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 > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
