BAND
3
CONVERTER
Measurement of a signal in Band 3 is accomplished by down converting from the microwave range to
approximately 125 MHz. This is accomplished by mixing the input signal with a known reference fre-
quency which is found by selecting a VCO harmonic in the range of 400 to 500 MHz. The VCO frequency
can be selected in 50 kHz increments by using a microprocessor controlled phase lock system, while re-
taining the accuracy and stability of the counter's time base reference.
A simplified diagram of the Band 3 converter
is
shown in figure 4-5. There are two major assemblies. The
Converter Control assembly (A1 08) and the Converter Assembly
(A203).
CONVERTER CONTROL
A108
The Converter Control assembly contains the interface between the microprocessor bus system and the
Converter
(A203). A digital-to-analog converter and a precision current (YIG) driver provide a
2
MHz
frequency resolution for setting the YIG filter of
A202.
A108 also contains the programmable VCO phase lock control system. This system lets the microprocessor
interface select any VCO frequency between 400 and 500 MHz, in increments of 50 kHz.
CONVERTER
A203
'The Converter assembly consists of three subassemblies.
A201A. Voltage Controlled Oscillator (VCO) Assembly
A201B, IF Amplifier Assembly
A202, Microwave Assembly (yig)
The A202 Microwave Assemblycontains the YIG filter, mixer and comb generator.
The input signal (1
GHz
-
18 GHzI26.5 GHz) passes through a YIG filter on A202. The filter
is
an
electronically tunable
bandpass filter, with an operating frequency proportional to its tuning current.
This filter determines the approximate frequency of the input signal, and filters out any undesired
signals, making it possible to count a signal at one frequency even if a larger signal is present at another
frequency.
When tuning the
YIG filter to the input signal, the mixer is used as an RF detector, and its output
is
ampli-
fied in the video amplifier on the IF assembly
.
The output of the Video amplifier
is
maximum when the YIG filter
is
tuned to the input signal. In the
case of multiple input signals, the video amplifier output determines which signal
is
largest.
Scans by ArtekMedia © 2007
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