[Elecraft] Roofing Filters

[Ron D'Eau Claire]

Can someone explain the meaning of "roofing filters". I have searched
everywhere and can't find a good explaination. Lisa recommended the FAQ
secftion of Elecraft and there they just mention them. Maybe I should order
an updated Ham bible from ARRL.

--------------------------------------------

The K3 receiver, like many current superhetrodyne receivers, has more than
one high-performance filter in the system. 

I don't know how long you've been tinkering with superhets, but a few
decades we ago we just called the "roofing filter" the first I.F. filter. 

You probably know  that a superheterodyne receiver takes the selected
incoming signal, no matter where it is in the tuning range of the receiver,
and converts it to one fixed frequency before it is demodulated. That fixed
frequency is called the intermediate frequency (I.F.). 

>From a design standpoint, it's a good idea to put all the selectivity as
early in the receiver chain as possible so the first I.F. filter has been
the most important in the system. Indeed, we normally tried to do all  the
significant filtering there. That's who the K2 is designed. 

Nowadays higher performance amplifiers and mixers allow more signal
processing before it's necessary to use a filter to strip off all but the
exact signal we want to hear. 

We use the first I.F. filter to reject signals that are completely outside
the range of interest, then use a second filter further along in the signal
path to finish the filtering to set the final bandwidth to just what we
want. 

The first I.F. filter, therefore, defines how far off each side of the
center frequency we can "hear". That is, it sets the limit or "roof" on the
bandpass. (I think of it as the upper and lower "sides" to the bandpass, but
no one checked with me before choosing the name "roofing filter".) 

The K3's second filter is provided by digital signal processing (DSP) and
allows controlling the bandpass and center frequency to set it anywhere
within the range of the first I.F. (roofing) filter bandpass. 

Even though mixers and amplifiers are much better today than a few years
ago, it's still a good idea to reject all unwanted signals as early in the
signal path as possible. That's why the K3 offers up to five "roofing
filters" of various bandwidths to fit the sort of signal you're receiving
from very wide FM signals down to a very narrow CW/PSK signal. Within the
bandpass of the roofing filter, the DSP filter provides additional
flexibility to set the exact upper and lower frequency limits, provide a
notch filter to remove a specific heterodyne, etc.

Ron AC7AC



-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of AnnLatz
Sent: Wednesday, February 20, 2008 11:38 AM
To: elecraft at mailman.qth.net
Subject: [Elecraft] Roofing Filters


Can someone explain the meaning of "roofing filters". I have searched
everywhere and can't find a good explaination. Lisa recommended the FAQ
secftion of Elecraft and there they just mention them. Maybe I should order
an updated Ham bible from ARRL.

Thanks, Alan KA9UCP _______________________________________________
Elecraft mailing list
Post to: Elecraft at mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft    

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com