[TenTec] Older Ten Tecs and Roofing filters

[Ken Brown]

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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