S meter still not correct on FLEX6600M with Version 2.3.9

WX7Y · July 2018

On Version 2.3.9 the S Meter and Pre-Amp is NOT working correctly.
On the 6600M to see what I am saying, terminate the Antenna and go to the Attinuator and Pre-AMP settings and what I am seeing here is that -8 gives me a S4, 0 gives me S3, +8 gives me a S2, +16 gives me a S1, +24 gives me S1, +30 gives me S1.
So where it is the most important on the lower bands to NOT run the Pre-Amp the S Meter reads 2 to 3 S unites higher then it is supposed to.
Test where run on 40Meters.

My FLEX 6700 when doing this exact same test with the Pre-Amp settings up or down all settings read S1 like it is supposed to.

I have done a hard reset of course and find this the same either Direct connect or through SmartLink connection's.

Thanks
Bret
WX7Y

Bill-N6RV · July 2018

Actually it worked the same way on my 6600M when I had it. Rob Sherwood also confirmed this.
What is being measured is the noise floor which is a function of the receiver noise figure. The lower gain settings have higher noise figure hence higher noise floor. The noise figure in the lower gain settings is very large. According to K5SDR's analysis the noise figure in the -8 gain state is 40 dB (10 meters). The noise figure in the +32 dB gain state is on the order of 10 dB. There could be as much as a 30 dB variation in noise floor over the gain settings.

Another difference between this reading and readings from older radios is the Flex Radios actually reference the signal level at the antenna input port. When gain is removed the signal level is adjusted proportionately. For instance, a signal at -73 dBm (S9) reads -73 dBm regardless of the gain setting. In older receivers the signal level would vary. In fact the noise varied just the opposite. With the preamp on the noise would read higher and as the gain is decreased and attenuation added the noise reading would drop.
The real test is signal to noise. Whether or not in a quiet area the gain must be set so to minimize the receiver contribution to the total noise.

In a noisy environment the noise from the antenna should exceed the noise from the receiver so the S meter will not drop below the value you are seeing when the receiver is terminated. I think Flex has stated that the terminated receiver noise should 16 dB below the noise with the antenna connected. That would give a 0.1 dB degradation to the SNR. In any event on the lower bands the noise is usually above S4. Also as you narrow the bandwidth the noise reading decreases.

As far as the 6700 maybe the noise figure is better in the lower gain states (The gain distribution is different). I never looked at a 6700.
BillN6RV

Steve-N5AC · July 2018

Most radios don't adjust for the gain in the preamp when you turn it on. As a result, the gain makes the noise floor RISE. This is actually not what is happening in the radio. A preamp should lower the noise floor. All FlexRadio transceivers account for this and so the noise floor will generally go down, like it actually does, when you enable the preamp.

However, there are two things that will stop the noise floor from going down: 1) atmospheric noise or 2) you've reached the limit on how low the noise floor can go given the noise figure of the radio and the cascade noise figure of the preamplifier with the receiver.

S meter still not correct on FLEX6600M with Version 2.3.9

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