70907B External Mixer Interface

The HP 70907B module allows the use of external mixers in an HP 70K spectrum analyzer system. External mixers allow frequenices reaching up to 110 GHz to be analyzed, far higher than those natively supported by the HP 70K RF modules. When introduced, the 70907A/B was intended for use with the HP 11974 and HP 11970 preselected and harmonic mixers.

Like the RF modules typically used in an HP 70K system, the 70907B is controlled by the 70900A (firmware 860203 or greater is required) or by the 70900B (firmware 890606 or greater is required). The 70907B can be used alone, or in conjunction with another RF module.

Inside the 70907B

The major components visible on this side are:

A1 — Miscellaneous Bias Board Assembly
A2 — Calibration Source Board Assembly
A5 — Leveling Amplifier Assembly
A6 — Isolator Assembly
A7 — Front Panel Board Assembly
A8 — Attenuator Assembly
K2 — RF Switch Assembly

The rear panel LO IN is routed to a leveling amplifier (A5) that regenerates the rear panel LO OUT, and sends the LO to an RF switch (K2) that selects between the rear panel LO IN and an internally generated calibration signal to send to the front panel LO OUTPUT.

The front panel IF INPUT is routed through the Calibration Source Board Assembly (A2) to the Attenuator Assembly (A8), then into the Last Converter Board Assembly (A3), on the other side of the module.

The front panel TUNE SPAN is connected to the rear panel TUNE SPAN, and the front panel MIXER BIAS is generated by the Miscellaneous Bias Board Assembly (A1).

The major components visible on this side are:

A3 — Last Converter Board Assembly
A4 — Power Supply/Control Board Assembly

The RF magic that processes and converts the 321.4 MHz IF input to a 21.4 MHz video signal happens inside the Last Converter Board Assembly (A3), of which a block diagram is shown below:

Here we can see that the rear panel 300 MHz OUT clock is regenerated from the rear panel 300 MHz IN clock, and that the rear panel 21.4 MHz OUT is switched between the down-converted 321.4 MHz IF from the front panel IF INPUT (with an aux 321.4 MHz OUT being sent to the rear panel) and the rear panel 21.4 MHz IN. This allows the 70907B to be switched back and forth with another installed RF module.

Interesting Experiments

Knowing the internal organization of this module, one interesting experiment that can be made is to connect an E4406A VSA with option 300 (321.4 MHz IF output) to the IF INPUT of the 70907B, fix-tune the 70900 to the centre frequency that the E4406A is tuned to, and see if that allows the E4406A to be used as an RF front-end for the HP70K system.

A second interesting experiment is to see if the 321.4 MHz to 21.4 MHz downconversion functionality of the 70907B is still active when the module is not configured as a slave to a 70900, and no LO IN signal is present. If this is the case, this module can be used as a stand-alone downconverter, albeit a rather specialized one.

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