[Elecraft] Tuning L34 on K2 - The 'Signal to Noise Ratio' method

[Tom Hammond NØSS]

Ron:

Thanks for a MUCH deeper analysis than I gave it.

73,

Tom  N0SS


At 02:04 PM 10/4/02, you wrote:
>Tom, N0SS, wrote:
>
>... I was dinking around with L34 (no recollection why), and even
>'earballing' the adjustment, I did notice what I perceived to be a
>distinct
>drop in relative background at a point just inside the point of maximum
>received signal... maybe 1/2 turn (into the can) before reaching max
>signal. I was so surprised to hear it that I tried it several times,
>just
>to confirm that I was hearing what I thought I heard, and each time I
>noticed the same thing. So I now do believe that there was some thought
>behind the statement.
>
>NOTE that I have NOT (yet) performed any quantitative measurements to
>confirm my beliefs.
>==========
>
>I've heard that too, but decided that it was an artifact of my hearing
>based on the frequency spectrum of the noise. Looking at the K2 audio
>spectrum I can see the noise peak produced by L34 move across the audio
>range as the coil is adjusted. This seems to a produce a subjective
>response in our ears (and in an voltmeter on the audio amplifier output)
>that makes the best signal-to-nose ratio appear when L34 is tuned off of
>its real peak.
>
>To see the effect on the background noise using Spectrogram requires me
>to turn the Attenuator On to put the 7 MHz birdie down near the internal
>noise level so I can see both the peak of the birdie signal and the
>noise floor on the display without driving the K2 into compression. It
>is important to turn the K2 AGC Off so it isn't trying to maintain an
>even audio output level as I make the adjustment.
>
>Then, as I adjust L34 I can see the "hump" of noise move back and forth
>in frequency as the spike of the "birdie" signal moves up and down in
>amplitude. To my ear, the background noise sounds lower when L34 is
>tuned to the low-frequency side of the peak. That produces the
>impression that the signal-to-noise ratio is better there, but it looks
>the same on Spectrogram. That is, the difference in dB between the
>signal peak of the birdie and the peak of the noise stays the same on
>the Spectrogram display. Only the frequency spectrum of the noise is
>slightly different.
>
>I found that trying to measure the "peak" with an audio voltmeter at the
>speaker jack produced a false peak when L34 is tuned off on the
>high-frequency side of the actual peak.  The voltage the meter shows is
>the total voltage at all frequencies across the spectrum that is being
>amplified by the audio amp in the K2, and it appears that by detuning
>L34 to the "high frequency" side of the actual resonant point produces
>greater wide band noise for the meter to register, producing the false
>"peak".
>
>It seems to me that our ears, like that a-c voltmeter, are responding to
>a wide spectrum of noise that masks the actual "sweet spot".
>
>And, bottom, line, I can't see how that has any detectable effect on the
>K2's performance in the "real world" on the HF bands. These noise levels
>seen when doing the adjustment are WAY below what is coming in from my
>antenna in actual use. Still, it's a satisfying feeling to have the
>adjustment "peaked".
>
>Ron AC7AC
>K2 # 1289
>
>
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