Attached is a plot showing the measured floor of the sine wave loop in the 5061A/B and also in the 5065A, along with plot of an HP 5061B with a high performance tube. The tube is the limiting factor. As you can see the margin provided by the current design is substantial. Also if you look at the FTS 4050 and 4060 that use square wave modulation they are a bit noisier than the HP 5061A/B. If you can provide an Allan deviation plot using the existing sine wave system along with a plot of the square wave modified system doing better I'll be surprised! If you really want to make your Cesiums STS better then I'd suggest building a disciplined clean up oscillator like the Microsemi 4145C. If you look at the Allan deviation plot of the combined 5071A and 4145C you can see it works pretty good. However I'm not sure how good a "cherry-picked" 10811 or FTS 1200 at 6 parts in 10-13th or so will compare to the BVA! Corby