Flex 6400M noise floor

N4QM

My new 6400M has a -120dBm noise floor which equates to a constant S4 on the S meter with a terminated antenna input on 80M and 0dB gain.  On 10M the noise floor is -114 and the S meter reads a constant S5 under the same conditions.  Is this normal?  I have not connected the radio to an antenna yet, I am still learning all the menu options with the help from friends who have 6000 series Flex radios.  They all tell me that under the same conditions their radios are around -130 to -135 dBm, which is what I was anticipating seeing on my radio.  Is this a "normal" condition, or is something else going on with my radio?

73, de N4QM

WX7Y

Where is your pre-amp settings on those bands?
When you say "terminated antenna input"  how is it terminated?  50 Ohm Dummy load right on the back of the radio is the only way to test this correctly. 
Could be noise right in your shack getting into to the extremely sensitive receiver of the 6400 if you have any cable at all or the Antenna port is open, I have much more noise when I have  the Antenna port open to the world and not terminated.

73's
Bret
WX7Y

Rich McCabe

For what its worth into a dummy load my 6600 is almost -140 and under S1 on the bands you mentioned.

Mike KE4U

S-meters indicate Received Signal Strength per IARU standards.  My Flex 6400M running on shack emergency battery power with a shorted antenna input and -8dB of RF gain on 75M with NO back panel connectors except power is S6, so like yours is measuring something else.  What that is, I do not know. I was gonna raise this question but I was having too much fun with the rig.  Thanks - we'll have to see the technical explanation of the noise floor in these rigs.

Bill -VA3WTB

Yes, that is very normal, if you were to do a search on this there is a lot of discussion on this that explains  things, Steve at Flex has explained this in good detail, how ever some just do not believe him,,interesting...

Mike KE4U

Thanks Bill - so high S-meter readings with no antenna port signal present are normal. That's weird but I can get used to it. 

S-meter implementations 'get no respect' as Rodney Dangerfield says. My K3 S-meter changes with AGC gain! By design!

I am new to Flex and did not realize that the S-meter was not IARU compliant ( with no signal presented at the antenna port). I understand that the 6400M S-meter implementation is processing data / integrating the total RF power in the FFT bins within the receiver's passband.

I guess my remaining question is "what the heck is in the FFT bins when there is no signal present at the antenna port that integrate to a S5?"

Jay Nation

Remember also, that you need to be measuring a 500 Hz passband, when you take your noise floor measurement.
 
#Flexradio

73, Jay / NO5J

Rich McCabe

I would ask Flex to get a real answer.  It's noise from somewhere. Could be local but I would not consider it "very normal" as Bill posted.

Steve - K6SRS

This is normal 6000 operation per Flex and how the software processes things. That said, it is obviously impossible to get noise out of a dummy load.

N4QM

Thanks for all the replies.  Yes, terminated by a 50 ohm dummy load, gain set at 0dB.  I did search the community for this and got no search hits, that's why I asked.  I'm new to Flex, so there are going to be hiccups.  Radio has now been integrated into my station.  Next tasks are getting it connected to HRD and my ALS-1306.  I have leads on accomplishing those tasks.  Thanks again everyone who responded.

Rich McCabe

Enjoy Dennis.  I am sure you will love the radio.

Bill -VA3WTB

As I said this is very normal, there is no such thing as no noise. The FFT bins make noise without an antenna. If you really want the technical answers then search this subject, Steve and Tim explains all this.
for some that do not care to search i will past a few things from Tim and maybe Steve.
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.

And you dBm to S unit numbers are a little off too for frequencies below 30 MHz

S1 = -121 dBm
S2 = -115 dBm
S3 = -109 dBm

The S meter isn't broken so it doesn't need fixing.  The "fix" you are asking for would actually break the S meter by assigning an arbitrary signal strength value to the meter based on user perceived signal or comparison to an inaccurate standard (traditional radios).  Let me explain.

First, there is always signal present.  Blackbody, atmospheric noise, cosmic noise and thermal noise are "signals" at RF energies and all contribute to the RF noise floor.  Even with a dummy load connected, there is still signal being digitized.  There is no such practical thing as "no signal" (unless the radio is in an RF isolation chamber at absolute zero).

The signal strength meter design in the FLEX-6000 is based on how all lab grade digital spectrum analyzers operate; by accurately integrating the total RF power in the FFT bins within the receiver's passband.  This is a very important point.

Where a traditional received estimates signal strength by calculating a value from the AGC voltage, the FLEX-6000 actually measures the signal power in each FFT bin. Since S units are a standardized unit of measure for RF power as defined by the IARU, we calibrate the S-meter based on a standard, making the FLEX-6000 very accurate in this regard.

In the second paragraph, I emphasized an important point about measuring in the FFT bins, as opposed to estimating, RF signal within the receiver's passband.  This is very easy to demonstrate.  With your dummy load connected to the antenna port, change the slice to CW mode and change the filter to 50Hz.  At this RX filter passband size, there are only a few FFT bins measuring RF power and the S meter is barely moving.  If you put your cursor on the meter bar to display dBm, it may be as low as -134 dBm.  Now the IARU defines S0 as -127 dBm.  So the S meter reading in this case is actually less than S0!
  

