Frank, It looks as if your friend is trying to set the K3 up for PSK31 using DATA A. If so, I believe he is approaching it the wrong way.
The procedure I would recommend (subject to any correction from the gurus at Elecraft) is as follows: 1. Set the PWR control to approximately 50 watts. You do not want to transmit an average power in PSK31 higher than 50-60 watts, because if you do, instantaneous peaks will exceed the PA's linear range, resulting in gain compression and splatter. 2. Set the K3's meter to display ALC. From your PSK31 software, transmit an idle PSK31 signal at approximately 1500 Hz audio frequency. Advance the LINE IN level (using the MIC control) until the ALC meter displays 4 bars or perhaps slightly more. YMMV, but as far as I can tell once the display shows 4 bars, increasing the LINE IN control further seems to have no significant effect on the output, although the ALC meter may continue to increase to about 6 bars. The value of the LINE IN level needed will depend on the amplitude of the audio signal coming out of the sound card. 3. Now set the K3's meter to display RF power. While transmitting an idle PSK31 signal, adjust the PWR control as necessary to reduce the power indication to 50 watts (5 bars). If you prefer, you can program an offset into the CONFIG:TXG VCE configuration menu item to adjust the relationship between the setting of the PWR control and the K3's power output. For example, I find that with firmware version 2.73 and CONFIG:TXG VCE set to 0.0 dB, I have to reduce the PWR control to between 35W and 40W to hold the power reading to 50W. If I set CONFIG:TXG VCE to -2.0 dB (negative value), I get 50W indicated output with the PWR control set at 50W. 4. The actual power output will depend on the firmware version, the audio frequency used in the PSK31 software and the frequency response of your K3's transmit filter. I would suggest adjusting CONFIG:TXG VCE so that a PWR setting of 50W produces a maximum output reading of no more than 60W (6 bars) at any audio frequency you are likely to use in PSK31, and no more than 50W (5 bars) at the audio frequencies you use most often. Explanation (based on my imperfect understanding of how the K3's ALC works): In most radios with analog ALC circuits, the ALC meter indicates whether or not there is any gain compression. Your goal is to operate below the point where the ALC meter starts to move. You can set the power control on these radios to full power without ill effects in PSK31, as long as you keep the audio drive low enough to avoid causing any ALC indication. The K3's DSP ALC does not work by clipping the output signal; instead, it adjusts the drive level to bring the output power back to the requested level. Therefore, you want the K3's ALC to be in control to keep output power at the desired level. When there are no bars on the ALC meter, that means that the DSP ALC is not in control. A reading of 4 or more bars on the ALC meter indicates that the DSP ALC is controlling the K3's output. If you are able to produce an idle PSK31 signal whose average power is higher than one-half the K3's maximum power capacity, the PA will clip instantaneous peaks. In order to avoid this, you want the DSP ALC to be controlling the output such that the average output power is low enough to avoid significant gain compression on instantaneous peaks; I would recommend a value of no more than 50-60W average power output. On my K3, observing the output with a KF6VSG PSKMeter, a 50W output signal is acceptably clean (around -28 dB IMD). IMD increases rapidly as power increases; it is not as good but still tolerable at 60W, and increases to unacceptable levels at higher powers. The difference in signal power between 60W and 50W is 0.8 dB, or about one-eighth of an S-unit - unlikely to be observable at the receiving station. The corresponding increase in IMD is about three times this large (2.4 dB), and at 60W output the IMD is reaching the point where it is becoming observable on the waterfall. If I have got this wrong, I would certainly appreciate corrections. 73, Rich VE3KI DD7ZT said: > Hello, > > a friend of mine found a strange problem with his new K3. His setup: > > 1. > PSK31 (DATA-A), LINE IN = 5. PWR OUT set to 120 W, measured PWR at > PWR-meter: 5 W. > > 2. > Then tap A/B. Switch to SSB. Measured PWR=30 W. After about 25-30 sek of > whistling into the microphone PWR out is 120 W. > > 3. > Tap A/B again and switch back to DATA-A, PWR now is 25 W. > > 4. > Switch off and back on the Rig and you will start again at point 1...??? > > Any idea? > > ATU is on the whole time, however SWR is about 1,5. Tested was on 80 and > 40m with no difference. > > vy 73! > > Frank, DD7ZT _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
