Some Applications of FPGA in Custom Waveform

Generation and Triggering for Metrology

Department of Instrumentation and Control Engineering

Netaji Subhas Institute of Technology, New Delhi

Authors

Dr. K.P.S. Rana Dr. Vineet Kumar

Mr. Sumit Kumar Shakya

Mr. Nishant Mittra Mr. Neel Pramanik

Department of Instrumentation and Control Engineering Netaji Subhas Institute of Technology Sector-3, Dwarka, New Delhi -110078

Abstract

Precise custom triggering and waveform generation are necessities in several

metrological applications these days. With the advent of high speed computation and

precise electronics fabrication, it is now possible to generate customized triggering

and waveforms meeting precise timing and frequency standards with the help of FPGA

technology. These are reconfigurable real time devices available at relatively low costs.

FPGAs also have advantages over their counterparts as all of the logic can be

implemented in hardware alone thereby, gaining maximum processing speed.

Additionally, multiple tasks can be scheduled to run simultaneously because each task

will utilize a different set of logic on the FPGA hardware. FPGA finds potential

applications in real time control, signal processing, digital communication and custom

timing and triggering.

A LabVIEW based approach to custom waveform generation and triggering

applications of FPGA is demonstrated in this work. LabVIEW offers FPGA toolkit which

allows graphical programming of FPGA hardware for a desired application. Such an

interface allows the user to program FPGA hardware without necessary knowledge of

VHDL/Verilog. In this work, the logic for waveform generation and triggering were first

simulated using FPGA Simulation Toolkit of LabVIEW and the simulation results were

then verified with the hardware implementation. Testing of the generated waveforms

and triggers was performed with the help of a high bandwidth digital storage

oscilloscope. Some selected waveforms including arbitrary waveforms are presented

in this paper in addition to customized triggers. The results thus obtained

demonstrate that FPGAs can be used to generate precise waveforms and triggering.

Field Programmable Gate ArrayThe FPGA target used in the experiment is NI PXI 7833R . The device is

equipped with 3 million gates and is capable of delivering high-precision, real-

time applications such as custom waveform generation and triggering.

• C.P.U Independent Operation: FPGA can operate independently and

continue to run even if the host computer crashes thus serving as a

dedicated and reliable controller .

• Parallel operation: FPGA is capable of handling and executing multiple

tasks simultaneously without compromising speed or accuracy. This is

done by utilizing an entire different set of logic on the hardware for

different applications.

• High Clock Rate : High clock rate (40 MHz- 160 MHz) not only enhances

the speed and accuracy of operation but also widens the scope for

generating waveforms of considerably high frequencies .

• Real time applications : FPGAs are perfectly suitable for applications in

time-critical systems. In contrast to software based solutions with real

time operating systems, FPGAs provide real deterministic behavior. By

means of the featured flexibility even complex computations(limited to

fixed point mathematics) can be executed in extremely short periods.

• A user can configure the behavior of FPGA hardware to match the

requirements of a specific measurement and control application.

• Compiled VI (Virtual Instrument) is directly deployed onto the FPGA which

is executed independently by the target device while sharing only power

supply with the onboard C.P.U .

• Due to such advantages that FPGA offers , its applications in industry are

increasing constantly.

LabVIEW and FPGA Toolkit• The presented work has been developed on LabVIEW 8.6 platform using

its FPGA Toolkit and National Instruments’ NI PXI-7833R as the hardware

FPGA target.

• The Virtual Instrumentation approach offered by LabVIEW enables the

users to learn a variety of instrumentation techniques and control

applications through an easy and simple to understand graphical

programming approach.

• Such a flexible system allows user to deploy the LabVIEW code directly

onto the target device (7833R) which allows the output to meet precise

timing and frequency specifications as required in metrology.

• Overall, FPGA toolkit enables its user to perform many tasks in a rugged,

high-performance, and modular architecture without knowledge of low-

level EDA tools or hardware design.

Experimental Setup

1. PXI-1031 - M/S National Instruments

2. FPGA Module 7833R - M/S National Instruments

3. LabVIEW 8.6 with FPGA toolkit - M/S National

Instruments

4. Digital Storage Oscilloscope – TDS 2022 – M/S Tektronix

5. Arbitrary Waveform Generator – AFG 3022 - M/S

Tektronix

6. Connector(SCB-68) - M/S National Instruments

ArbitraryWaveform Generator

FPGA(NI PXI-7833R)

Digital Storage Oscilloscope

UserInterface

Experimental Layout

Case Studies

TRIGGER• Trigger is a precise clock of defined frequency which is used to initiate

an event or a sequence of events.

• Trigger can be of two types:

- Pre-defined: If an event is to be periodically initiated

after some interval of time.

- Initiated by some other waveform.

• In this work, a pre-defined trigger with a customizable duty cycle and

two other triggers of rising and falling edge types initiated by a high

frequency sinusoidal waveform have been presented.

• In case of a rising edge trigger, trigger is generated when sinusoidal

wave crosses the time axis with positive slope.

• In case of a falling edge trigger, trigger is generated when sinusoidal

wave crosses the time axis with negative slope.

Trigger (LabVIEW Code)

START

Increment the loop iteration variable i, of the while loop

Input the signal from the waveform generator through the input pin AI0

Subtract the new value of the signal from the old value (memory block)

If the value of the above subtraction is negative and signal

strength decreases below zero then Output High. Else output is Low

Output the value to the output pin AO0.

Go back to the starting point and repeat continuously

SQUARE WAVE GENERATOR

( With customizable duty cycle )

Square Wave Generator (LabVIEW Code)

START

Initialize the loop iteration variable i, of the inner for loop

Increment the value of i

For i <Duty Cycle, the output of the case structure is High. When i > Duty Cycle, the output of the case structure is Low.

The output is taken at the output pin AO0.

Go back to the starting point and continue to get a periodic wave

Front Panel

ARBITRARY WAVEFORM GENERATORS

Arbitrary Waveforms

SINE WAVE GENERATOR

Sine Wave Generator(LabVIEW Code)

START

Initialize and increment loop iteration variable i, of the internal for loop

Normalize the iteration variable i, between [-1, +1]

Input the normalized iteration variable to the sin (x) block

The output is taken at the output pin AO0

The for loop will run for n iterations giving one full cycle at the output

Go back to first step

SAWTOOTH WAVEFORMGENERATOR

Sawtooth Waveform Generator(LabVIEW Code)

START

Initialize the loop iteration variable i, of

the inner for loop

Scale i to give appropriate voltage at

output

Output the value to the output pin AO0

Keep incrementing for n iterations

Go back to starting point and continue so as to get

a periodic waveform

PARABOLIC WAVEFORM GENERATOR

Parabolic Waveform Generator(LabVIEW Code)

START

Initialize the loop iteration variable i, of the

inner for loop

Square the iteration variable i

Scale the output after squaring by an

appropriate value

The output is taken at the output pin AO0.

Increment the iteration variable to generate rest

of the waveform

Go back to starting point and continue to generate

a periodic wave

Chirp

Sequence of sine, square and sawtooth

Conclusion

• Some selected standard and arbitrary waveforms and triggers were generated

using LabVIEW FPGA toolkit and PXI-7833R as the FPGA hardware target.

• The coherence between the expected and obtained results clearly suggests that

FPGA can be used for precise triggering and custom waveform generation.

• Unlike conventional techniques, the proposed method does not require any

complex hardware circuitry and also relieves the user from learning any hardware

description language.

• FPGA, being a real time target, along with its several merits can be easily

integrated in metrological applications.

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Thank You !!