Thanks to all that responded. I am playing with a receiver for Navtex. Problem there is that the nearest station only transmits for 10 minutes (max) every four hours so that makes tuning the receiver part a bit awkward.
Hence I need to be able to use a RF generator instead and a counter to make sure my LO and IFs are correct. The detector has narrow filters in DSP so accuracy is important. Nevertheless 1/1000 should suffice for me.
With the simple setup I have now (simple GPS, 1kHz) I know that the counter is fairly accurate so I assume that if it is acurate on 1kHz, then it should be accurate on 518kHz as well.
Thanks Stuart for the tip of the eBay receiver. I have seen those, but did not know if they were any good. I take your comment as a vote of confidence, so I might end up buying one.
Anders