The SARK-110 provides a single frequency measurement mode that presents the complete impedance parameters at a user selectable frequency. To facilitate the reading, the analyzer displays graphically the R-L-C as both series and parallel equivalent circuit and the SWR as a convenient bar graph.

The analyzer uses an internal 2MB flash disk for the storage and recall of measured parameters, screenshots, analyzer configuration and firmware updates. This disk is accessible via USB so the measured parameters can be downloaded to a PC for analysis using the ZPLOTS spreadsheet program or the SARK Plots client software for Windows.

The SARK-110 is as well a hackable device. You can experiment with the device by writing an application using the provided open-source SDK. Besides the SDK includes a device simulator for the PC so you can easily debug your application before loading to the device. For more advanced users, the SARK-110 board provides connections to the STM32 SWD interface so you can use the ST-Link in-circuit debugger or equivalent to program or debug your software.

With its pocket size, features, and simple user interface the SARK-110 offers a perfect device for HF/VHF impedance measurements for the RF experimenter and for the amateur radio operator. Whether you are at home in your shack or in the field during a field day or expedition.

• Pocket size and lightweight
• Solid aluminum case
• Intuitive and easy to use
• Operating modes: Scalar Chart, Smith Chart, Single Frequency, Cable Test (TDR), Field Mode, Multi-band, Signal Generator, Computer Control and Band Scan
• Excellent accuracy over a broad range of impedances
• Resolves the sign of the impedance
• Manual and automatic positioning tracking markers
• Transmission line add and subtract
• Circuit models function: transmission line, inductor, capacitor and crystal
• Internal 2MB USB disk for the storage of measurements, screenshots, configuration and firmware upgrade
• Exports data in ZPLOTS-compatible format for further analysis on a PC
• SARK Plots client software for Windows
• Lifetime free firmware upgrades
• Open to community requested features
• Open source Software Development Kit (SDK) including a device simulator for development of user applications

   

• Theory of operation

  The block diagram below illustrates the main functional blocks of the SARK-110 Antenna Analyzer:
   
  
  
  
  The 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 channels output is frequency dependent and it is reduced with increasing frequency following a SIN(X)/X function. The SARK-110 software compensates 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 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 analyzers 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.

   

• Specifications

• General	· Frequency range: 100 kHz to 230 MHz · Frequency resolution: 1 Hz · Frequency stability: ± 30 ppm · Sine wave output · RF Connector: MCX socket · Output power: »-10 dBm (0.1mW, 70.7mV rms) into a 50-ohm load · Sweep time (Scalar Chart/Smith Chart/Field Mode): 3 seconds (Sampling: Normal), 5 seconds (Sampling: Double), 1.5 seconds (Sampling: Normal - Res. 1/2), < 1 second (Sampling: Normal  Res. 1/4 and 1/8) · Sweep time (FDR): 6 seconds · Frequency Resolution: 258 points per sweep (258 to 10000 in deep sweep mode) · Return loss dynamic range: 0 to -60 dB · VSWR dynamic range: 100:1 maximum · Impedance reading in open circuit: 60000 (@1 MHz) · Measurement limits: |Z| < 100K, |R| < 100K, |X| < 100K, |Rho| < 0.98, C < 100 nF, L < 100 mH, -180 < ? < 180
  Measured Parameters	Complex impedance (series and parallel) and reflection coefficient in rectangular and polar form, VSWR, return loss, reflection power percentage, quality factor, equivalent capacitance, equivalent inductance
  Operating Modes	Scalar Chart, Smith Chart, Single Frequency, Cable Test (TDR), Field, Multi-band, Signal Generator, Computer Control and Band Scan
  Features Common To Most Modes	· Presets for amateur radio bands · Adjustable reference impedance · Save to disk and recall functions · Three available fixed scale options and automatic scaling · Presets for popular coaxial cables · Add/subtract transmission line · Black or white color schemes · Adjustable plot trace widths
  Scalar Chart Mode	· Graphical plot of two user-selected parameters in a rectangular chart · Two markers with manual or automatic positioning · Display detailed parameters for center frequency or any of the two marker positions
  Smith Chart Mode	· Plots complex reflection coefficient in Smith Chart form · Two markers with manual or automatic positioning · Display detailed parameters for center frequency or either of the two marker positions
  Single Frequency Mode	· Display all parameters for a single frequency · Graphical representation of series and parallel impedance equivalent · VSWR Audio feedback · Circuit models and automatic LC matching network calculator
  Cable Test Mode	· Range: about from 2 to 250 m · Displays step and impulse responses
  Field Mode	· Graphical plot of one user-selected parameter in a scalar chart with enhanced legibility · Display max and min values
  Multi-band Mode	Display rectangular charts for four bands simultaneously
  Signal Generator Mode	· Frequency range: 1 kHz to 230 MHz · Programmable output level from -73 dBm to -10 dBm into a 50-ohm load · Continuous, frequency sweep, AM and FM modulation modes · Linear, logarithmic, bi-linear, and bi-logarithmic sweep modes
  Markers	Tracking modes: Peak Min, Peak Max, Absolute Min, Absolute Max , Value Cross Any, Value Cross Up, Value Cross Down
  Interface	· Full color 3 TFT LCD 400 x 240 pixels · 4 dedicated buttons · 2 navigation keys
  PC Interface	· USB Mini-B receptacle · USB 2.0 Full Speed Composite Device: o Mass Storage Class (internal disk) o HID Class (Computer Control mode)
  Storage	· 2 MB internal disk FAT compatible · USB Mass Storage · Screenshot save and recall feature · Measurement data save and recall feature · Possibility of files with user definable frequency presets, scale settings, and three custom cable types · Measurement data files compatible with SARK Plots and ZPLOTS programs
  Calibration	· Automated Open/Short/Load calibration with up to eight stored profiles · 400 calibration points · Frequency calibration
  Measurement Architecture	· Single conversion superheterodyne · Two independent measurement channels for simultaneous voltage and current    measurement for precise phase measurement · Two synchronized 12-bit analog to digital converters
  Processor	72 MHz STM32 MCU with 256KB Flash and 48KB SRAM
  Power	· 3.7V 1000mAh Internal Lithium-Polymer battery · USB for operation and charging · Automatic Power Off functionality (disabled, 5, 10, or 30minutes) · Autonomy: approx. 2.5 hours · Charge time: approx. 3.5 hours
  Environment	Operating temperature: 0ºC to 50ºC
  Dimensions	98 * 60 * 14.5 (mm)
  Weight	120g
  Package Content	· SARK-110 x 1 with built-in battery · MCX to SMA female connector adapter x 1 · SMA plug to SMA female 8 (20-cm) cable adapter x 1 Shark-100 Manual