Table of Contents
Advertisement
User Interface
Normal operator interaction with the Instrument begins at the A6 Front Panel which
includes a keypad (A6A2) for entering Instrument configuration and control data and a
display (A6A1) for verifying the selected configuration of the RF output signal. Both the
keypad switches and the display are included on a plastic bezel assembly which
comprises the majority of the Front Panel Assembly. A rubber keypad, whose keys
include a conductive backing, provides switching information to the A3 Digital PCA. A
controller on the A3 Digital PCA provides the addressing, strobing, and storage of data
coming from and going to the display.
Frequency Synthesis
Primary frequency synthesis takes place within the A1 Synthesizer PCA (Figure 6-2),
where a combination of phase lock, frequency translation and direct digital synthesis
(DDS) techniques are used to generate signals of appropriate resolution, phase noise and
spectral purity. Four main outputs are created, covering the ranges 20 kHz to 9 MHz,
1 GHz to 2.7 GHz, 2.7 GHz to 4 GHz and, for wide deviation FM only, 62.5MHz to
125MHz. To achieve the required frequency accuracy, these outputs are derived from a
10 MHz Oven Controlled Crystal Oscillator (OCXO), but the instrument will also accept
an external reference clock source of between 1 MHz and 20 MHz in 1 MHz increments.
The four A1 Synthesizer PCA output signals are fed to the A2 RF Output PCA
(Figure 6-3) where additional frequencies are derived: frequencies covering the range 9
MHz to 1 GHz (with the exception of wide deviation FM signals at carrier frequencies
below 125 MHz) are derived by applying signals of between 1 and 2 GHz to a binary
divider chain, and frequencies spanning the range 10 Hz to 20 kHz are generated by
means of a wave table and associated DAC. Frequencies covering the range 10 Hz to
4 GHz are thus available from the A2 RF Output PCA.
Amplitude Control
Signals from the various sources are amplified, filtered and selected as appropriate to
produce a sinusoidal output at the required frequency.
To achieve the necessary dynamic range on the output, up to 138 dB of switched
attenuation, comprising 60 dB on the A2 RF Output PCA (Figure 6-3) and 78 dB in the
A9 Leveling Head (Figure 6-4), can be applied to the output signal.
For frequencies greater than 20 kHz, the output amplitude is controlled by a feedback
loop which compares the output level with that of a set value and maintains it by
adjusting voltage controlled attenuator circuits in the forward signal path. At higher
output levels all switched attenuation is applied within the A9 Leveling Head, and in this
condition the signal level is sensed in the A9 Leveling Head prior to attenuation. Two
level detectors are used: an LF Detector (20 kHz < frequencies > 125 MHz) and an HF
Detector (frequencies > 125 MHz). For output levels below approximately -60 dBm, the
60 dB attenuator in the A2 RF Output PCA is switched in, and in this condition the level
is sensed by LF and HF Detectors on the A2 RF Output PCA prior to attenuation.
To offer the greatest noise immunity when connecting the Instrument to a UUT, the
A1 Synthesizer PCA and the A2 RF Output PCA power supplies are isolated from Earth
so that the instrument output is floating.
Theory of Operation
Overall Functional Description
6
6-5
Advertisement
Table of Contents
