SteppIR
SARK-110
User’s Manual
Rev 1.1.7 October 29
th
, 2014
- 63 -
© Melchor Varela – EA4FRB 2011-2014
for this amplitude rolloff effect by using the capability of the DDS to adjust the amplitude level of the
output signal, so the analyzer maintains a flat output amplitude.
The output of each of the DDS channels is differential and is amplified by a dual high speed current
feedback amplifier working in differential input mode and with output in single ended mode. Most
DDS designs use a broadband balun transformer to convert to single end mode but because of the
restricted height available in the
SteppIR
SARK110 enclosure a silicon-based solution was chosen.
The output of each amplifier is followed by elliptic low pass filters with a cut-off frequency of 230
MHz. These filters reduce the level of spurious high frequency components that appear in the output
of the DDS. These high frequency components consist of aliases at multiples of the internal clock
frequency as well as other spurs.
For impedance measurement a resistive bridge is used because of its simplicity and good frequency
response, working down to DC. In the bridge a voltage across one resistor is proportional to the
voltage being applied to the circuit under test and the voltage across another resistor is proportional
to the current flowing into the circuit connected to the analyzer’s test port. Both the magnitude and
phase are measured. The ratio of the two corresponds to the impedance we want to measure.
One of the mixers is used for the voltage measurement and the other for the current measurement.
The output of the mixers is the 1 kHz I.F. signal which is then amplified and filtered with a bandpass
filter before digitizing. Identical mixer and amplifier circuits are used for both the voltage and current
sensing paths. Any small differences in the gain and phase shift of these two signal paths are taken
care of by the calibration process.
The core of the analyzer is an STM32F103 microcontroller from STMicroelectronics. This
microcontroller incorporates the high-performance ARM Cortex M3 32 bit core operating at 72 MHz,
a Flash memory of 256 KB, SRAM of 48 KB, and an extensive range of I/O and peripherals
including a USB device controller and three 12-bit ADC converters. The digitizing of the 1 KHz I.F.
signal is done by two independent 12-bit ADC converters contained in the STM32 MCU. These two
converters operate simultaneously and are synchronized, so providing good accuracy for the phase
measurement.
The two sets of digital data from the voltage and current sensors are analyzed using an optimized
implementation of the discrete Fourier transform that works with a single bin. This produces the
amplitude and phase of the 1 kHz fundamental signal and cancels out any dc component due to
offsets in the operational amplifiers. The load impedance magnitude is the voltage amplitude divided
by the current amplitude. The phase angle of the impedance is the difference in the phase angles of
the voltage and current. Knowing these two parameters, we can calculate the equivalent resistance
and reactance of the load impedance. The rest of the parameters such as VSWR, reflection
coefficient, etc. are derived from the measured impedance value.
Summary of Contents for SARK-110
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