This looks like a good beginner's GPSDO on eBay: 172148560746
"This is GPS Disciplined Clock made with trimble GPSDO Board.Full tested
by Agilent 53132A with US-012 option and Ex-ref from trimble thunderbolt
GPSDO."
The seller says it has a sine wave output and is accurate from
10e-11 to 10e-12.
Any thoughts?
Richard