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Model 11848 A
Service
Principles of Operation for the Block Diagram
General
The HP 11848A Phase Noise Interface is the central instrument in the HP 3048A Phase Noise
Measurement System. It is a collection of circuits configured under computer control to make accurate
phase-noise measurements on a wide variety of devices-under-test and using several test methods.
Besides the Interface, the System includes:
• a Fast-Fourier-Transform (FFT) spectrum analyzer (the HP 3561A Dynamic Signal Analyzer)
that measures the demodulated phase noise as a function of frequency offset from the carrier,
• optionally, an additional RF spectrum analyzer that extends the frequency-offset measurement
range,
• optionally, reference sources with high spectral purity, and
• the system controller.
Some of the interface circuits in the HP 11848A Phase Noise Interface include:
• numerous signal-routing switches,
• an RF phase detector and optionally a microwave phase detector (1) for demodulation of phase
noise to be measured by the spectrum analyzers and (2) to provide a tune voltage to phase lock
the input sources in quadrature when the phase-noise-using-a-phase-lock-loop method is used,
• various networks which process the phase detector output for voltage control of the input sources,
• various networks which process the noise for measurement by the spectrum analyzer,
• detectors which sense erroneous measurement conditions,
• four RF sources for system calibration verification, and
• control circuits which interface with the controller via HP-IB.
The maximum frequency offset from the carrier that can be measured by the system is 100 kHz for
the standard system and 40 MHz with an additional RF spectrum analyzer (such as the HP 3585A)
supported by the system.
NOTE
In the text that follows, the primary discussion of a functional circuit
is indicated by lettering the circuit description in boldface, for example,
RF Phase Detector. The term "Interface" refers to the HP 1184&A
Phase Noise Interface; "System" refers to the HP 3048A Phase Noise
Measurement System. An RF spectrum analyzer is assumed to be connected
to the System.
Measurement Modes
To clearly understand the functions of the Interface, it is helpful to see the relationship of the Interface
to the other System instruments and the device-under-test in a phase noise measurement. Refer to
Figure 4.
Figure 4 shows the most basic measurement mode where the noise on the source has been demodulated
externally and fed into the front-panel NOISE INPUT. In this mode of operation the Interface serves
mainly to condition (amplify and filter) the noise. The conditioned noise is then measured (in the
frequency domain) by the spectrum analyzers. The measured noise is then analyzed, mathematically
manipulated, and plotted by the controller. The noise need not be exclusively phase noise; it could also
be AM (from an AM detector), FM (from an FM discriminator), or baseband (0.01 Hz to 40 MHz)
noise.
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