Using National Instruments Data Acquisition in a Microgravity Environment

Author(s):
Jacob Stich - Student, Pittsburg State University
Randy Winzer - EET Program Coordinator, Pittsburg State University

Industry:
University/Education, Aerospace/Avionics

Products:
LabVIEW,

The Challenge:
Designing a method to test the acceleration-induced frequency sensitivity or shift of a crystal oscillator as the foundation for creating an acceleration compensation system.

The Solution:
Using the National Instruments DAQPad-6016 data acquisition device for USB and LabVIEW to develop a method for testing acceleration-induced frequency sensitivity of a crystal oscillator

NASA often uses sounding rockets to study the atmosphere. These rockets offer a cost-effective method for conducting experiments and gathering information. Data collected during these flights is sent back to the launch site for analysis via an FM-based communication system.

The FM-based communication scheme NASA currently uses is very expensive but also very forgiving in terms of center frequency drift. On the other hand, using a Tracking and Data Relay Satellite System (TDRSS) transceiver on board the rockets would allow for greater launch site flexibility and more robust communication from the rockets back to the launch site. However, the TDRSS communication scheme loses communications (COM) lock if a shift in the local oscillator (LO) frequency occurs. This shift can be caused by several factors, including acceleration, which this project addresses. The sounding rockets could expose the LO to as much as 30 g’s of acceleration. If the TDRSS loses COM lock with the rocket, valuable information could be jeopardized.

Testing Frequency Shift in Crystal Oscillators during Rocket Flights

Using the DAQPad-6016 data acquisition device and LabVIEW software, we have developed a method to test the acceleration-induced frequency sensitivity of a crystal oscillator as a precursor to designing an acceleration compensation system. This project, coordinated in conjunction with the NASA Wallops Flight Facility near Chincoteague Island, Virginia, was approved for a November 2005 NASA C-9 Reduced Gravity Aircraft flight. The C-9 has the ability to establish periods of 0 and 2 g for 15 s and longer. The data collected by this application is essential to designing an accurate system to compensate for acceleration-induced frequency variations.

A device with the ability to monitor and quantify the acceleration sensitivity of a crystal oscillator is necessary to implement a cost-effective, stable TDRSS transceiver. This project involves the first step in developing a procedure to stabilize the 10 MHz local oscillator of a TDRSS transceiver on board a sounding rocket.

Basic Design Development Procedure

During this step, we first introduce a suite of 10 MHz crystal oscillators to various accelerations using a vibration table. The output signal of the crystal oscillator under test is then mixed with a 10 MHz (non-accelerated) reference signal via a frequency mixer.  We then monitor the height of the sidebands with the DAQPad-6016 data acquisition device for USB.

Due to the nature of the shake table facility being used, the two variables of most concern (besides acceleration) are magnetic field and electromagnetic interference (EMI). This is because the vibration table is essentially a giant speaker configured to vibrate a platform. Therefore, extraneous magnetic fields and EMI could potentially contaminate the experiment. In an attempt to null this effect, we surround the crystal oscillator with mu-metal to shield it from magnetic fields, and we enclose the entire apparatus (oscillator and environment board) in a specially designed EMI shielded box. After we conduct the tests on the shake table, we are going to put the device through real-world assessments on board the NASA C-9 microgravity aircraft. During this flight, the microgravity plane subjects the unit to periods of 0 to 2 g’s and generates results we can compare with the shake table facility results.

Flight Tests

During the second phase of testing, we hope to correlate the data obtained from the vibration table with that of the C-9. Although the 2 g data will be the most useful, the results obtained from the periods of 0 g are very important in understanding and creating a full acceleration response graph. To accommodate the data collection during phase two, some changes to the test setup will occur. Rather than a single setup, we will implement two synchronized setups. The ground setup will be located at Ellington Field and will provide control conditions with respect to acceleration. It will include a reference oscillator with LabVIEW and the NI DAQPad-6016 for monitoring variables such as temperature, humidity, magnetic field, and acceleration.

The test setup onboard the C-9 aircraft is a duplicate of the ground setup, and the difference in acceleration as well as the frequency response of both oscillators will be the primary data of interest during the test.  In order to make sure data from both oscillators is properly correlated for analysis, the data acquisition operations from the reference setup and from the crystal onboard the C-9 need to be synchronized.  For this phase of testing, the research team will add two GPS receivers to provide time synchronization between the systems. 

Our hypothesis is based on papers by Raymond L. Filler for the IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control¹and R. B. Haskell and P. E. Morley of Vectron International, a leader in the crystal oscillator industry.² They have determined that the acceleration sensitivity depends on the direction of acceleration and is effectively a vector quantity. To find acceleration sensitivity, scalar measurements are typically performed with equal accelerations in three mutually orthogonal axes. The spatial properties then can be derived from those three measurements.³ The instantaneous frequency of a crystal integrated into an oscillator circuit can be described by:

                      w(t)=w₀{1+ya cos(w_vt)}                          (Equation 1)                                                                                                                                                              

Instantaneous Frequency³

Where w_v  is the vibration frequency, w₀  is the resonator frequency, y is the component of the g-sensitivity vector in the direction of interest, and a is acceleration. Filler, Haskell, and Morley have shown that if the oscillator output is mixed with the output of a stable reference, the y sensitivity can be related to the resultant first-order sideband levels relative to the carrier power. This equation is:

                                        y = 2Wv  10^L/20      (Equation 2) 

                                             aw₀                          

Oscillator Acceleration Sensitivity³

L is the relative sideband level in dBc (dB in relation to the carrier power). The resultant mixing and relative power levels of these two frequencies can be observed using a spectrum analyzer centered at the reference oscillator’s output frequency.⁴

According to IEEE Standard 1193-1994,⁵ crystal oscillators are sensitive to not only acceleration but also other repetitive stimuli such as temperature, pressure, electromagnetic effects, humidity, and neutron radiation. This project monitors the first four, excluding neutron radiation, with an environment board, which allows any nonacceleration-related results to be discarded from the data. After performing our ground-based tests, we can plot the output of the crystal oscillator under test against the acceleration and, using Equation 2, determine the acceleration sensitivity (Γ) of each crystal that we subject to the experiment.

The goal of this research was to establish a cost effective, repeatable test to quantify the amount of frequency shift under a given acceleration condition.  This system will gather data that can be used to facilitate the design of a low-cost TDRSS transceiver based upon a crystal oscillator reference.  The next step would be to use the data collected to develop such a TDRSS transceiver that could provide compensation for any acceleration induced frequency shift.

For more information, contact:

Randy Winzer

Pittsburg State University

1701 S. Broadway

Pittsburg, Kansas 66762

Tel: (620) 235- 4370

Fax: (620) 235-4004

E-mail: rwinzer@pittstate.edu

 

---

Browse All Case Studies »