A Streamlined Signal Preconditioner
1 U (1 3/4" = 1 Rack Space) For 19" Rack Mount
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The Type-6 Preselector designed to occupy 1 RU and complement 3 RU Professional Receivers for installation in a 4 RU travel case
(Road Case). Typically, this unit will be packaged with the Racal shown below.
Close view of left side of the Type-6 panel.
Close view of right side of the Type-6 panel.
Details of the Noise
Filter section.
Each of the 5 types of
filters have variable
"Depth" control.
Some of the initial
R&D was conducted
with the "all-out"
Type-3 Preselector.
Note that the Null (or
Notch) function is not
used in the Type-6.
See more of the
Type-3
here.
A Racal RA6790/GM is
used for all final
testing.
This is the Type-5
Preselector in use
with a JRC NRD-515.
Some of the higher-Q
selectivity is used
from this design. See
more about the Type-5
here.
This is the Type-2
Preselector, which is
the simpilist version
of the circuitry. It
usually sits atop this
NRD-535 in the shack.
See it
here.

Likewise the Type-4
(not shown here)  
accompanies the
NRD-545 in the family
room. See more
here.
The boiled-down
schematic.
(If your
screen settings don't
allow a legible view, just
right-click the image and
paste it in your photo
viewer and enlarge as
necessary.)

Note that all functions
may be bypassed
however, even when
all are in that mode,
the Attenuator
remains active,
unless you use the
System Bypass
switch. This design
philosophy allows the
"set-and-forget" use
of the controls, which
I have come to prefer
over the
re-adjustment of all
the functions every
time they are needed.
At left is the standard
Noise Abatement
circuit I designed a
few years ago. The
ANL allows the usual
diode clipping of
transients riding
above the signal's
carrier in the
traditional way, but
with an
amplitude-selective
function which
fine-tunes the clipping
activity in width,
depth, sensitivity and
threshold.

The DCB section
mimics the tried and
true NB operation, but
is biased to allow a
knife-edge adjustment
of Sensitivity and
Threshold. More
details
here.
A Note on Analog Noise Abatement
The above described circuits are entirely executed without the use of DSP - there is no signal conversion in the modern sense
where A-D, and subsequent D-A, conversion is employed. There are a few reasons for this, but primarily; (1) philosophical
reasons to keep the entire signal chain in its natural analog form. and (2) to not have any digital processes near the signal path,
which may possibly insert additional noise, or distortion from digital artifacts, into the path. Of course DSP has been used for
many years with every range of utility, from poor to excellent. Those devices are known and available, and that is why the
above circuitry is shown separate and optional to the Type-6 Preselector.
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