On Sat, Jan 11, 2025 at 07:28 PM, Art Delibert wrote:
>Jim accurately describes the manner in which IMD is typically tested and reported ¡ª i.e., when two close-in signals produce an IMD product that exceeds the noise floor.? But it seems to me that the IMD inherent in just >about any amplifier will actually raise the noise floor, because any noise within the passband is causing some low-level IMD products that also appear within the passband.??
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>So the noise floor might be said to consist of two components, which we'll call Original Noise and IMD Noise.? When we reduce the RF gain, we're reducing the Signal and the Original Noise by the same amount, but >reducing the IMD Noise faster, because of the 1:3 ratio between the Original Noise and the IMD Noise.
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>If we're considering just the noise floor of the receiver circuitry, this effect would be pretty small.? But if we consider band noise as a component of the Original Noise,? and consider how the IMD products of that band >noise contribute to what we're calling IMD Noise, the effect may not be so small, particularly on the low bands where most of us experience a lot of noise.? In this case, turning down the rf gain could make a noticeable >contribution to the S/N ratio.? I think this is why Bob Heil's video is able to show a marked increase in readability of an 80-meter QSO by turning down the rf gain.
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>Thoughts welcome.
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>Art Delibert, KB3FJO
I find it real hard to? believe that band noise alone will? create enough IMD to? degrade anything.? ?With band noise like S4 to S8....any imd would be through the floor.? ?You wouldn't even be able to measure it, if say using 2 x test tones, and each? S8.?
When turning down the RF gain, the agc threshold increases.? Assume a? S7 to S8 noise level on 75m.? ?And the SSB stations you are talking to are all S9 + 10 db.? ?Reduce the RF gain till the S meter swings up to? S9.? ?The agc threshold has risen greatly,? and any signal <S9? will drop off rapidly.? ?Ok, the S7-S8 band noise just vanishes.? ?Why, because the noise is now below the agc threshold.?
And none of this has anything to do with IMD.? ?Heil didn't invent anything new.? ? This technique about reducing the RF gain control a bunch, on the lower bands, when loads of band noise is present.... is old news.?
Your only other option would be to run the RF gain at MAX.....then introduce enough RX padding,? such that the S7-S8 band noise is inaudible.? IE: add say aprx? 30-40 db of padding, such that the band noise is below the S2 agc threshold.? ?Your S9 + 10 db SSB signals will drop by 30-40db.
