The R-4A HAS a 'roofing' filter, as do the R-4, R-4B and R-4C. It's T6,
right after the 1st Mixer.
If you want a flexible receiver, it needs to be wide enough to pass both
sidebands of a signal to allow the Passband Tuner to work. It also
needs to be wide enough to pass impulse noise without stretching or
delaying it to allow the Noise Blanker to work. You can keep it wide,
and steepen the skirts for some improvement, by adding additional
poles. However, this increases the group delay to the point that the
Noise Blanker is no longer effective as the pulse and blanking signals
arrive at different times!
For a special purpose receiver, you can make it narrower if you wish,
only ONE sideband's width (2.4-2.7 kHz) or narrower still (600 Hz), and
give up useful Passband tuning and/or a Noise Blanker. There are
situations where that is desired, such as high density, low band
contesting, but is rarely of much value in today's relatively uncrowded
bands.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Neil M Califano wrote:
Where in a schematic would a roofing filter be located?
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