Re: : K3 receiver noise - my test results (long)

Posted by Julian, G4ILO on Dec 19, 2009; 11:32pm
URL: http://elecraft.85.s1.nabble.com/Re-K3-receiver-noise-questionable-test-results-tp4192141p4193141.html

Don Wilhelm-4 wrote

My conclusions: I am not certain what K3 owners are perceiving as noise
any more than I  had been before these tests.   The flatter passband of
the K3 may be sub-consciously perceived as being more 'harsh', but I
could not equate that to 'noise'.  The other filters with a high end
in-passband response that drops off similar to a "pink noise" response
*could* be causing a perception of additional noise *if* their local
noise has significant content in the 2 kHz to 3 kHz range - yes, the K3
will make this section of the audio spectrum louder than the other
receiver I measured.  The "audio hiss" is not coming from the high
frequency spectrum outside the passband because in that area, the K3 and
the K2 are much more quiet than the others measured.

I used Spectran rather than Spectrogram which has the advantage that it can also be used interactively as an audio filter. I found that decreasing the bass content by increasing the frequency of the high pass filter made the audio seem more noisy. I presume this is because high frequency noise components then comprised a greater proportion of the signal and were emphasized.

I think that if people were using low impedance headphones and the original DSP board with small value coupling capacitors then they would find the audio over-bright and harsh, as I did.

I tried different AGC settings and agree that lower AGC THR settings make the radio sound more noisy, so that may also be a factor. I am now using AGC THR = 8 which is the maximum and might well use a higher setting if it was available. I am also using AGC SLP = 1.

Having identified the problems and made some adjustments to settings the K3 sounds really good now on high impedance phones and on the internal speaker.