Flex 6400M noise floor

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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
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Dennis ODell

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Posted 6 months ago

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WX7Y

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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
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Rich McCabe

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For what its worth into a dummy load my 6600 is almost -140 and under S1 on the bands you mentioned.
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Mike KE4U

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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.
(Edited)
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Jay / NO5J

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

73, Jay / NO5J

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Bill -VA3WTB

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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...
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Mike KE4U

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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?"
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Rich McCabe

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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.
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Bill -VA3WTB

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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!
  
(Edited)
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Steve - K6SRS

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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.
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Rich McCabe

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Just asking for a friend.


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Bill -VA3WTB

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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.
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Steve - K6SRS

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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.
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Bill -VA3WTB

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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.
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Steve - K6SRS

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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.
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Dennis ODell

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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.
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Tim - W4TME, Customer Experience Manager

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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.
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Dennis ODell

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

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


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Rick WN2C

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k0eoo

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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.
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Bill -VA3WTB

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And S5 is fine. As Flex mentions.
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Wayne

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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
(Edited)
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Bill -VA3WTB

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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
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Elliot Skultin

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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
(Edited)
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N5LB - Lionel B

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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. 
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Elliot Skultin

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The video wasn't mine but I followed the recommendation. 
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Rex K0KP

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Well done Elliott.  Exemplary. 
(Edited)
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Dennis ODell

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That's quite impressive, but wouldn't have any effect whatsoever on what I am experiencing.  My question has been resolved, thanks.
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Rex K0KP

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It appears that the following phenomenon is evident.  The design of the Flex makes for an S Meter that is incapable of displaying, on the meter that is, a signal below S 4 in strength.  This is because the receiver is displaying, on the S meter, noise generated internally within the filter via FFT bins.  Is this correct?

Also, what is the matter with having the option to click in a relative reading S meter?   I think a whole lot of Flex customers would appreciate that ability.  We use all kinds of methodologies to measure things, peak, quasi peak, peak to peak, RMS, weighted average, PEP, bandwidth limited, relative indications and so forth in electronics all the time.  

This is ham radio folks, not the National Institute of Standards Laboratory.

Successful business models value their customers opinions and desires.  I'm sure Flex could conjure up an algorithm or two to simulate a relative analog S meter indication as an OPTION to click in if desired. 
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N8SDR

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You want to see a signal below S4 or S5 simple, narrow up your receiver bandwidth! 
(Edited)
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Bill -VA3WTB

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You first comment is almost correct. What you are seeing as you pointed out the FFT bins are being sampled, this is not internal noise as such,,but the sampling we can  see in the pass band.
If you narrow the pass band down to nothing you see the S meter read around 1 or 0.
so what we see is the bins in the pass band sampling.
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Martin S. W7RU

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I am starting to understand why FT8 might become a way to establish a contact better than CW ;-)