Posted by
Joe Subich, W4TV-4 on
Feb 15, 2009; 8:36pm
URL: http://elecraft.85.s1.nabble.com/K3-Beta-firmware-rev-2-80-Improvements-to-AFSK-A-DATA-A-SSB-AM-FM-DVR-tp2305650p2331360.html
> So, neglecting the ~25 W vs 30 W error, it's clear that
> something is keeping a tight rein on things in LSB, but
> is not doing so in DATA-A.
> Presumably, this is the ALC action, which we are told is
> minimized in DATA-A mode.
It looks like the ALC uses the wattmeter as an input in
SSB but not in data mode. I tried the same set of tests
and found that the SSB power would increase (or decrease)
over the course of 10 seconds or so to bring the power to
the set point. That same effect was not present in DATA.
This would explain the differences between SSB and DATA
and would - depending on the time constants in the ALC
circuit - explain some of the IMD effects in SSB (and the
recommendation against the use of amplifier ALC).
Here are my power measurements in DATA and DATA-R modes.
I have the 8-pole filters in both of my K3s but I have
not checked the other one to see if there s any difference
in "flatness" ...
Set: 60 W
Measured: 58.2 W (Tune)
NOR (USB) REV (LSB)
-----------------------------------
200 0.3 (-22.9) 0.1 (-27.6)
400 22.0 (-4.2) 15.2 (-5.8)
600 46.4 (-1.0) 39.2 (-1.7)
800 55.8 (-0.2) 50.5 (-0.6)
1000 57.0 (-0.1) 55.8 (-0.2)
1200 45.9 (-1.0) 49.0 (-0.7)
1400 43.8 (-1.2) 46.5 (-1.0)
1600 52.9 (-0.4) 50.8 (-0.6)
1800 64.5 ( 1.1) 61.5 ( 0.2)
2000 69.0 ( 1.2) 72.0 ( 0.9)
2200 59.0 ( 0.6) 68.2 ( 0.7)
2400 50.0 (-0.7) 63.0 ( 0.3)
2600 48.5 (-0.8) 64.9 ( 0.5)
2800 15.4 (-5.8) 26.5 (-3.4)
3000 0.0 0.0
A sweep of the filter using K3FilterTools in the receive
mode shows very close correlation with the USB power
measurements including the broad peak at 1800-2000 Hz,
the roughly 7 dB decline from peak to 2800 Hz and the
2 dB slope between 500 and 2000 Hz. The greater than
expected roll off below 600 Hz is apparently due to the
audio input response.
> -----Original Message-----
> From: Joe Planisky [mailto:
[hidden email]]
> Sent: Saturday, February 14, 2009 7:46 PM
> To:
[hidden email]
> Cc: elecraft list
> Subject: Re: [Elecraft] K3 Beta firmware rev. 2.82
>
>
> Hi Joe,
>
> > Further, single tone power calibration is only accurate for the
> > specific frequency within the IF filter on which the calibration is
> > performed. If I calibrate power in CW mode that
> calibration point is
> > approximately 300 Hz from one "corner" of the 2.8 KHz
> filter thanks to
> > the way the K3 offsets the filters.
>
> If I understand you correctly, you're essentially saying that
> in DATA-
> A (where, according to Elecraft, ALC action is intentionally
> minimized
> to minimize distortion), we're seeing more of the true shape of the
> filter passband. Whereas in a voice mode (USB or LSB, where ALC
> action is more pronounced,) the ALC action will tend to make the
> filter passband appear flat if the input is a single tone.
>
> To test this, I used an audio signal generator to input single tones
> of various frequencies into my K3 in both LSB and DATA-A
> modes. I set
> the power control for 30W. Here are my measurements:
>
> AF input freq. Po-DATA-A Po-LSB
> TUNE 25.7 25.2
> 200 4.6 6.1
> 400 27.0 24.8
> 600 34.5 24.8
> 800 45.6 24.8
> 1000 53.1 24.8
> 1200 47.5 24.8
> 1400 37.8 24.8
> 1600 32.3 24.8
> 1800 30.9 24.8
> 2000 31.9 24.8
> 2200 34.3 24.8
> 2400 37.9 24.8
> 2600 34.9 24.8
> 2800 13.9 24.8
> 3000 nil nil
>
> So, neglecting the ~25 W vs 30 W error, it's clear that something is
> keeping a tight rein on things in LSB, but is not doing so in
> DATA-A.
> Presumably, this is the ALC action, which we are told is
> minimized in
> DATA-A mode. The data also seem to corroborate your statement that
> the calibration point is 300 Hz from the filter corner.
>
> If I'm really wrong about something, please correct me. If I'm NOT
> wrong, I think I might finally understand why power output in DATA-A
> doesn't seem to track the requested power closely.
>
> 73
> --
> Joe KB8AP
>
> On Feb 14, 2009, at 11:36 AM, Joe Subich, W4TV wrote:
>
> >
> > You CAN NOT make those tests at 50 Watts! The power calibration
> > circuitry will be constantly changing the internal gain.
> If the ATU
> > is enabled and/or your dummy load is not flat the changes
> invalidate
> > the measurements.
> >
> > Further, single tone power calibration is only accurate for the
> > specific frequency within the IF filter on which the calibration is
> > performed. If I calibrate power in CW mode that
> calibration point is
> > approximately 300 Hz from one "corner" of the 2.8 KHz
> filter thanks to
> > the way the K3 offsets the filters. When I switch to DATA
> A, and try
> > to test the power level with a 1500 Hz tone, I can see 2 dB more
> > power simply because my 2.8 KHz filter has a 4 dB slope across
> > the passband and the K3 gain calculation includes compensation
> > for the loss at the calibration frequency. If I check the
> > power with a 1 KHz tone, the difference may be 3 dB (DATA A
> > is USB, CW calibration is LSB so the slope changes direction
> > with tone). If I switch to AFSK A (REV to stay with USB)
> > and check power with a 2295 Hz (USB Mark) tone, the difference
> > may be down to one dB or so.
> >
> > There are too many uncompensated variables to expect the K3 to
> > maintain absolute control over the power output with changing
> > antennas, power levels, modes, (relative) frequency within the IF
> > passband and possibly even different filters. ALC controls
> the audio
> > level into, and the RF level out of the DSP (modulator)
> process - it
> > is an ALC (automatic LEVEL
> > control) NOT an APC (automatic POWER control).
> >
> > The ALC is an open loop system - the loop gain is
> calibrated once at
> > the 5 watt level and once at the 50 watt level for one specific
> > frequency in the IF passband. Once that is done, ALC
> simply controls
> > the DSP gain. If you change the system gain following the DSP, the
> > output power level WILL CHANGE. I'm sure it would be theoretically
> > possible to calibrate the system gain from 100 Hz to 2900 Hz offsets
> > from the carrier (for both the USB and LSB cases) and apply
> > that mask to the modulation process but does the DSP system
> > have enough horsepower to do so and is it worthwhile simply
> > so the operator can avoid the need to check his power level
> > when changing mode or modulating frequency?
> >
> > 73,
> >
> > ... Joe, W4TV
> >
>
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