Rick WN2C

Here is some additional reading on the subject.

https://community.flexradio.com/flexradio/topics/why_does_my_s_meter_read_s5_all_the_time

https://community.flexradio.com/flexradio/topics/6300-shows-s-meter-1-3-on-various-bands-with-dummy-...

I had the same question but the answer makes sense. 

k0eoo

Remember guys, as Jay mentions above, receiver noise floor measurements using the S-meter depend on the filter bandwidth setting!!  I usually put my 6500 into CW mode and the filter to 50Hz if I want the S-meter reading to come close to the dBm scale on the right side of the slice panadapter.

The S-meter reading is the rms sum of the data bins within the filter bandwidth. The wider the bandwidth the higher the S-meter reading....

So, if your Flex is reading a high noise floor with a 50Hz bandwidth then something is probably wrong, be it the radio or some noise being generated in the shack.

Bill -VA3WTB

And S5 is fine. As Flex mentions.

Steve - K6SRS

I agree with your points, but all this molecular noise is far far below the S levels flex shows. The high S unit readings are a Flex abstraction uncomfortable to many of us.

Bill -VA3WTB

Steve, there really is not anything to be uncomfortable about, but I do know it is hard to wrap our heads around. this is much different than how the older receivers work. Direct sampling technology simply works this way as it always samples the FFT bins witch always have rf in them all the time.
This is how a digital signal generator operates.

Rich McCabe

So do you suggest those with S1 noise levels fill out a help desk ticket since that is not how a digital signal generator operates??

Rich

Rich McCabe

Just asking for a friend.

Bill -VA3WTB

I really don't know Rich, you can ask Flex. I am simply repeating Tim and Steve.  I have done some research into this because I find it interesting.

The S signal will change depending on you band pass settings as that changes what it samples.

My radio, 6500 is commonly around s3.  At s1 it shows there is RF there at some level.

Steve - K6SRS

I have a simple view on this. When I have no external RF energy (ignore the noise because I’m operating above absolute zero) at the radio’s input, I want to see S1 levels, not S3, 4 or 5.

Wayne

If it aint broke ... dont fix it! If you dont want an s meter reading with dummy load put tape over it of course you could got to i can only monitor those s readings were always 1/4 scale on the meter under any conditions.


YMMV .... The more I use it the more I love it ... 6400M

Bill -VA3WTB

Steve, it is not sensing at the radio input when an antenna is not connected. It is sensing the RF in the FFT bins. All FFT bins have RF in them, internal.

Bill -VA3WTB

Wayne, so true. It really is hard for folks to understand what they are seeing even after Tim and Steve try's to explain what is going on and how this technology works.
nice to hear about your 6400M

Steve - K6SRS

Oh, I’ve got that Bill. And accept this is an SDR parameter we may have to live with. I’m just making the point that I don’t care how much internal RF (or the digital equivalent) gets generated and measured. The only signal strength parameter useful to me is SIGNAL energy at the input. As it has been for decades.

Tim - W4TME

Dennis, that you "see" as the noise floor visually represented on the panadapter is affected by several factors, that being zoom level (the bandwidth of the FFT bin), whether or not the RF Preamp is enabled when you have a noise-limited antenna (or dummy load) attached and the amount of RF noise in your environment (the radio receives RF to a certain degree even with an unconnected antenna).  

The Slice S-meter is not an indication of the noise floor but is the integrated (sum) of the RF power of each FFT bin in the RX passband filter and it will always be higher than the noise floor.

N4QM

Yes, I empirically figured this out, and should have realized what was going on based upon my experience with my IC-7300.  Thank you!

Elliot Skultin

I have the 6400M and my noise floor now only seems to follow what the band is doing but I have taken a few measures to isolate my station.
   
First: Being a dipole guy, I installed a 1:1 toroid choke with 9 wraps RG400 on 2 stacked FT240-43 chokes at the feed point. Not only does it stop my feed line from radiating, it drastically kills feed line noise.  Here is the supporting test I used to make the decision: https://www.youtube.com/watch?v=QFOx7vjchy0

Second: I followed best practice and installed a full compliment of power line and coax chokes on every component and connection in my station. 
  

Last & best:  The wonderfullest feature, in my opinion, with no noticeable impact on my receive sensitivity, is WNB. Wide Noise Blanker.  Wow wow wow!  Turn it on and watch the algorithms analyze and clean up background noise in a few seconds.   

My S-Meter seems to just follow the band noise for a given bandwidth these days and switching off the antennas takes me to peace and quiet every time. 

Note: I live in a somewhat remote area on a mountain top.

I hope this helps.

Elliot
KE0PPC

Lionel

Very interesting, thank you.  Qualitatively I can say chokes make a difference in noise levels from in the house sources.  Mostly wallwarts.  There are more than most realize in the house. 

Two points:

1. Mountain top
2. R&S signal generator on the bench

Sweet. 

N4QM

That's quite impressive, but wouldn't have any effect whatsoever on what I am experiencing.  My question has been resolved, thanks.

Elliot Skultin

The video wasn't mine but I followed the recommendation.