Why does my S-meter on my 6400M show S-7 with a dummy load?

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  • Updated 4 weeks ago
With a dummy load with no coax:
160 meters S6
80 meters S7
30 meters S3
20 meters S4
10 meters S2
6 meters S0
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Burt Fisher

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Posted 1 month ago

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Bill -VA3WTB

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Did you check to make sure the heatsink is in place in the radio?
Is it the same while the antenna is connected?
(Edited)
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Thomas NE7X

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I am seeing the same issue, with no antenna, 6400M connected to a 50 ohm dummy load, I see S5 on 40 meters. I performed the same test on my ICOM IC-7700 and FT-2000, and they both show S0. Yes, same with or without an antenna connected.
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KY6LA - Howard, Elmer

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This is a VERY OLD SUBJECT. - suggest you do a search of the community n

The Flex S Meter reads exaxt Value of the receiver Noise Floor.

The Icom’s give you a FICTICIOUS MEANINGLESS NUMBER

The Flex is CALIBRATED S0=-127dBm. The Icom is NOT calibrated but gives you a meaningless feel good zero reading when shorted

If on say 20M the Noise Floor is -103dBM the SMeter should read S4 NOT S0
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AA0KM

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Bill -VA3WTB

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But I would not expect an s7,,,an s4 yes. An s7? is why I think about checking the ADC heat sink.
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Burt Fisher

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Where is that heat sink?
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Bill -VA3WTB

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The heat sink is on the ADC board near the front of the radio.

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K1UO - Larry

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Bert,  This picture is actually showing you a heatsink that has fallen off from the ADC chip (large black chip on the vertically mounted board) and onto the board below.   The 6400 will have only one of these ADC circuit boards.  This is a picture of a 6600.
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Joe N3HEE

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I get the following when connected to a dummy load via short coax cable.  Mode CW.

160 = S2 (-113dbm) with RF Gain at 0
80   = S1 (-119dbm) with RF Gain at 0
40/20   = S2 (-113dbm) with RF Gain at 0
15   = S0 (-122dbm) with RF Gain at 0
10/6   = S2 (-114dbm) with RF Gain at 0


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WQ2H - Jim Poulette

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The 6600M here reads between S1 and S2 with a 50-ohm dummy. I think it may have to do with the environment of the operating position. We live in a rural area, and the shack is in a fairly quiet location - in the corner of the basement, underground, surrounded by concrete on three sides. I can make the noise floor jump a little higher by turning on appliances around the house. Especially the wife's crappy little hand-held vacuum cleaner.

Even with the quiet location - when the guy on the other side of town uses his arc-welder.....I still may be the first one to notice......

73
Jim, WQ2H
(Edited)
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Bill -VA3WTB

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It is supposed to read at least an S4 without an antenna connected. The receiver is sampeling the receive bins all the time, even without an antenna there is always RF energy in the bins.
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Paul

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Bill, speaking of understanding how SDR's work; I believe the bins are in the digital domain and contain data which represents the signal & noise, not RF energy. It would be a bad thing if RF were leaking into them.
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Bill -VA3WTB

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Paul, I know you know this..lol
With our radios we measure the actual signal strength by integrating the digitized RF power contained within the FFT bins inside the RX filter.  This is how a digital signal generator operates.

What we are saying is there is always RF power inside the filter and it is additive.  The filter bandwidth is a dependency in the power measurement process.  If you make the RX filter wider, the S-meter reading increases.  If you make it smaller, it decreases.  The standard is to measure integrated power in a 500 Hz filter.  And it is frequency dependent too.

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Paul

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Sorry Bill, I couldn't resist. 73, Paul
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Thomas NE7X

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Here is a suggestion:

On most of the older vacuum tube receivers, they gave the user a pot to adjust the zero of the S-meter to their local environment. Maybe Flex could do the same. Give the user a menu selection of DEFAULT or ENABLE an adjustment slider to adjust the S-meter zero. 

Most users are not looking at S-meter readings through the eyes of an engineer, they are amateur radio operators looking for what the signal strength is above their local noise floor. Let the user decide how they want to utilize their S-meter. 
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Burt Fisher

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You totally got it. I want the meter to read zero on a dummy load as far as the heat sink issue wasn't that addressed by Flex?
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Bill -VA3WTB

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Burt, yes it was addressed by Flex and the effected radios have been fixed, I was saying this because that could possibly account for a high noise floor.

I know some people want their state of the art radio to work the way older radios did that takes the signal from the AGC circuit. In those radios, any non SDR radio, there are no receiver digital bins to sample, so the S meter reads 0 on a dummy load.

In our Flexes, there is always sampling going on internally, there is always digital noise being converted to RF,,always. And every radio has it. And the Flex reports it.

Perhaps Flex should turn the signal off so It reads 0 to make people feel better.
(Edited)
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Bill -VA3WTB

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It seems to be more about understanding SDR and how real SDR works. I don't think Flex is about to hide it's accuracy in order to make the radio look and work like non SDR radios. Keep in mind even though their are other companies that apear to have an SDR radio, they take the S meter readings from a different source. Or chop off the signal so people can't see it. Flex takes it directly from the ADC as it samples the bins, the bin reading is what we are seeing.
And why adjust the meter to zero? it's never zero. Unless we fake it.
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Homer1952

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My Airspy HF+ & my SDR Play show 0.5 S units into a dummy load with SDR Console & SDR Sharp software.  I think they are real SDR Radios.  I like their approach to the "S meter", but have learned to accept (maybe too strong of a word) the Flex method.  I find the Flex method much less useful to me.  The whole S meter below S5 is useless, although Lab accurate.  No wrong answer here.  Just what you accept or expect.  - Bob W8RMV