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Optional 500 and 300 Hz filters must be
turned on in the menu before they will
activate. See ENABLE 500 Hz, ENABLE
300Hz in Section 4.
I-F FILTER AND DSP BANDWIDTH
FILTER THEORY AND USE
OMNI-VII is equipped with a series of
"distributed roofing filters" at the 70 MHz first
I-F and 455 kHz second I-F.
There are stock filters at 20 kHz bandwidth
at the 70 MHz first I-F and 20, 6, and 2.5
kHz bandwidth at the 455 kHz second I-F.
Optional filters at 500 Hz and 300 Hz
bandwidths can be also installed in the 455
kHz second I-F.
Some basic discussion on what is and what
is not important for optimum receiver
performance.
An HF receiver must have adequate
sensitivity to hear signals. The receiver also
must have reasonable dynamic range to
hear weak signals in the presence of loud
signals (the definition of dynamic range
being the ratio between the strongest signal
that can be handled without front-end
distortion and the weakest detectable
signal). Design considerations should keep
internally generated local oscillator phase
noise to a minimum.
It's no difficult feat to achieve these
requirements – most any HF receiver used
for ham radio purposes today, on face value
has adequate (or even too much) sensitivity
and enough dynamic range that a single
loud signal that appears on an HF frequency
doesn't immediately overload the receiver.
The problems start, potentially, when there
are multiple loud signals within a small
distance of a desired frequency. You may
be listening to 14.200 MHz, but that S9+40
signal on 14.210 that you're not hearing? It
still has the potential to compromise overall
receiver performance, even when it's not on
the frequency you're listening to. You might
not even realize it – a loud signal 10 kHz up
the band limits dynamic range, making a
weak signal you want to hear at
14.200...just not be there. You'd never
know – the receiver may sound "fine", but no
signal will be on the frequency you're
listening to. Worse perhaps would be not
only is no desired signal present, but that
588 manual
Part #74409
Printed in USA
loud signal and other ones near it might
even be throwing spurious distortion
products onto the frequency of interest.
What to do? Loud signals are a fact of life in
ham radio.
The answer is: find a way to protect the
overall receive chain, within the limits of the
design of the radio, to maintain adequate
sensitivity, dynamic range and third order
intercept point.
What does third order intercept point sound
like? It doesn't sound like anything – you
can't hear it, because it's calculated from a
formula that uses both the minimum
discernable signal and dynamic range to
predict a "point" at which the radio will go
non-linear and break into distortion.
The most optimal receiver protection
scheme is one that does not affect receiver
sensitivity, and maintains dynamic range
even in the presence of nearby undesired
loud signals.
Second I-F roofing filters can help limit
undesired signals from having an impact on
overall receiver performance. OMNI-VII
uses a 20 kHz wide monolithic filter at the
first I-F of 70 MHz. This filter is of little
practical value for maintaining receiver
performance – it is generally for allowing AM
and FM operation only.
Filters at the 455 kHz I-F are 2.5 kHz
(standard), 500 Hz and 300 Hz (both
optional). These filters are cascaded
together with the 20 kHz wide 70 MHz first I-
F filter when one of then is in use.
The net effect of using 455 kHz second I-F
filters over only the 20 kHz monolithic at the
first I-F is to increase blocking dynamic
range over what the receiver would be
capable of without the second I-F filters
installed. Third order intercept point remains
fairly constant with only a small degradation.
The front-end AGC in the transceiver is after
the 455 kHz I-F filters; having these filters
installed also prevents the radio from
attempting AGC action on a signal that is
outside the bandwidth of the 455 kHz I-F
filter.
These filters are not to be confused with
bandwidth filtering, which is done in DSP
and is adjusted from the front panel BW
Second release – August 2007
29
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