[K3] 10 dB audio drop with 50/100 Hz DSP filters

[George Dubovsky]

All,

I have a relatively new K3, s/n 4545; I just upgraded to the latest FW using
the latest utility, but the problem predates that. I have 2.7 KHz, 5 pole
filters, 1.8 kHZ and 250 Hz 8 pole filters in both the main and sub
receivers. I have zero offset set for all of the 8 pole filters. When I
first was learning my way around the radio in September, I noticed that the
audio level would drop noticeably when I decreased the DSP filtering from
150 Hz to 100 or 50 Hz, both on the main and the subreceiver. A quick check
with a friend indicates that his K3 doesn't do this. I hung a scope across
the headphones to verify that it wasn't my imagination, and then I found the
AFV readout. It confirms that the audio out drops by almost exactly 10 dBv
going from 150 Hz to 100 Hz, and then does not drop further going to 50 Hz.

My test setup is a stable 8640B set to 14.100 MHz, -73 dBm sig level, AGC-S,
centered up in the passband. When I switch in 100 or 50 Hz DSP filtering the
audio drops, and slight retuning either side only make it worse. This is all
with the 250 Hz 8 pole filter switched in. Because both the main and sub
behave identically, I assumed it wasn't just an 8 pole filter with non-zero
offset, but just to test, I offset the 250 Hz filter in the main rx by + and
- 0.25 and the step audio change was still there, although the absolute
audio level was lower with the filter offset from zero, indicating (to me)
that zero offset was close to correct. I have also tried switching between
FIR and IIR filtering to no effect.

I keep thinking this is something I have accidentally changed somewhere in
the initial setup of the K3, but several readings of the Menu and Config
sections of the manual only made my headache worse.

Please, what am I missing?

73,

geo - n4ua
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[KK7P]

You probably have the Rx equalizer set to boost audio at the tone
frequency you are using for these measurements.  Set it to flat - Rx EQ
is not in effect at 50 Hz and 100Hz selectivity of the K3.  It is on my
list of possible DSP code changes.

Meanwhile, if you really want to use Rx EQ in CW, set the PEAK to 0 dB
at and just use CUT to shape the passband you want on either side of
your chosen PITCH.

73,

Lyle KK7P

> Please, what am I missing?

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[George Dubovsky]

John, Lyle, RX equalize was it! Thank you very much - I never would have
found that one on my own.

73,

geo - n4ua

On Sat, Oct 30, 2010 at 8:18 PM, Lyle Johnson <[hidden email]> wrote:

> You probably have the Rx equalizer set to boost audio at the tone
> frequency you are using for these measurements.  Set it to flat - Rx EQ
> is not in effect at 50 Hz and 100Hz selectivity of the K3.  It is on my
> list of possible DSP code changes.
>
> Meanwhile, if you really want to use Rx EQ in CW, set the PEAK to 0 dB
> at and just use CUT to shape the passband you want on either side of
> your chosen PITCH.
>
> 73,
>
> Lyle KK7P
>
> > Please, what am I missing?
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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