SteppIR SARK-110, User Manual

Page: 62 / 78

SteppIR
SARK-110
User’s Manual
Rev 1.1.7 October 29 th , 2014 - 62 - © Melchor Varela – EA4FRB 2011-2014
Appendix A: Theory of Operation
The block diagram below illustrates the main functional blocks of the SteppIR SARK-110 Antenna
Analyzer:
The SteppIR SARK-110 comprises four main sections: a signal generator used as an active source,
a bridge to provide signal separation, two tuned receivers that downconvert and detect the signals
and a microcontroller and display for calculating and reviewing the results.
The signal generator is provided by a single chip dual direct digital synthesizer (DDS) AD9958 from
Analog Devices, which generates a sinusoidal signal for impedance measurement and a local
oscillator signal for the tuned receivers (mixers). One of the DDS channels operates at the specified
test frequency and the other is programmed to operate just 1 kHz above it, which is the value of the
intermediate frequency. The DDS has an internal oscillator driven by an external 24 MHz crystal and
is able to multiply this clock internally by a user configurable factor of 4 to 20, so the maximum
internal clock frequency is 480 MHz. In general the DDS can be configured to generate a frequency
of up to one third of the clock frequency but in this design, due to the external reconstruction filter, it
is possible to achieve an output frequency of up to 230 MHz.
The amplitude level of the DDS channel’s output is frequency dependent and it is reduced with
increasing frequency following a SIN(X)/X function. The
SteppIR SARK-110 software compensates