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70900B Local Oscillator

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The 70900B is the master control module for the 70000-series modular spectrum analyzers and surveillance receivers. It is also a component of the lightwave signal analyzers. The 70900B LO has two main functions in the HP 70000 spectrum analyzers: as the synthesized local oscillator (LO) and as the master control module for the system.

As a spectrum analyzer local oscillator, it affects phase noise, stability, and frequency accuracy. The 70900B is the reason that the analyzer has excellent phase noise performance of -108 dBc/Hz at 10 kHz offset from the carrier. The synthesis technique has millihertz resolution for accurate frequency control. The fast tuning speed of the LO directly relates to the tuning speed of the analyzer.

As the master control module, the 70900B determines system features, slave module control, command processing speed, and GPIB speed. The combination of hardware and processing power provides fast tuning for surveillance applications and automatic testing.

signal_and_spectrum_analyzers:70900b.jpg signal_and_spectrum_analyzers:70900b_front.jpg signal_and_spectrum_analyzers:70900b_rear.jpg


Available Options
Option 0D2 Factory Refurbished Demo Equipment
Option 1BN Military Standard 45662A Calibration Certification
Option 1BP Military Standard 45662A Calibration with Test Data
Option 512 1 Mbyte RAM upgrade PC board
Option 655 5.25 Operation Verification Software
Option 8ZE Factory Refurbished
Option H01 Rear Outputs

There are also some options relating to various extra manuals.


For further information on the 70900B, several HP manuals are freely available: Service Manual (part no 70900-90116) Installation & Verification Manual (part no 70900-90314), User's Guide (part no 70900-90286), Programming Manual (part no 70900-90284), Firmware Note (part no 70900-90312) and Service Guide (part no 70900-90293).

Artek Media have the schematics / CLIP (component level information package) available to purchase for the 70900A & 70900B, also the 70900B service manual.

Module Faults & Issues

Two of the most common issues with aging 70900B local oscillator modules are (a) power supply failures, and (b) YIG-tuned oscillator failures.

Power Supply Failures

The switching power supply in the 70K systems runs at 40 kHz, with 40kHz AC power is distributed to all the modules, where it is then converted to the DC voltages each module needs. There is a transformer operating at 40kHz in each module, with various secondary windings, followed by rectifiers, filtering and liner regulators. The capacitors in this power supply circuitry are prone to failure, having reached end of life.

If there are issues relating to the power supply in the 70900B, it can be worth just replacing all the capacitors on the power supply board,

Removal of A3 Power Supply PCB requires first the removal of A1 Host /processor PCB consult Service Manual 70900-90116 Vol1 Page 6-3


Use electrostatic discharge (ESD) precautions when working on this assembly. Refer to “Electrostatic Discharge Information” in Chapter 1.


The controller assembly can be damaged if it contacts a conductive surface. This possibility exists because of the board-mounted battery.

A1 Removal. Remove the top cover of the module. It may be useful to take a picture as a reassembly guide. Disconnect the cables, be careful with the ribbon cable at the back edge of the PCB. Remove three screws with nylon spacers (don't mix these up, as they are different lengths and are color-coded), then the board can then be removed.

Removal of A3 Power Supply PCB consult Service Manual 70900-90116 Vol1 Page 6-8

Exact replacements for the capacitors are not necessary, but try to match both the required voltage and capacitance, and search for low ESR types in a similar can size, rated for SMPS service, preferably at 105 degrees C.

YIG-tuned Oscillator Failures

There are a number of error messages relating to the YIG-tuned oscillator that can appear

  • 7012 - Cannot lock YTO
  • 7013 - Cannot finetune YTO
  • 7015 - YTO unleveled
  • 7016 - YTO is unlocked
70900b_local_oscillator.txt · Last modified: 2022/07/06 08:50 by paul

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