Is it perhaps some kind of bang-bang servo?  It seems to have more than eight 
but fewer than 16 states, which suggests a 4-bit control word perhaps from a 
microcontroller.

John F5VLF

*********** REPLY SEPARATOR  ***********

On 30/09/2006 at 20:00 Gary - G4WGT wrote:

>Hi All,
>
>I now often check around 500KHz for what is going on, I usually see on
>SpecLab (sometimes hear) a continuous carrier very  near to 500KHz. On
>looking/listening tonight the signal frequency is varying in small steps
>(see attachment). I can't decide if it is somthing local to me or a genuine
>MF signal. Zero on the capture is 500KHz. Any comments please.
>
>Gary - G4WGT.