Posted by
Guy, K2AV on
Oct 24, 2010; 7:25pm
URL: http://elecraft.85.s1.nabble.com/K3-Sub-RF-gain-tp5659334p5668484.html
I assume that you are always making this test after hitting A>B two
times in quick succession.
Listening to daytime 160m noise I set both RF gains to max and adjust
the AF to get equal irritation levels in both speakers, I get the same
11 oclock for both RX. If I then come up from RF min to where I just
barely hear the noise equally in both, I get an identical 1 oclock
setting for both.
But remembering that this ain't your daddy's analog radio...
The RF gain calibration is only made from the MCU to the output. It
determines what internal number results in what gain. It has no means
currently to test the RF gain pots themselves because it does not ask
you to provide full CW and full CCW as part of the calibrate.
The RF gain pots are doing a linear divide on an accurate 5 volts
which then goes to ADC pins on the MCU to convert to numeric advice.
If the pot itself was not linear, then the numbers advising position
would likewise be biased. Currently they are not running a test for
linearity of the pots themselves -- it is presumed they are linear,
min produces a zero numerical advice, max produces a 255 or some
equivalent to that. There could be a missing SMD solder joint keeping
the divide from happening properly. If the top of the resistor
surface was connected and the wipe was connected, and the bottom of
the resistor surface was NOT connected, this could result in that
behavior, because you could never get to zero, just to pot max in
series with what ever sink resistance in the ADC circuit in the MCU
chip.
It would have to be the ground connection on VR2REAR that was bad. If
you can get at it somehow, you should be able to see the resistance on
the wipe go down from 5 volts to zero if it's good. If it's not the
pot...
It could be that the VR2REAR ADC pin on the MCU has a bad embedded ADC
connected to it. Can only fix that with a new board.
This control sequence emerges on the KRX3 side of the MCU et all as
VIFGAIN2. With no signal on input, taking RF gain from max to minimum
should swing the voltage on the U3 side of R22 from zero to 3 volts.
If you grab the schematics you may be able to find these voltages and
check them out. Using VR2REAR and VIFGAIN2 will find you the
schematic spots in question.
Maybe this will help.
73, Guy.
On Sun, Oct 24, 2010 at 2:22 PM, Mike Scott <
[hidden email]> wrote:
> Joe (W4TV)
>
> Yes I have performed the gain calibration many times on both receivers using
> the K3 Utility with the XG2. I recently did it after loading the new K3
> Utility that supposedly fixed some issue about writing receiver gains to
> memory. So now you make two people whose RF gain tracks.
>
> I can't begin to speculate anymore as to the difference in RF gain behavior
> in the two receivers. So to repeat: same antenna (to XG2), same stock
> filters with matched offset, same (0) filter gain, same ATT on, same PRE
> off, same IF filter width, same NB off, same NR off. Sub Rx gain set to
> minimum I can still copy signals and hear band noise, Main Rx RF gain set to
> minimum is dead band.
>
>
>
> This difference in RF-gain tracking makes it difficult to use reduction in
> RF gain as a method to manage high band noise conditions. I can't just grab
> both controls and move them the same, like I can with AF controls which
> track very well.
>
>
>
> Stan KR7R Main Rx RF gain behavior is bizarre. In the days of old
> potentiometers this would be explainable but not with gain controlled by
> firmware. Maybe a bad RF gain control with odd switch bounce behavior at
> some gain setting... I would try to move the RF gain knob while K3 is shut
> down to a new position and then turn it back on to see if the RF gain
> nonlinearity location changed to someplace other than 9 o'clock.
>
>
>
> AE6WA
>
> Mike Scott
>
> Tarzana, CA
>
>
>
>>>Joe W4TV said.
>
>
>
> Both of mine operate as expected (same RF/AF gain response for main
>
> and KRX3). Have you performed the RF Gain calibration on both of
>
> the receivers using a calibrated 50 uV signal source (e.g., XG-2)?
>
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