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Spectra VX
3
User’s Guide
response, so that a strong ground and weak quarter response
would look like:
The detector will see both signals at the same time, and the
combination of the two can be represented with a third vector as
follows:
The resulting signal appears to be a fairly strong ferrous tar-
get instead of a quarter. This is the downfall of the old TR-dis-
criminator designs. Fortunately, since the VDI response for
ground is usually far away from the response of desirable tar-
gets, there are ways to deal with it. In a modern VLF motion
discriminator, the receiver determines what part of the signal is
the ground response and, using special filter techniques, nor-
malizes the whole VDI scale to the current ground signal,
resulting in the ground signal being ignored. Graphically, this
looks like:
Weak
Quarte
r
Strong Grou
nd
-95
+95
0
Weak
Quarte
r
Strong Grou
nd
-95
+95
0
Resultin
g Signa
l
