8600 Noise Floor with Preamp Gain set to "0db"

Pat W5VY · February 12

Had the F8600 on 10M early one morning before the band started to open. I was doing other things and looked at the SSDR screen and noticed the S-Meter was hovering around S5 on 10M with essentially no signals and low noise. I have moderate power line noise at times so I'm used to seeing the S-Meter hanging around S4-5 depending on orientation of the SteppIR. I switched to XVTA RX (not connected) input and didn't see much change. I recalled the Preamp test method that Gerald outlined years ago and decided to go through that to see what I could discover. Here are the results:

Flex 8600 Noise Floor Measurements - Jan 30, 2025

	SCU-A ANT 1 50ohm Termination
Band	0db Gain	+8db Gain	+16db Gain	+24db Gain	+32db Gain
20M - 14.2MHz	-104dbm	-110dbm	-118dbm	-125dbm	-131dbm
10M - 28.4MHz	-103dbm	-110dbm	-118dbm	-124dbm	-129dbm
6M - 50.3MHz	-105dbm	-112dbm	-120dbm	-126dbm	-131dbm

	SCU-A XVTR Port A - No antenna
Band	0db Gain	+8db Gain	+16db Gain	+24db Gain	+32db Gain
10M - 28.4MHz	-102dbm	-110dbm	-118dbm	-125dbm	-129dbm
	SCU-A ANT 1 Connected to Antenna
Band	0db Gain	+8db Gain	+16db Gain	+24db Gain	+32db Gain
20M - 14.2MHz	-98dbm (S4)	-102dbm	-103dbm	-103dbm	-103dbm
10M - 28.4MHz	-101dbm	-106dbm	-108dbm	-108dbm	-108dbm
6M - 50.3MHz	-105dbm (Antenna 2)	-112dbm	-118dbm	-121dbm	-122dbm

50 Ohm termination was directly connected to the Ant1 Jack
50 Ohm load tested with better that 26db Return Loss from 1 - 100MHz
Antenna 1 was a SteppIR DB18E tuned to the test frequency. Antenna 2 was a 6L LFA Yagi
I have moderate Power Line noise (HF through 70cm). SteppIR was pointed away from source.
Feedline to SteppIR 110ft of LMR600, 6L LFA Yagi 135ft LMR600.
Flex 8600 connected to PGXL connected to TGXL (SO2R -Bypassed)

In general, the F8600 noise floor was considerably higher that values recorded on the F6500/6700 in previous test.

Today I powered up my F6700 (3.8.21) with no antenna connected, 28.2MHz Panadapter 24KHz wide, No WNB, NB, NR, etc. Antenna Gain set to "0", Mode=CW, BW=400Hz.

Panadapter noise floor was around -130dbm. Slice showed -118dbm (S1)

Set up the F8600 with same parameters.

Panadapter noise floor was around -122dbm. Slice showed -102dbm (S4).

The 16db difference in Slice noise power doesn't seem normal. S4 with no signal looks odd as well. Is this a calibration issue? The 8600 RX sounds fine. Anyone else see a high noise floor/S-Meter reading with no signal input and Antenna Gain at "0db"?

Pat, W5VY

Trucker · February 12

I did a similar test with my 6600M some time ago. With the radio connected to a 50 Ohm dummy load my readings are similar to yours. Same with the antenna disconnected. With the antenna connected, it is higher. But, that is because of my local noise sources and current band conditions. As you know, S2 to S3 is normal ( at least with the 6000 radios) with the antenna disconnected because you are seeing the internal noise from the ADC's. I remember people being upset that their new Flex radios S meters didn't read S0 with the antenna disconnected like a traditional superset S meter would. Thanks for sharing your test results.

James

WD5GWY

Pat W5VY · February 12

James,

Thanks for the reply. I just looked at the QST Product Review of the 6700 in 2015, SSDR V1.3.8. It's MDS on 20M with 500Hz BW was -119dbm (S1). That's within a db of my 6700 on V3.8.21. The Product Review of the 6600M in 2020, V3.1.8, showed its MDS on 20M with 500Hz BW was -114 (S2). Rob Sherwood's test of the 6700 and 6600M agree closely with the ARRL Lab measurements. The 6700 and 8600 have similar ADC's. 16bit/250msps. No guess as to the quantizing noise contributed from ADC. Maybe a design change to shift the levels up to better accommodate strong signals and have preamp gain available when things are quite. Still seems odd to see S4 with no signal input.

73,

Pat, W5VY

Mike-VA3MW · February 12

@Pat W5VY

This is an excellent discussion and it comes up ever so often.

If you are new to this discussion, FlexRadio decided to do the math. SuperHets, not so much.

In many designs, the S‑meter is also linked to the Automatic Gain Control (AGC) system. The AGC adjusts the gain of the receiver to keep the output level relatively constant, and the voltage used for AGC often correlates with the signal strength. The S‑meter uses this voltage as an indirect measure of how strong the incoming signal is.

In short, the old school S‑meter in a superhet radio works by sampling the signal, rectifying it to produce a DC level, and then displaying that level on a meter calibrated in logarithmic S‑units, giving the operator an intuitive indication of signal strength. It didn't really calculate the noise floor.

When comparing 2 different radios, the BW is a key to make sure you are comparing apples to apples.

Which begs the question. What bandwidth setting did you use for the first part of the test? In order to compare it it needs to be the same as FLEX-6700 test at 400hz. Or, read on:

50Hz is the best value to use for noise floor calculations. You also want to disconnect all wires/coax going to the radio and make sure you have chokes on the LAN and Power cable to eliminate any common mode current influences. The antenna port and all others should also be terminated with 50 ohm loads. This is what we do in the factory if we are working on something such as this.

This is an example of what you should use as a test on my FLEX-8600. I would grab my FLEX-6700 reading but one of my friends is using it at the moment at the remote site.

This is BW @ 50Hz and no preamp.

These articles explain it further.

https://helpdesk.flexradio.com/hc/en-us/articles/14443135006235-Why-Doesn-t-the-S-Meter-Read-S0-when-the-Antenna-is-Disconnected

https://www.flexradio.com/videos/understanding-flexradios-s-meter-readings/

Pat, if you still see a huge discrepancy, are you able to use a signal generator to see if the ability to hear a signal is actually different? If so, get a ticket opened so we can get it looked into.

73

8600 Noise Floor with Preamp Gain set to "0db"

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