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If a mode-appropriate roofing filter is
substituted for the 15 or 20 kHz wide roofing
filter at the first I-F, the result is that close by
loud signals do not compromise dynamic
range or third-order intercept point. A 2.4
kHz crystal filter will not allow loud signals
that are 3 or 5 kHz away from the target
frequency to compromise the overall
performance of the receiver. This is where
every other HF transceiver that has come
before ORION II is deficient. Imagine how
much worse the receiver performance of a
competitor's radio can be in the presence of
many loud signals across the band (like in a
major contest).
ORION II is equipped with a total of seven
available crystal roofing filter slots. Four of
the seven crystal roofing filters are standard;
three are optional. The standard roofing
filters included are 20 kHz, 6 kHz, 2.4 kHz,
and 1 kHz. Optional filters are available at
1.8 kHz (model 2000), 600 Hz (model 2001)
and 300 Hz (model 2002).
Note that the 20 kHz and 6 kHz roofing
filters are of limited utility for maintaining the
overall receiver performance level of a high
end HF transceiver like ORION II. They
were included only because AM and FM
operation would require them, and because
some operators with an interest in "hi-
fidelity" SSB audio will require receiver
bandwidths higher than the typical 2.4 kHz
communications grade roofing bandwidth
would allow. Certainly the use of either a 20
kHz or 6 kHz wide roofing filter has the
potential to allow overall receiver
performance (dynamic range and third-order
intercept point) to be seriously compromised
by loud close by signals. For serious
receiver use, like weak signal DXing and
contesting, a much smaller roofing
bandwidth than 20 or 6 kHz is necessary. In
ORION II for SSB use, it can be as little as
1.8 kHz for roofing. For CW, it can be as
little as 300 Hz, depending on the
installation of optional filters.
For some recommended real-world
examples of how roofing filters affect overall
receiver performance, please look at recent
ARRL Product Reviews from QST magazine
where dynamic range and third-order
intercept are measured at 20 kHz and 5 kHz
566 manual
Part #74410
Printed in USA
signal spacings. For our competitors'
transceivers, the 5 kHz spacing numbers are
always significantly worse than the 20 kHz
spacing numbers – this is because of the
presence of test signals under a 15 to 20
kHz wide roofing filter vs. outside the filter.
FREQUENCY STABILITY NOTES
Optimal frequency stability in multi-
conversion super-heterodyne receivers like
ORION II is a function of design. It is
affected by the choice of high or low-side
placements for the various local oscillators.
All local oscillators are first locked to a 1
part-per-million master TCXO. In the
ORION II, only the 1
oscillators have a significant effect on the
frequency stability. The 3
subsequent frequency translations in the
DSP) contribute only sub-Hz temperature
drift and can be essentially ignored.
The first LO (developed by the PLL) is
placed approximately 9 MHz above the
operating frequency. It tracks the 1 ppm
drift of the TCXO, so its maximum frequency
error is 10.8 to 39 Hz for operating 1.8 to 30
MHz respectively. To cancel most of this
error, the second LO is then placed below
the 9 MHz first IF at a fixed frequency of
approximately 8.545 MHz (locked to the
same TCXO). With this low-side placement
of the second LO, the frequency drift in the
455 kHz second IF is the difference
between the two LO errors or 10.8 - 8.545
Hz = 2.255 Hz. This is 1.25ppm @ 1.8 MHz
over the entire temperature range of the
TCXO. Conservatively specifying a TCXO
with a larger temperature range than
required results in an overall frequency
stability of better than 1 ppm over 0 to 50°C.
As most ORION II's are used at room
temperature your real world stability is
substantially better than 1 ppm at any
operating frequency.
Version 4 – October 2006
st
nd
and 2
local
rd
LO (and
6
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