Hi John,
Yes all those Ten-Tec rigs have a first IF of 9 MHz, which allows the
use of economical narrow crystal filters right after the first mixer,
before most of the active circuitry of the IF. This is a great advantage
compared to most rigs capable of general coverage receive, which usually
have a first IF of 45 or 70 MHz, where narrow filter are more difficult
to make.
The first filter after the first mixer in a superheterodyne receiver was
not always called a roofing filter.
Once upon a time the term "roofing filter" was used in FM microwave
radio systems with analog multiplex audio channels. These systems were
designed for a certain maximum bandwidth occupied by the FM microwave
signal, and a maximum number of 4 kHz wide audio channels (actually SSB
signals) modulating the microwave carrier. There was a low pass filter
used on the output of the receiver's FM discriminator, and also probably
one at the input to the wobbly oscillator in the transmitter, that
limited to top multiplex channel frequency that could be used on that
system and kept higher frequency noise from the discriminator out of the
baseband, or from the baseband out of the modulator. Since the filter
cut off anything ABOVE a particular frequency, it made sense to call it
a ROOFING filter.
There are probably similar low pass used in digital microwave systems
between the IF and the ADC, and no doubt the terminology carried over
into the newer digital systems. All these microwave systems I refer to
have bandwidths of hundreds of kilohertz or even several megahertz.
Ten-Tec may be the first to use the term "roofing filter" for a narrow
bandpass filter used in front of an ADC in a narrow band (relatively at
least) receiver.
DE N6KB
John Geiger wrote:
In looking over the reviews and other on-line sources
for rigs, it looks like some of the older Ten Tecs
(Omnis, Corsairs I and II, but not the Paragons) use a
low first IF (9 mhz or so) with crystal filters there,
as well as in the 2nd IF. So the filters in the first
IF would be a roofing filter, correct? Just like what
Icom and Yaesu are now discovering, if I understand
the roofing filter concept. This should make the close
in spacing on the Corsair II and these other rigs very
good, correct?
73s John W5TD
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