http://elecraft.85.s1.nabble.com/K3-Beta-firmware-rev-2-80-Improvements-to-AFSK-A-DATA-A-SSB-AM-FM-DVR-tp2305650p2330464.html
around 1000 Hz is rather startling. I'm not sure what could cause that, but
at least to me that seems to be screaming for an explanation. There's an
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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