SC5305A Operating & Programming Manual
Rev 2.1.0
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S C 5 3 0 5 A
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H E O R Y O F
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P E R A T I O N
Overview
The SC5305A operates on the principle of heterodyning, a process whereby an incoming RF signal is
mixed with specific oscillator frequencies in stages, producing both sum and difference frequency
products. At each stage the summed frequency product (or image) is removed through low-pass
filtering, allowing the difference frequency product to continue through the signal path. Repeating this
process several times using carefully selected local oscillators (LOs) and well-designed band-pass
filtering, the original signal is translated or “downconverted” in frequency low enough for inexpensive
digitizers to acquire the signal with reasonable bandwidth. The resultant output signal of a heterodyne
downconverter is known as the intermediate frequency (IF). Using a tunable LO as the first mixing
oscillator allows the downconverter to translate a broad range of frequencies to a common IF output.
When combined, a tunable LO and extraction of the lower mixed frequency product creates an
important and useful variant of the heterodyne process known as superheterodyning.
The SC5305A is a three-stage superheterodyne downconverter that delivers superior image rejection
over single stage conversion and offers both high signal-to-noise dynamic range and high spurious-free
dynamic range. The RF input ranges from 1 MHz to 3.9 GHz, and the IF output is fixed at 70 MHz. When
the input frequency is lower than the intermediate frequency, the device technically behaves as an
upconverter. The SC5305A up-converts when the input frequency ranges from 1 MHz to 70 MHz. The
converted spectrum polarity may be inverted or non-inverted by programming the device accordingly.
Fundamentally, each conversion stage consists of a frequency mixer that mixes two input signals and
producing a wanted third. The
wanted
third component is selected, via a frequency filter, among other
signals generated in the mixing process. The three primary components of the signals in each conversion
mixer are commonly known as the local oscillator (LO), radio frequency (RF), and the intermediate
frequency (IF) as shown in Figure 2.
Figure 2. Frequency conversion stage using a mixer.
Where R represents the RF component, L represents the LO component, and I represents the IF
component. The LO is resident in the downconverter and is either frequency tunable or fixed in
frequency depending on the stage.
The first IF stage is an upconversion stage - all input signals are converted to an IF higher than the
highest input frequency specified. The second and third stages successively convert this high first IF
down to the final IF of 70 MHz. Having a high first IF allows the downconverter to achieve very high
image rejection ability. This image-free architecture achieves high image rejection without the need for
sharp cut-off pre-select band-pass filters. Having high image rejection makes the SC5305A suitable for
L
R
I
RF
LO
IF