The 8644A signal generator is closely related to the 8643A, 8645A, 8664A, 8665A/B . Most general information can be found on the 8643A page.

In that family, the 8644A, 8645A and 8665A were all released together in 1989 (see also HP Journal issue from October 89). The 8665B appeared in the 91 catalog. The 8643A, 8644B and 8664A appeared in the 92 catalog.

The 8644B was marketed as having superior spectral purity in that range; the 1992 catalog says

The HP 8643A 1 GHz/2 GHz and HP 8644B 1 GHz/2 GHz RF signal generators are replacements for the HP 8644A. […] The HP 8644B builds on the HP 8643A's performance by lowering SSB phase noise (-136 dBc/Hz vs. -130 dBc/Hz) and lowering spurious ( -105 dBc vs. - 100 dBc).

• 252 kHz to 1030 MHz, 2060 MHz (Option 8644B-002)

• SSB phase noise -136 dBc/Hz (where ?)
• Non-harmonic spurious -105 dBc, >15kHz offset, 0.25 to 1030 MHz

• Opt 001 High stability time base
• Opt 002 2 GHz doubled output
• Opt 003 RF connectors on rear panel only
• Opt 004 Enhanced spectral purity
• Opt 005 Electronic attenuator (5 year warranty on attenuator, cannot be used with Opt 002)
• Opt 007 Synthesized audio oscillator
• Opt 010 Reduced leakage configuration

• 001 High-Stability Timebase
• 002 2 GHz Doubled Output
• 003 Rear-PanellnputiOutput
• 004 Low-noise Option
• 005 Electronic Attenuator (N/A with Opt
• 008 Pulse Modulation
• 009 Specified VOR/ILS
• 010 Reduced-Leakage Configuration
• 011 2 GHz Internal Frequency Counter