The 6521 is a Radiation Tolerant, Space Qualified, Voltage Controlled SAW Oscillator (VCSO) governed by Hi-Rel Standard DOC206559. When ordered, flight units utilize Class K Element Evaluation IAW MIL-PRF-38534 and Class S Screening IAW MIL-PRF-55310. Engineering Models (EM) are fit, form, and functional equivalents of the Flight Models (FM) and can withstand the same environmental envelope less radiation. Please see DOC206559 for the multitude of design, assembly, inspection, and test options that can be selected or added by purchase order request.