I have a Hewlett Packard 8640B RF Signal Generator for sale. Unit features Option 003, (Reverse Power Protection). The Unit also has a built in frequency counter. It will work with external signals from 20 Hz to 550 Mhz, via its' 6 digit counter. The unit is in very good condition, is under calibration, and I will warrant for 60 days. $395.00 shipped, (USA & lower 48 only). Picture upon request. Money order, check, (must clear), or Paypal. snipped-for-privacy@att.net