70904A RF Section

The 70904A RF Section provides lower-frequency RF conversion and acts as the front-end for one configuration of an HP 70K-based spectrum analyzer. An RF signal is received from a device under test or a probe, is attenuated as needed, and is passed through a sequence of three IF converters, resulting in a 21.4 MHz video output that can then be used to analyze the presence of frequencies as the spectrum analyzer sweeps through the frequency range of interest.

The 70904A is a lower frequency RF section, capable of analyzing signals from 100 Hz through 2.9 GHz. Two unique characteristics of the 70904A is the ability to scan all the way down to 100 Hz, and the ability to support both AC and DC coupled signals.

One example of a minimalist HP 70K spectrum analyzer is a 70004A mainframe, populated with a 70904A RF section, a 70900A (or B) Local Oscillator, and a 70902A IF section.

Inside the 70904A

The major components visible on this side are, from left to right clockwise:

A8 Input Attenuator (70 dB)
A9 Limiter
A11 First Converter
A12 Leveling Amplifier
A1 Misc Bias Control Board
A14 Low Pass Filter
A10 Isolator
A6 321 MHz Matching Network

The front panel RF INPUT is connected to the Input Attenuator (A8), then through the Limiter (A9) to the First Converter (A11).

The First Converter passes the signal through a 2.9 GHz low-pass filter to eliminate higher frequencies and harmonics, then the input signal is mixed with a 3.6 GHz through 6.5 GHz local oscillator output from the LO module that controls the spectrum analyzer sweep frequency. The resulting mixing product is passed through a 3621.4 MHz band-pass filter, then through a 6.2 GHz low-pass filter. This effectively up-converts the frequency of interest, then filters out all other unwanted frequencies.

The resulting 3621.4 MHz signal is passed through another 6.2 GHz low-pass filter (A14), an isolator (A10), and into the Second Coverter.

The Second Converter down-converts the signal to 321.4 MHz after passing the signal through a second 3621.4 MHz bandpass filter. This signal is then passed through a 321.4 MHz matching network (A6).

Finally, the 321.4 MHz signal is passed to the Last Converter (A3), where it is down-converted to 21.4 MHz.

The Last Converter includes several amplifiers, filters and a mixer, following a similar down-conversion approach seen earlier in the RF signal path. It also regenerates the 300 MHz reference clock that is also used for the Second Converter PLL.

The major components visible on this side include the Last Converter (A3), Second Converter, and the Power Supply/Controller board (A4) on the bottom.

A full block diagram can be found in Module Resources section below.

Module Resources

Service Manual:

Block Diagram: