Section 1 Description of Module Operation
1-2
1.1 Outline
1.1.1 Outline for MS2681A
Please refer to Fig. 1-1 Overall Block Diagram.
The MS2681A is a superheterodyne system scanning-type spectrum analyzer.
For RF input after passing through the 8 GHz STEP ATTENUATOR frequency of 9
kHz to 3.0 GHz the signal pass through the Coaxial LPF and then to A10 3 GHz
CONVERTER.
In the A10 3 GHz CONVERTER, RF input signal gets mixed with first Local
Oscillator signal to generate a 4060.69 or 4066 MHz first IF signal by first mixer.
After passing several filters and amplifier, First IF signal gets mixed with second
Local Oscillator signal (4000 MHz) to generate a 60.7 or 66 MHz second IF signal
by second mixer, and fed to A06 IF.
The first Local signal generated at YTO (YIG tuned oscillator) is frequency-swept by
scan signal from A05 SCAN/AD after phase-lock to reference signal (its frequency is
26 to 32 MHz) generated on A08 LOCAL02 at the center frequency of its sweeping
range, in normal sweep condition.
The YTO output is passed through an amplifier, and then divided into two paths with
a directional coupler. One of divided signal is fed to sampler circuit, sampling
signal (its frequency is 194 to 206 MHz) generated on A08 LOCAL02 is frequency
multiplied, and mixed with YTO output to generate sampler IF signal with a
frequency of 26 to 32 MHz.
The 4-GHz second local signal is phase-locked to 100-MHz VCXO signal, of which
the frequency is also phase-locked to 10-MHz crystal oscillator.
In the instrument, high accuracy 625-kHz/50-MHz signals are present for calibration.
These signals are generated by dividing the reference signal, and the CAL ATT
varies power level of 625-kHz signal with 1-dB steps.
Internal calibration operation being carried out, these calibration signal is fed to RF
signal-route through the switch in 8 GHz STEP ATTENUATOR.
66 MHz second IF signal is selected when resolution band width (RBW) setting is 1,
3, 5, 10, or 20 MHz. For other settings, 60.7 MHz is selected.
