Jim. That was good! Now, define "conversion". Sounds like at least 2 to me
in the Corsair. Input freq. to 9mhz then 9mhz to 6.3mhz. Then again, with
mixers at each end of the 6.3mhz filter....maybe triple conversion?
Can you give any credit to that passband tuning gizmo for enhancing that
weak signal? I think it adds a bunch of white noise. Maybe a variable BFO
would have been a better design. On those older TT rigs I always found
myself wanting a variable frequency audio filter and not being stuck with
750hz which is nerve racking to my ears. A built in Autek QF-1A would have
been ideal!
Steve Ellington N4LQ.
----- Original Message -----
From: "Jim Reid" <jimr.reid@verizon.net>
To: "George, W5YR" <w5yr@att.net>; <geraldj@isunet.net>
Cc: "Stuart Rohre" <rohre@arlut.utexas.edu>; "N1EU" <n1eu@yahoo.com>;
<tentec@contesting.com>
Sent: Tuesday, April 02, 2002 8:46 PM
Subject: Re: [TenTec] what is "quiet"?
> 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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