Posted by
Guy, K2AV on
Feb 07, 2010; 6:56pm
URL: http://elecraft.85.s1.nabble.com/K3-in-a-cw-pileup-needs-work-tp4523884p4530500.html
Thanks Lyle,
Some follow-on questions:
1) When you say "The DSP outputs a voltage to control the IF gain of
the radio." are you referring to VIFGAIN1 input to U3B on the RF
board?
2) Assuming so, is there anything that influences the voltage on
VIFGAIN1 other than on behalf of the RF gain pot, and is the voltage
appearing at VFGAIN1 on behalf of the RF gain pot "processed" away
from a linear replication of the pot wiper voltage?
3) The hardware AGC appears to have fixed slope, attack and decay
constants. What would be the DSP slope and decay config constants to
mimic the hardware AGC constants?
4) Is there a table that provides a dBm equivalent of each of the
threshold values? Or alternatively are the values on the Clifton Labs
site correct?
( 2 -117 dBm
3 -110.5 dBm
4 -105 dBm
5 -103.5 dBm
6 -102.5 dBm
7 -101 dBm
8 -99 dBm )
73, Guy.
On Sun, Feb 7, 2010 at 11:10 AM, Lyle Johnson <
[hidden email]> wrote:
>
>> ...Lyle would have to comment on whether the DSP
>> AGC is developed before or after the IF gain is applied...
>
> Assuming ATT and PRE are OFF, hardware AGC voltage begins to have an
> effect on IF amplifier gain with a signal at the antenna in the region
> of -60 dBm, plus or minus a few dB. The hardware AGC voltage is
> developed at the final IF of 15 kHz and applied to the 8.215 MHz IF
> stage. Its purpose is to prevent the 15 kHz A to D converter at the DSP
> from being driven beyond its input limits (called over-ranging).
>
> The "onset of hardware AGC" level is influenced by PRE (which increases
> gain ab out 10 dB, decreasing the signal required at the antenna by the
> same 10 dB), ATT (which reduces signals by 10 dB, thus increasing the
> required signal at the antenna), and RF Gain when backed off sufficiently.
>
> What is "sufficiently"? The DSP outputs a voltage to control the IF
> gain of the radio. This voltage is compared with the hardware AGC
> voltage, and the higher voltage is applied to reduce the IF gain. If
> the RF Gain is backed off enough so that the resulting gain control
> voltage is grater than the hardware AGC voltage derived from the 15 kHz
> IF signal, then the criterion for "sufficiently" has been met.
>
> The DSP reads the hardware AGC voltage, regardless of the source, and
> uses the value as part of the S Meter calculation.
>
> The DSP AGC algorithm is computed based on the 15 kHz IF signal applied
> to the DSP's A to D converter. Thus it is after the IF gain is applied.
>
> Enjoy!
>
> 73,
>
> Lyle KK7P
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