The measuring system is for radio-technical measurements of avionics. The requirements placed on it are largely conform to the features of the Rohde&Schwarz CMS54 tester and merely extend its range or accuracy in certain directions. The instrument has logically become the basis of the system and whole systems allows automated measurement. The system is complemented by several modules extending the field of application, measuring ranges, etc. The high-frequency input is common to both receiver and transmitter tests and must be done without cable handling.

There is a pair of low-frequency generators with a minimum frequency range of 20 Hz to 100 kHz (CMS54 has a range of up to 30 kHz), a phase-sensitive voltmeter to measure voltage 3 mV to 300 V and frequency range 20 Hz to 100 kHz (voltmeter built into CMS54 has a range up to 30V and up to 20kHz), high-frequency generator with a resolution of 1 Hz (CMS54 has a resolution of 10 Hz) and last but not least memory oscilloscope with frequency range up to 100 MHz for input level 10 mV to 10 V (CMS54 contains only single oscilloscope with range up to 20kHz) in the system.


VXI

The Pentek type 5020 is used to generate low-frequency signals. It is a four-channel generator controlled by a built-in signal processor. The frequency is synchronized from a precision frequency generator and allows to generate a signal in the DC to 20 MHz range with a resolution of 0.012 Hz. The output signal is +10 dBm at a resistance of 50 Ohm, sine wave purity is guaranteed in the range of -55 dBc.

The Marconi 3002 high frequency generator generates frequencies from 9 kHz to 2.4 GHz with a resolution of 1 Hz, the output is -137 to +25 dBm / 50 Ohm with a resolution of 0.1 dB. Residual frequency modulation is 4.5 Hz, harmonic distortion is less than -30 dB and non-harmonic is less than -60 dB. Frequency modulation is possible in the range of 0 to 100 kHz, with distortion rate 3% (typically 0.5%). Amplitude modulation is up to 500 MHz in the range of 0 to 99.9% and with an error of 1.5% to 2.5% (depending on the modulation depth).

The frequencies of both the low and high frequency generators are synchronized with the Racal Dana 3351E frequency normal. It is a standard based on a thermostatically protected oscillator generating frequencies of 1MHz, 5MHz and 10 MHz with an accuracy of 5×10-10 per day. If even higher accuracy is required, a version with a rubidium oscillator can be used, which increases the accuracy by more than 2 orders of magnitude. The module contains three separated outputs.

The VXI227 is a phase sensitive voltmeter that measures voltages and alternating phases at frequencies of 100 Hz to 100 kHz with 4½ digits accuracy, phase accuracy is ¼ of degree. Measured voltages range from 3 mV to 300 V with automatic range selection.

The Tektronix TVS621 dual channel digital oscilloscope module is used to monitor faster waveforms. The module has a bandwidth of 250 MHz and is equipped with an 8-bit converter with a maximum sampling rate of 1 GSa/s, which meets the requirements. The length of the recording memory is 15,000 samples. The module is equipped with considerable computational capacity, which enables evaluation, transformation, digital filtration etc.

The Tektronix VX4357 switch is used to switch individual channels. These are 32 individually controlled SPDT relays that can carry up to 250V ~ 2A. This arrangement of the switch allows to define the connection at the moment of creating the measuring program, or to modify it when preparing a specific test.

The modules are placed in the VXI frame of VX1410 type from Tektronix. This frame is equipped with 13 positions and is designed so that the faulty module can be replaced as quickly as possible in case of failure. For example, the power block can be replaced within three minutes.

The system also includes a Pentium Pro computer operating at a frequency of 200MHz. The VXIpc-860 computer, in the form of a VXI module, is inserted into the system frame. The computer will collect data and generate excitation waveforms.

The solution has the following main advantages:
  • highly reliable solution,
  • an open solution allows to combine the most suitable modules into one unit,
  • the system will become morally obsolete very slow,
  • one partner for solving possible complaints of technology and programs, domestic service,
  • one compact system with built-in computer,
  • all common requirements are implemented by a standard radiotester, while VXI modules implement required extensions, resulting in the possibility of successive purchase of individual parts according to requirements and possibilities,
  • generator properties exceed requirements and thus have sufficient margin to deal with future stricter requirements,
  • programmable configuration,
  • connection to the surroundings via a computer network,
  • modules are VXIplug & play compatible, with full VXIplug & play support from National Instruments (de facto from the creator of the standard),
  • possibility to lend modules during repair or calibration period.

More information request here.