[TenTec] Rev. 3.033XA and receive noise level question

[Trevor Smith]

Thanks for your inputs Barry,
I spent the last two evenings continuing my quest to understand the issue (or non-issue)

1. Noise v. bandwidth - no argument here - my last 20 years has been happily doing DSP stuff for the global
mobile phone/Infrastructure industry wrestling with Mr. Nyquist, truculent PLL's, quantisation, system snr's etc etc.

2. The O-II is connected to the PC via the line out signal on "low" O/P level. Pre-Amp Off, AGC Off.
Configured the O-II with 3.029, ant #1 terminated with my XG-2 at 1uv 14080mhz. Ant #2 terminated with 50 ohm. BW >2.4khz
Centered the ant#1 XG-2 signal peak at 600hz in LCW, S-meter at S5 - captured noise floor and sig amplitude. Switched to ant #2.
No change in noise floor, S-meter at S0. Cycled through the attenuator "off to 18db" - No change in noise floor.
Cycled through default AGC - No change in noise floor.

3. Loaded 3.033XA - did not remove 50 ohm termination or turn off XG-2 or touch SpectrumLab. AGC & Pre-amp Off.
Completed S-meter cal. Ant #2 with 50ohm termination shows S3 at above 2.4khz, S0 at 100hz.
Spectrumlab noise floor display identical to 3.029

Ant #1 with XG-2 - Spectrumlab Signal/noise floor display identical to 3.029. S-meter at S5.
Cycled attenuator - noise floor raised by 10db at 18db of attenuation as seen on Spectrumlab.
(This was the noise floor difference I saw in my mail below before I realised it was on)
Cycled through default AGC - No change in noise floor.

So on *my* O-II the following observations are shown with 3.033XA:
1. A calibrated S-meter with zero signal (50 ohm terminated) rests at S3 at >2.4khz
2. The attenuator raises the noise floor 10db at 18db (but does not on 3.029)
3. AGC setting has no impact to displayed noise floor in the zero signal state for either 3.029 or 3.033XA.
4. For the same signal input SpectrumLab displays the same signal & noise floor for both 3.029/3.033XA.

Disclaimer - SpectrumLab/XG-2 is not a calibrated environment but just a basic comparison tool.

So I am going to "fake" the S-meter cal to remove the S3 and as I only use the attenuator on 160m anyway
its probably a none issue. Just curious why the attenuator introduces noise though.
I know its hot here in Texas but I doubt its a Johnson-Nyquist issue!
And one final question...why my O-II? or does every O-II with 3.033XA do this?
- 73 Trevor AB5EU

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I'll take a stab at it - you're not MISSING something with 3.033XA, you're
GETTING something.  Noise level is proportional to bandwidth and that's
what you're now seeing.  AGC threshold masked the low level s-meter action
in earlier versions of firmware.  The new firmware introduced full scale
s-meter action independent of AGC threshold, so you're now able to see the
bandwidth affecting noise level.

73, Barry N1EU

On Thu, Jul 11, 2013 at 3:07 AM, AB5EU <ab5eu at wildblue.net  <http://lists.contesting.com/mailman/listinfo/tentec>> wrote:

>/  I finally decided to try 3.033XA as it advertised a fix for the rx audio
/>/  in 3.029. Successfully updated the radio and completed the S-meter
/>/  calibration.
/>/  Master reset/memory clear - enabled RX-366.
/>/  Noticed that without the antenna connected the S-meter was at S3 in USB
/>/  mode. Further investigation determined it was not mode specific but
/>/  bandwidth specific.
/>/  Above 2.4khz = S3 resting level, 2.4 to 1khz = S2, Below 1khz to 100hz S1
/>/  to S0.
/>/  No difference between the main or sub (rx-366) receiver.
/>/
/>/  Observed the noise floor with SpectrumLab so its not just the S-meter.
/>/  Went back to 3.029 and all is well.
/>/  No noise or S-meter reading at any bandwidth. Am I missing something with
/>/  3.033XA?
/>/  - Trevor AB5EU/