Ok, so which is more quiet: my RX-340 or my Corsair II?
Quick answer, just after turning the two rigs "on": the Corsair II.
But, now some experiments, and discussion.
The receiver "plans":
RX-340 (First deliveries were in July of 2000, desinging
continued well into the Spring of 2000)
The 340 antenna input signals enter a bank of 0.5 octave
filters covering from .05 to 30 MHz; then selectable preamp,
15 dB atten or bypass followed by 6 push-pull FETS driving
the 1st mixer going up to 45.455 MHz. Next come 6 more
push-pull FETS to drive two 2-pole 20 kHz wide filters. Out
of the filters into a 12dB gain amp to drive mixer 2. Now at
455 kHz, next enter a series of 4 single ended FET amps and
into a ceramic 16 kHz BW, 455 kHz filter. From here, the
signal enters the first AGC gain controlled amplifier which is
controlled to have gain between 0 and 80 dB. The output
of these AGC's amplifiers enters another 16kHz BW, 455 kHz
ceramic filter, and out into the 3 rd mixer down to the final
analog IF at 16 2/3rds kHz. Out of this mixer, the signal enters
an "anti-alias" low pass filter and enters the A/D converter.
The 340 BFO's are synthesized.
The DSP of the 340 provides a selection of 57 bandwidths:
from 16 kHz down to only 100 Hz.
Corsair II (First deliveries Spring/Summer 1986)
The Corsair is basically a single conversion rig. All IF
amplifiying, filtering, etc. goes on at 9 MHz. However,
there are two more mixers used! They are on the Pass Band
tuning board. These mixers provide an added up/down
conversion to sweep signals across the bandwidths of
the "optional" filters which follow the "standard" Ten Tec
8 pole crystal filter which establishes the basic IF bandwidth
on receive AND acts as the SSB filter during transmit. From
the pass band tuning/filter board, the signal passes through
two AGC gain controlled stages which are on the IF/AF board
which has the final BFO injection stage, "RF" gain control, audio
notch, S-meter cpntrol, audio gain, CW sidetone and an audio
Band Pass filter control.board unit.
The Corsair uses a PTO LO, a la Collins and others of the era.
This audio Band Pass filter control deserves special attention in
this instance of conern of quietness on reception of very
weak signals in the presence of noise. This audio frequency
filter has a fixed center frequency of 750 Hz ( the CW offsett
frequency delta) but can be controlled to produce a flat audio
response or one that increasingly peaks at 750 Hz until it
reaches a bandwidth of about 200 Hz with skirts rolling off
at 24 dB down per octave.
Comaprison Test Signal I used.
I tuned to a 10 meter beacon signal of only 10 watts located
about 4800 miles from my QTH. The receivers are here at
my QTH on Kauai, the beacon is 12 or so miles West of
Pittsburgh, PA, signs as W3HH/B on 28.269 MHz.
The rcvd signal level here from the beacon varied from the noise
level, in whatever bandwidth was tried on up to around -120 dBM
or so; -120 dBm is just about S 1 if a 50 uV (near - 75 dBm)
signal at 50 ohms is S 9. The noise floor from the RX-340
during this comprison test, using 100 Hz bandwidth, was
at the bottom of the scale, just a bit of jiggle around -140 dBm
on the meter, so at moments the S/N ratio was nearly 20 db
using the 340 from the 10 watt beacon in PA; and the same
from the Corsair.
Ok, after all that: when first turning the rigs on, tuning
in to the beacon and using the "normal " CW settings
for both radios, the noise from the 340 speaker was LOUD,
while the noise from the Corsair was very noticable. There
was frequent QSB and often the CW beacon would disappear
into the noise.
The ONLY way to obtain 100% copy on either rig was as follows:
As the Corsair has a fixed CW offset tone of a 750 Hz audio note,
I set the 340 for a BFO offset also of 750 Hz.
RX -340 was set all the way down to 100 Hz bandwidth for solid
copy; the signal was "eaten" by the noise as the bw was increased
to about 500 or so Hz. The IF gain was full up on the 340 for this
comparison.
An essentially identical S/N was found using the Corsair when
both the 250 Hz CW filter and the audio Band Pass filter
was used. Amazing was the near complete disappearance
of noise with the signal, except on very deep fades, with
the Corsair BP Filter control at full clockwise setting or with
the most narrow 200 Hz bw audio setting. I would judge
the "sounds" from the two rigs at the above settings to
be essentially identical. Seems they should be, as probably
the "effective" bandwidth of the Corsair with both IF and audio
filters at the most narrow was near 100 Hz as well.
HOWEVER, when the signal got really weak (no wiggle at all
on the 340 S meter), it could not be heard from the 340,
but could "just" be heard from the Corsair.
So, will I abandon thed 340 to the Corsair.....no, it does so
many other things well besides the very weak CW signal copy
case. The two radios are really an apple and an orange, and
probably should not be so compared. But then the question:
should the RX-340 be used as an amateur radio receiver?
That is up to the user/owner to decide, hi.
Hope this is interesting/useful.
73, Jim KH7M
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