FT-991A Difference between FM and SSB S-Units

Discussion in 'General Technical Questions and Answers' started by PY2RAF, Mar 16, 2019.

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  1. PY2RAF

    PY2RAF XML Subscriber QRZ Page

    Hello, there!

    After a few weeks observing my new FT-991A, I have found that, at the same signal (or background QRM) level, the FM mode tends to report a lot more S-units more than SSB mode.

    For example, at the time I'm writing this post, I have S7 in SSB mode in VHF band. If I switch it to FM mode, I get S9+20, over the very same signal, same frequency. I am tending to understand that the SSB S-meter is the good one, as very frequently I have S9+60dB and the waterfall doesn't get to the red tones.

    See the video: https://c.rf01.co:8443/q/s-meter-fm.mov

    Is it me or can you also reproduce it? Is it expected/normal or some S-meter misalignment?

    Appreciate the comments!

    73s de PY2RAF
  2. K8JD

    K8JD Ham Member QRZ Page

    On FM you have to rate signal level by "Quieting" of the no-signal IF noise level.
    So, the stronger the incoming signal of the unmodulated carrier , the closer to zero noise the receiver audio becomes.
    The S meter is not a good indication on FM.
    I have had several multimode rigs and the, FM mode, IF noise wih no signal shows on the S meter like a strong AM or CW signal on the air.
    PY2RAF likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think what you're seeing is very normal.

    It takes more sophisticated circuitry to make an "S" meter read properly (e.g., 6 dB/S unit, with 50uV as S9) on FM and most rigs don't have anything like that. Of course, most "commercial/professional" 2-way FM rigs don't have any kind of S meter at all, so having "anything" is better than that, I guess.:p

    Problem is, FM detection is very different and what reaches the detector follows a limiter which is intended to be saturated by any kind of strong signal, whether it's just 20 dB quieting or "infinity" quieting, and the meter circuit is usually tied to the limiter.

    "Some" multimode rigs actually have S-meters that function on FM; my 1987-vintage FT-736R actually does. It's driven by the signal level way before limiting, a similar AGC circuit as used on SSB/CW. A slightly noisy signal will read S1-S2. A full quieting signal is about S9, and it takes an enormously strong signal equivalent to a mobile parked across the street to be able to "pin" the meter. A few high powered line-of-sight repeaters can do that, but that's a very, very strong signal.

    Unless you're using the meter for critical beam antenna steering, it really doesn't matter. The FT-736R also has an RF gain control that works fine on FM just as it does on the linear modes, so if a signal is way upscale and I want to orient a beam more towards it anyway, I can just turn the RF gain down, which will drive the S-meter reading lower, and then I can peak a signal from even a mile or two away (not that there's ever any reason to do that).

    I've never seen a "mobile" ham rig that doesn't exhibit the same meter issues as described above. Most of the units I've owned will go from "just barely moving the meter" to "completely pinning the meter" with only about a 10-12 dB change in signal level.

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