Bob, While directly attaching a counter without a 10X probe can pull the frequency of the measured circuit (and that can also happen with a 10X probe), the beauty of comparing the resulting external counter frequency count with the K2 internal frequency count is that it does not depend on the exact frequency of the frequency source (in this case TP1). Even if the external counter (or the internal counter for that matter) does pull the frequency at TP1, that frequency is not the parameter being adjusted - one is adjusting the clocking frequency for the K2 internal counter, and if that clock is set so both counters read the same, the internal counter will be almost as accurate as the external counter (within the uncertainty range of the last display digit).
I have also cautioned about attempting to measure the 4 MHz oscillator directly - the indirect methods recommended are more accurate and less prone to errors similar to those you mentioned. It may also turn out that the 'real' frequency for the 4 MHz oscillator is a few Hz different from the nominal frequency, but even if that is true, it is circumvented by measuring the displayed frequency rather than the master clocking reference itself, and any other hidden variables are also compensated out of the 'equation'. 73, Don W3FPR > -----Original Message----- > > Hi Carel, > > Have a care connecting an extra frequency counter in parallel > with the K2 > counter as the loading capacitance of some counters may pull the > readings off > what they really are. The K2 frequency counter via the supplied > probe gives > relatively light loading on the circuits you are measuring. The > extra frequency > counter may not cause a problem in the case of TP1, but better > to play safe > and isolate the parallel frequency counter with a high impedance probe. > > I fell into this trap when setting my K2 4MHz control board clock > oscillator > with a Racal Dane counter. This has a ovened xtal clock, hence retains a > fair degree of accuracy from calibration. However the extra load > capacitance of > the frequency counter input undid all that and gave an erroneous > reading. A > high impedance scope probe gave enough level to operate the > frequency counter, > yet still provided sufficient isolation not to affect the K2. > > May not be causing problems in your case, but something to be aware of. > > Regards, > Bob, G3VVT > _______________________________________________ > Elecraft mailing list > Post to: [email protected] > 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 > > > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.1.408 / Virus Database: 268.13.3/474 - Release Date: 10/13/2006 > > _______________________________________________ Elecraft mailing list Post to: [email protected] 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
