Elecraft K2 Control Board 4MHz Oscillator Adjustment k2-4mhz-set.txt 07 August 2003 ©2003 Steven Gibbs GU3MBS mailto:sgibbs@guernsey.net Here are two methods of correctly setting the K2 4MHz clock frequency with greater confidence than the methods suggested in the K2 Owner's Manual, Chapter 6, 'Alignment and Test, Part II' and Chapter 9, 'Frequency Calibration Techniques'. Direct comparison of the MCU clock and a standard frequency is made, so these methods avoid pitfalls which commonly trouble K2 builders: 1. Neither method is iterative - you don't have to refine your adjustment by chasing around a loop. 2. Neither method calls for specific frequency readouts on the K2 display - the 10Hz dial uncertainty is not a factor. Method A requires that assembly of the K2 is complete. It requires reception of a standard frequency transmission, and needs a PC running the Spectrogram program. Latest Spectrogram version is available at http://www.visualizationsoftware.com/gram.html Freeware Spectrogram version 5.1.7 is available at http://www.guernsey.net/~sgibbs/gram50.zip Method B can be performed at the Chapter 6, 'Alignment and Test, Part II' stage, or when assembly of the K2 is complete. It needs an oscilloscope and a 1MHz frequency standard. Lab- standard signal generators & counters commonly have a 1MHz square-wave reference output. I used an off-the-air standard derived from the high-accuracy carrier frequency of the BBC's 198kHz LW transmission. ________ METHOD A A1. Set the K2 to USB. Identify and tune (approximately zero-beat the carrier) a standard frequency (WWV) transmission on 10MHz, 15MHZ or 20MHz. (To improve reception on 15MHz you could temporarily peak C21 & C23; to improve reception on 20MHz you could temporarily peak L10 & L11). Tune the K2 dial about 1kHz lower. On Spectrogram you should see the WWV carrier as a tone of about 1000Hz. Disregard any modulation tones that WWV may also be transmitting. A2. Disconnect the antenna; you should see a weak signal somewhere on Spectrogram - this is the 10th (or 15th, or 20th) harmonic of a 1MHz signal derived by the MCU from its 4MHz clock (you may recall that a similarly-derived 7MHz signal is used in initial 40M alignment). You can verify that you are looking at the right signal by touching the Control Board MCU crystal X2 or its associated C22 trimmer and noting that the frequency on the Spectrogram display changes slightly. If you don't see the MCU signal you can tune the K2 dial up or down a couple of hundred Hz until you locate it. A3. If WWV is coming through strongly there may be enough breakthrough to show WWV's signal also. Use an insulated tuning tool (sorry, but the Elecraft tool is rather poor for this job) to adjust Control Board C22 so that the two signals coincide - final proximity may show a beat which can be reduced to zero. If WWV isn't very strong you won't see its breakthrough signal so you'll have to alternately connect and disconnect the antenna until you have the two signals showing the same frequency on the Spectrogram display. NOTE: the exact setting of the K2 dial and the exact frequency on the Spectrogram display doesn't matter. The sidetone frequency which you have set doesn't matter either. These are the important features of this method - the MCU oscillator is being compared DIRECTLY with the standard frequency transmission. If you have a KAT2 or a KAT100 you can switch to an unconnected ANT2 instead of disconnecting your antenna. A4. Perform CAL-PLL. A5. Perform CAL-FIL. CAUTION: remember that the K2 manual says 'Whenever you switch modes or filters, the K2 will first record your new settings, IF THEY HAVE BEEN CHANGED' [my caps]. This means that to properly calibrate the BFO to the new MCU crystal frequency, the BFO control parameter must be changed, and then changed back to its original setting. A6. SPOT the WWV carrier in CW-nor and again in CW-rev; in each case the K2 dial should show 10000.00kHz (or 15000.00kHz or 20000.00kHz) probably plus or minus about 0.03kHz (30Hz). NOTE: significantly greater error means that either your MCU 4MHz isn't properly set to the WWV (re-check), or that you forgot CAL-PLL, or that CAL-FIL wasn't properly performed (re-do). If CW-nor & CW-rev dial accuracy is OK, then LSB & USB should look after themselves, provided CAL-FIL has been correctly done. ________ METHOD B If you have access to a 1MHz frequency standard, here's a way of setting the K2's 4MHz Control Board oscillator with greater certainty than trying to beat with WWV. You will need a 4MHz bandwidth oscilloscope having a high- sensitivity y-amp (5mV/div worked for me), with an independent trigger input derived from your frequency standard. B1. Temporarily reduce the y-amp sensitivity, connect the probe to the trigger source, and adjust the 'scope's timebase and trigger level to stably display one or two cycles of the 1MHz standard frequency. B2. Connect the probe's ground connection to the K2 Control Board ground point. Move the 'scope probe to the vicinity of X2-C21-C22 on the Control Board, but do not touch anything, just rely on loose capacitive coupling. Switch the 'scope to maximum y-sensitivity, and look for a trace showing pick-up from the 4MHz clock oscillator. B3. Use an insulated trimming tool (sorry, but the Elecraft tool is rather poor here) to carefully adjust C22 so that the scope trace is resolved into a sine wave moving slowly or rapidly either to the left or to the right. Note that four cycles occupy the same time as one cycle of the 1MHz reference oscillator. Careful tuning will result in a near-stationary display when the trimming tool is removed, and the 'scope probe is gradually drawn away. This indicates very close matching of the K2's 4MHz oscillator to four times the standard frequency. (A left or right shift of 4 cycles in one second means the frequencies are matched to one part in a million. Temperature effects will degrade frequency accuracy, so it's probably not worthwhile trying for better than this). If you are still building your K2, continue with 'Alignment and Test, Part II, PLL Reference Oscillator Range Test'. Otherwise, continue with step B4. B4. Perform CAL-PLL. B5. Perform CAL-FIL. CAUTION: remember that the K2 manual says 'Whenever you switch modes or filters, the K2 will first record your new settings, IF THEY HAVE BEEN CHANGED' [my caps]. This means that to properly calibrate the BFO to the new MCU crystal frequency, the BFO control parameter must be changed, and then changed back to its original setting. B6. Loosely couple the frequency standard output to the K2's antenna. You will hear 1MHz harmonics at 2, 4, 7, 10, 14, 18, 21, 25 & 28MHz. SPOT these in CW-nor and again in CW-rev; in each case the K2 dial should show probably plus or minus about 0.03kHz (30Hz). NOTE: significantly greater error means that either your MCU 4MHz isn't properly set to the frequency standard (re-check), or that you forgot CAL-PLL, or that CAL-FIL wasn't properly performed (re-do). If CW-nor & CW-rev display accuracy is OK, then LSB & USB should look after themselves, provided CAL-FIL has been correctly done.