The most exciting new technology to hit the PC-based test market in recent years is the data acquisition cards for the PCMCIA interface. These credit card-sized boards slip into a Type II PCMCIA (Personal Computer Memory Card International Association) slot found in most laptop computers, allowing data to be taken anywhere a laptop can be carried–in planes, cars, around a lab, on a production floor or in thea field.
In addition to mobility, the cards offer the feature of hot swapability. After collecting data, for example, you can take the card out of its slot and load in a memory, fax, modem, communication or another data acquisition card without powering down or rebooting.
One company taking advantage of this nascent technology is JEF Consultant, Inc., of Belleville, MI. The company uses an eight-channel analog-to-digital (A/D) Type II PCMCIA card in a variety of characterization tests for an automobile.
This card has a set bipolar range of +5 V, a 25-kHz throughput and a 12-bit A/D converter. Data is taken at different lab setups, outdoors and in automobiles.
JEF designs test systems for automobile and other applications. Some of these systems are used on JEF’s antenna ranges to support the testing and development needs of their clients. Others are sold to automobile manufacturers and suppliers.
These systems are used for performance characterization testing of automobile antennas and other RF devices, such as remote keyless entry systems (super-regenerative receivers). The characterization testing of car antennas illustrates how PCMCIA cards can be used in static or dynamic environments.
Several typical automotive antennas are tested, including FM (88 to 108 MHz), remote keyless entry (240, 315 or 434 MHz), cellular telephone (820 to 895 MHz), and GPS (navigational systems between 1 and 2 GHz). Other possible applications are VHF and UHF television, collision avoidance and driver aid systems as well as other intelligent highway systems.
One of the most important parameters describing an antenna is its pattern or the coverage of the antenna.
The sample pattern in Figure 1 shows an omni-directional antenna with poor performance at 40o, 125o and 340o.
The shape of and the materials used in a vehicle affect the antenna pattern. As a result, an omni-directional antenna tested by itself has a different pattern when tested in the vehicle. Due to the complexity of the vehicle’s makeup, it is difficult to predict performance. The best way is to measure it.
Three types of tests are performed. While they differ in method, each test type installs and tests various antennas in many locations in the vehicle.
In one test, a car is rotated on a turntable and the installed antennas are evaluated on an outdoor antenna range using lab equipment and a transmit antenna to simulate the broadcast station (Figure 2). A signal generator feeds an RF signal to the transmit antenna, which then transmits a signal to the antenna to be tested.
The second test is done in the field using a slightly different setup. The car is taken to a remote location, driven around in a circle and the installed antennas are evaluated using ordinary broadcast signals. The third test is the same as the first except the antennas are evaluated as the vehicles are driven normally.
For the first test, the antenna-under-test receives the signal and feeds it to a spectrum or network analyzer. The analyzer detects the high frequency (1 MHz to 2,000 MHz, typically), takes the logarithm of its magnitude (readings are made in decibels for greater dynamic range), and converts it to a DC voltage output proportional to received signal strength (called a Y or vertical output).
That output is fed into the PCMCIA A/D card installed in the laptop. The linear relationship between the signal received by the analyzer (in decibels) and the vertical output is characterized by two calibration measurements before testing begins. By using this relationship, the DC voltages read by the A/D card can now be related to the RF signal levels.
Once this calibration is completed, the car is rotated on the turntable. Pulses from a shaft encoder attached to the turntable are used to trigger the A/D card. Conversions are made once every 0.1° or 1° , depending on the client’s preference.
Using the shaft encoder as the A/D trigger source instead of a timed trigger permits measurements to be made independent of the rotational speed of the pedestal. This is especially important for automotive work where the mass of an entire vehicle has to accelerate to running speed and then decelerate to a stop. As the antenna is rotated through 360o, the data collection system records the change in the received signal level vs angle, characterizing the antenna pattern of the antenna.
After all the data has been collected, RF signal strength is calculated by the computer from the DC voltages read by the A/D card from the vertical output of the analyzer. This data can be displayed in polar or rectangular form, printed on plotters or laser printers, and saved to disk.
Various reduction routines can be applied to the data to determine pattern averages, minimum/maximum ratios and standard deviations. For directional antennas, the side-lobe level, 3-dB beam width and other parameters can be determined.
The next part of the antenna characterization involves measuring the antenna’s response to actual broadcast signals. To do this, the laptop is brought inside the car. There, it is connected to a spectrum analyzer, an external trigger source and a directional gyroscope (Figure 3).
The directional gyroscope senses the angle of the vehicle and has a 0 to +5 VDC output that corresponds to 0 to 360o. As the car is driven around in a circle, the spectrum analyzer senses the amplitude of the broadcast signal by the antenna-under-test and again converts it to a DC signal. This DC output and the DC output from the directional gyroscope are fed into the PCMCIA A/D card.
The laptop computer monitors the received signal strength from the spectrum analyzer and the angle from the gyroscope through the A/D card as the car is driven around in a circle. Polar antenna pattern measurements are calculated from the resulting data of the amplitude of the signal vs the angle data from the gyroscope.
Amplitude vs time/distance also can be recorded while the vehicle is in motion or stationary (time only). This allows you to determine the signal level received by the vehicle antenna from a broadcast station.
An external TTL level trigger of known resolution, such as 1 pulse per foot, is fed to the A/D external trigger input to trigger conversions. The vertical output of the spectrum analyzer is fed to the PCMCIA A/D card to record the signal amplitude.
With this process, the broadcast signal amplitude received by the antenna-under-test is recorded each time the TTL trigger causes a conversion. Adding the directional gyroscope provides angle data in addition to the distance data and adds mapping capability so the route driven can be documented and related to the amplitude data through markers.
All data taken in these tests is stored in the laptop’s memory and can be exported later into spreadsheets or applications software for analysis, or stored on a disk. For large blocks of test data, all information can be downloaded onto a PCMCIA memory card. Then the memory of the laptop can be cleared and more testing performed.
PCMCIA is new for data acquisition, but its use is increasing as more is learned about the benefits–cost savings, mobility and performance–these cards can supply. This technology will soon be changing the look of both the data acquisition market and the PC market as faster, more powerful products appear and the installation of PCMCIA slots proliferates in both laptop and desktop computers.
Special thanks to Alan Miller at JEF Consultant for his help and technical input.
About the Author
Vincent Hebert has worked for six years in the field of data acquisition test and technical marketing. He holds a degree in engineering and is completing B.S.E.E. degree studies at Roger Williams University. ComputerBoards, Inc., 125 High St., #6, Mansfield, MA 02048, (508) 261-1123.
Automotive Electronics Test
Copyright 1995 Nelson Publishing Inc.