+++++++++++++++++++ Date: Fri, 22 Feb 2002 20:39:09 -0800 From: "John Grebenkemper, KI6WX" Subject: Re: [Elecraft] K2 Frequency Accuracy To: Elecraft Mailing List From: "Sherman Banks (W4ATL)" > If I measure my frequency error on 7000 KHz and make the proper calculation > of my frequency error, then Cal PLL and Cal Fil (moving the BFO one count > and then back) I get pretty good accuracy on 80 and 40 M throughput the > band. But the other bands seem to be off. 20 M is 50 Hz low. 15M is about > 80 Hz low. 10M error increases to about 120 Hz. 80M is right on. Ver 2 > firmware. Is this normal? Should I try to measure the error on the highest > band for more accuracy? > The absolute frequency error in the K2 will vary significantly by band if the 4 MHz oscillator is not precisely adjusted to 4 MHz. This error occurs because the BFO and the VFO frequencies are determined by the 4 MHz oscillator during CAL FIL and CAL PLL. A second uncertainty is created because the BFO step size is in the range of 20 to 30 Hz. If you measure the frequency error in both LSB and USB (or CW and CWR), you can compute the error for both the VFO and BFO which is very useful to determine why the error is occuring. Assuming that your data is for LSB on 80 and 40 meters and USB on 20, 15, and 10 meters, it fits quite well for the 4 MHz oscillator to be 17 Hz high (this offset predicts -59 Hz on 20 meters, -89 Hz on 15 meters, and -119 Hz on 10 meters). For those who are interested in the expected frequency error, I've computed the following table. There is roughly a 20 Hz uncertainty due to the finite step size of the BFO and VFO. The frequency error table assumes that the 4 MHz oscillator is 10 Hz high in frequency. Freq Mode Error (Hz) 3500 LSB +09 7000 LSB +18 14000 USB -35 21000 USB -53 28000 USB -70 It is very difficult to set the 4 MHz oscillator to a better accuracy than ~10 Hz, plus this oscillator does drift at a measured rate in my K2 of -1 Hz per degree F increase in temperature. The K2 internal temperature increases by about 10F over a 1 hour warmup period in receive mode which will shift this oscillator by -10 Hz as the K2 warms up. The most accurate way to set this oscillator is to drive the K2's internal counter with a known accurate frequency source in the 20 to 30 MHz range. You may also try to set the fifth harmonic (20 MHz) of this oscillator so that it is at exactly the same frequency as the 20 MHz WWV signal. You have to remove the K2's antenna to hear the fifth harmonic. Finally, I think it is unreasonable to expect a K2 to have an absolute frequency error better than the above table. My K2 can regularly achieve an absolute frequency accuracy of 40 Hz on 15 MHz WWV. The K2 was not designed with a temperature compensated master oscillator and a 1 Hz frequency synthesizer. While there are radios that have this level of performance, they tend to be expensive, heavy, and power hungry. Very few communication modes require an absolute frequency accuracy of a few Hz. -John KI6WX +++++++++++++++++++