than close in.
73, Jerry, K0CQ
On 8/20/2010 9:47 AM, JOSEPH DAVIS wrote:
What is a roofing filter and its advantages. thanks jjdavis
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