8400 S-meter readings strange behavior

RX3APM

Hi to all,
My new 8400 is finally arrived.
First of all I try to compare with two 6400 I have.
I note difference in S-meter readings.
On the same frequency, same signal (or even without signal), same bandwidth, same preamplifier settings 8400 shows about 12db (two S-units) more than both of 6400.
This goes against the IARU standard scale requirements as of -73 dBm for S9 and -141 dBm for S1.
Both 6400 S-meters shows around -140dBm for 400hz bandwidth, +32db preamp on 28mhz band without antenna connected.
8400 with the same setting shows just -128dbm.
When the antenna is connected and the signal is present the difference remains let’s say S7 on 6400 and S9 on 8400.
I checked this with two brand new 8400, mine and one of my friend.
Any ideas what’s going on and how to fix it ?

73, Sergey RX3APM

mstein4100

I'm having the same problem with my 8400M. The difference between my Icom 7300 and the Flex is about 2 S-Units.

Mike-VA3MW

The best way to test this is with a signal generator. And, you have landed in that unique problem. If you have 2 meters, which one is correct. And, S Meters readings are a great area for discussion on how do you actually measure the strength of a signal in the Receiver passband. It is hugely different between a superhet radio, hybrid radio and a direct sampling radio. Generally, direct sampling radios have it correct.

I can't speak for how Icom calibrates their S meters, but I can guarantee that your Flex was correctly calibrated when it left the factory.

FlexRadio calibrates its S‑meter (signal strength meter) according to a combination of industry standards and direct measurement within the digital signal chain of the radio.

I just hooked my newly calibrated IFR-1200S and set it to -73dBm.

1. Following the IARU Standard

• FlexRadio aligns with the IARU Region 1 S‑meter standard for HF receivers:
  • S9 = 50 µV at the antenna input (terminated into 50 Ω)
  • Each S‑unit ≈ 6 dB difference in signal strength
• Signals above S9 are indicated in dB over S9 (dB over S9+xx).

2. Digital Measurement in the SDR

Unlike traditional analog radios, FlexRadio’s S‑meter is derived directly from the digital signal chain:

1. The RF signal is converted to digital early in the receive path (ADC).
2. The radio measures RMS voltage or power in dBm across the selected bandwidth.
3. This value is then mapped to S‑units using the IARU scale.

• Example Mapping:
  • S1 ≈ ‑121 dBm
  • S9 ≈ ‑73 dBm
  • S9+20 dB ≈ ‑53 dBm

3. Factory Calibration

• During production, FlexRadio radios are factory calibrated:
  1. A known RF signal is injected (e.g., 50 µV for S9).
  2. The receiver’s measurement is compared and adjusted via software.
  3. Calibration ensures consistent readings across bands and preamp settings.

4. Dynamic and Accurate Measurement

• Because the S‑meter is computed from the FFT bins of the digital IF, it is:
  • Frequency-accurate (per slice receiver)
  • Independent of AGC (it measures the true signal level, not audio level)
  • Consistent across the entire passband

If you a stickler for accuracy, I would trust the FlexRadio measurement as being as close to a bench standard as you can get. Your Icom may be correct as well, but Icom may be using their own interpretation of what is correct.

BTW, the amazing thing about your FlexRadio is that that S Meter reading does not change regardless of the preamp setting. And, why should it?

73