HP 8648A 100 kHz - 1000 MHz Signal Generator

© Brooke Clarke 2007

HP 8648A Signal Generator


The motivation to get this signal generator was for testing military radios in the VHF low (30 - 90 MHz) and UHF (200 - 400 MHz) bands.  This series of generators are available with different upper frequency limits by using modular construction.  This model is also the one used for testing pagers when option 1EP is installed.  This one has the pager option and in addition to the built in pager modulation formats (POCSAG (512, 1200, 2400), FLEX & FLEX-TD (2 level FSK 1600 & 3200, 4 level FSK 3200 & 6400)) (5 fixed built-in & 1 user defined message) it also has an enhanced audio modulation capability ( 10 Hz to 20 kHz sine, square, triangle or sawtooth) when compared to a stock  HP 8648A (400 Hz or 1 kHz sine). 

The 8647 has 1 Hz resolution whereas the 8648 has 0.001 Hz resolution.  The switching speed and phase noise of the 8648 are also better.  These improvements were driven by the specs needed for pager testing.  The output can be adjusted from -136 dBm to +10 dBm in 0.1 dB increments by knob or keypad.

When working on my Precision Clock that accepts 2.5, 5, 10 or 20 MHz as it's input heartbeat instead of feeding the clock directly from a frequency standard I use the 8648 with an external reference connection to either the GPS disciplined PRS10 Rubidium or the FTS4060 Cesium standard.  The neat thing then is by using the frequency knob on the left the frequency can be changed easily.  This allows setting the 1 Pulse Per Second edge in a very easy manner.  It's also a great way to test the frequency range that the clock will accept without problems.  When the clock is too slow the program loops don't have enough time to complete which causes problems.  If the clock is too fast the PIC can not follow it.  Both these are easy to test with a knob for frequency.  In a similar manner the power levels & frequencies where the input circuit will trigger can easily be checked.


By adding boards various frequency ranges can be covered.  It's not clear how difficult it is to upgrade by just adding a board, or an EPROM?
F low
F hi
100 kHz
1 GHz
8648B 9 kHz
2 GHz
8648C 9 kHz 3.2 GHz
8648D 9 kHz 4 GHz

SSB Phase Noise 20 kHz offset

Agilent 8648A Discontinuance: March 1, 2007. Supported for 5 years after discontinuance date. Replacement Product N5181A MXG Analog Signal Generator

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page created 26 Oct. 2001.