Frequency stability under external reference

David McClain


Just a quick note about a finding of mine from tonight. I had the rare chance to monitor the frequency behavior from cold start up of the K3, after having shut down a few hours earlier for some thunderstorms in the area. (Rare because I almost never turn off the K3…)

Prior to power up of the K3, I ensured that the 10 MHz reference was active and fed into the K3EXREF port. The 10 MHz came from a PRS-10 Rb source. I used that same source to lock an Agilent 33522A Function Generator to produce a 7 MHz sine wave reference.

Then I fired up the K3, tuned in CW mode to 7 MHz, and I watched the 600 Hz CW tone from the K3 on Spectran at highest resolution (about 31 mHz resolution).

What I saw was that the 600 Hz tone was anywhere from 600-602 Hz. The manual states that the frequency will be within 2 Hz of the reference, and you might think this means +/- 2 Hz about the reference. But what actually happens is that it is 0 to +2 Hz from the reference. Or +/- 1 Hz plus a 1 Hz shift from the reference.

Hence, by using RIT/XIT and turning it to +1 Hz you are held within +/- 1 Hz of the correct frequency for both receive and transmit.

It was really interesting to watch the K3 oscillator warm up, but always held within that +/- 1 Hz band through the entire warmup period, even though the Ref Cal was steered by the K3EXREF over a range of 25 Hz during that same period. Very impressive!

73 de Dave, N7AIG

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