S meter reading

AJ5O
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S meter reading

Postby AJ5O » Thu Jul 12, 2018 5:22 am

I read this stickied post about S meter and calibrations etc but I still have a question. My 100B reads an s8 with no antenna connected. Light signals are reading 30 over on 80 meters. Does anyone elses read so high with no antenna connected?
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w-u-2-o
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Re: S meter reading

Postby w-u-2-o » Thu Jul 12, 2018 10:59 am

No, that definitely sounds wrong.

Have you tried resetting your database?
K9RX
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Re: S meter reading

Postby K9RX » Thu Jul 12, 2018 12:24 pm

I'd guess you have ATT set to -30 db?!
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w-u-2-o
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Re: S meter reading

Postby w-u-2-o » Thu Jul 12, 2018 12:30 pm

Attenuation is accounted for by the software. All readings, displays and indications are normalized to the levels at the rear panel connectors. So changing attenuation has no effect on measurement results (this is also true on Flex radios).

Put in enough attenuation and you will see the noise floor rise slightly, but that is due to the increased receiver noise figure with all that attenuation cranked in.
AJ5O
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Re: S meter reading

Postby AJ5O » Thu Jul 12, 2018 2:55 pm

I haven't reset the database yet as I was unsure if I would lose all of my current settings. If this doesn't fix it what would be my next step?
AJ5O
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Re: S meter reading

Postby AJ5O » Thu Jul 12, 2018 4:21 pm

Ok so I reset the database and still no fix. Still reading a 7 with no antenna connected.
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w-u-2-o
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Re: S meter reading

Postby w-u-2-o » Thu Jul 12, 2018 4:58 pm

OK, starting to grasp at straws...

Perhaps you have a bad power supply? What happens if you run the hardware off of a battery (if you can)?
AJ5O
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Re: S meter reading

Postby AJ5O » Thu Jul 12, 2018 5:03 pm

I know different radios read different things but I have a 7300 that doesn't do this on the same power supply
K9RX
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Re: S meter reading

Postby K9RX » Thu Jul 12, 2018 5:49 pm

ok....but if you have ATT set then the noise floor goes UP and the S meter goes up as well! So yes it is taken in to account which is what he is seeing!

I just set 30 db of ATT and my noise floor went up and my S meter went up ~ 4 S units or 21db roughly in the dbm reading...which implies I believe that I have 9 db roughly of 'safety' from where the system noise floor is and the ambient noise floor.

or what am I missing?


Gary
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Re: S meter reading

Postby K9RX » Thu Jul 12, 2018 6:07 pm

I tried the above (last post) on 17M ... and saw what I indicated ... of course those results will indeed depend on what kind of impact the NF has on the front end. Generally on the lower bands it won't impact the results ... on the higher/highest band it will... here on 10M I see 28 db difference when I engage 30 db ATT.

Gary
AJ5O
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Re: S meter reading

Postby AJ5O » Thu Jul 12, 2018 9:27 pm

Gary it seems you are right in my case. I had 30 db attenuation. Turned it to 0 and it dropped down to 2 or 3 s units with no antenna.
K9RX
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Re: S meter reading

Postby K9RX » Fri Jul 13, 2018 12:17 pm

well that's a start ... but that is still high. What band was this? And you're sure there is no antenna connected at the rear jack?

Also what filter width?

Gary
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w-u-2-o
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Re: S meter reading

Postby w-u-2-o » Fri Jul 13, 2018 1:12 pm

Into a dummy load, with no attenuation, and a 2.7KHz RX passband, the S-meter should be right around S2, S3 at most.
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DL2XY
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Re: S meter reading

Postby DL2XY » Fri Jul 13, 2018 5:26 pm

w-u-2-o wrote:Into a dummy load, with no attenuation, and a 2.7KHz RX passband, the S-meter should be right around S2, S3 at most.


Way too much.

Just checked on 80m, no ATT, 50Ohm termination at 26°C .

2700 Hz BW: -125 dBm (S-Meter, Avg)
1Hz BW: -159 dBm (Spectrum Display, AVG, 1Hz normalization active)

This is a NF of 15dB.

I did a fresh level calibration before.

73 Walter
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w-u-2-o
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Re: S meter reading

Postby w-u-2-o » Fri Jul 13, 2018 8:29 pm

S2 is not "way too much". However, my apologies for not clarifying that this is with the meter set to "Signal" (peak reading). Most people run their S-meter on "Signal", not "Sig Avg", and that is what you will get under the conditions you measured. From my second post here:

On the 8000 I have here, into a dummy load, using the standard 2.7KHz filter preset, I get -125dbm with the meter on Sig Avg, and around -117dBm on Signal (peak reading). You are probably using the Signal meter mode (peak reading) and S2 = -115dBm (close enough!)

Looking at this a bit more closely, the thermal noise floor is -174dBm/Hz and 2700Hz = 34.3dBHz. This gives a total noise power of -174 + 34.3 = -139.7dBm in a 2.7KHz bandwidth. Using the Sig Avg meter reading from above, that implies a receiver noise figure of about -125 less -139.7 = 14.7dB, which is about what you'd expect. Note that approx. half of that noise figure, 6.2dB, is from the pre-amp, which is an LTC6400.

With the S-meter in "Sig Avg" mode, it will, of course, display between S0 and S1.

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