OE1022 Digital Lock-In Amplifiers

Overview

Now OE1022 DSP Lock-In Amplifier is the newest product of core technology with high performance. Based on the digital demodulation, output filter and high-precision stabilized 24-bit ADC, it is ideal to detect the weak signals, which are overwhelmed by greatly larger noise components. And OE1022 could measure the magnitude and phase of a signal simultaneously. It could be comparable with the international leading technology in some core parameters, like accuracy of measurement, range of operating frequency, SNR, dynamic reserve and so on. Meanwhile, we have integrated other functions like multiple-harmonic measurement and FFT in OE1022, which makes this DSP Lock-In Amplifier could meet the needs of scientific research and industrial purposes.

Input Channel

With the low-noise analog pre-amplifier, the signal input of OE1022 can be switched to operate in the single-ended or differential voltage mode, and the input noise is 10 nV/√Hz. The input impedance is 10 MΩ and the full-scale input voltage sensitivity ranges from 1 nV to 1V. Besides, OE1022 can also be used for current measurements with variable current gains of 10 or 10 V/A. Two line filters (50/60 Hz and 100/120 Hz) are designed to eliminate line related interference. The programmable gain amplifier is provided to adjust the dynamic reserve of the system according to the magnitude of the input signal, so that OE1022 has inherently large dynamic reserve up to 100dB. The sampling rate of 312.5KSPS is determined by a precision 24-bit A/D converter and a specific filter is designed to avoid aliasing.

Reference Channel

In order to provide the reference signal for OE1022, an externally applied sine wave or square wave, or its owninternally synthesized reference source could be used. When the instrument is set to internal reference mode, the internal precision stabilized oscillator and the digital synthesized algorithm are used to generate sinewave output that multiplies the input signal, there is virtually no reference phase noise when choosing internal reference mode. Taking advantage of digital phase-shifting technique, the reference signal phase could be adjusted with 0.01° resolution. The
internal reference mode can operate at a fixed frequency from 1 mHz to 100 kHz. In addition, the external reference is also applicable to OE1022, including the sinewave reference signal and TTL logic reference signal. The rising and trailing edge of external reference signal are applied to trigger off the internal Phase Locking Loop (PLL). Based on the frequency of reference signal, harmonic detection can be performed by OE1022. The maximum frequency of the measurable harmonics is 32767 times of basic frequency, and it is also less than the maximum operational frequency 100 kHz.

Digital Demodulator and Output Filter

The core of Lock-In Amplifier OE1022 is demodulator. This digital demodulator could be effectively immune from the potential errors caused by DC drift and offset found in analog multipliers or switches. The coherent signal multiplication has been optimized for very low calculation error. The time constants of OE1022 could be configured extending from 10 µs to 30 ks. The roll-offs of the output low-pass filter on OE1022 are 6, 12, 18 and 24 dB/octave. This low-pass filter is carried out by the architecture of digital infinite impulse response (IIR) filters and updated at 312.5 kHz signal
sampling rate. The advantages of this design over common analog instruments are higher dynamic reserve (>100dB), accurate phase shifts (absolute phase error <1°), zero drift, and orthogonality. Meanwhile, below 200 Hz, OE1022 can perform synchronous filtering, which is used to notch out
multiples of the reference frequency. This function ensures that OE1022 works quickly at low frequency.

Display

OE1022 uses the 5.6 inch 640×480 TFT color display as its screen. Data measured by OE1022, such as, X, Y, R, θ, is stored in up to four traces. Trace values can be displayed as a bar graph or as a strip chart showing the trace values as a function of time.

OE1022 can also show the relationship of time and range in a XY graph and you can check the value of sampling point by using knob control cursor.

Besides, OE1022 can display polar plots, showing phasor composed by in-phase and quadrature components of the signal. All displays can be easily scaled by the manual operation, and the auto-scale feature is available to optimize display quickly. The screen can be configured as a single large display, or two horizontally-split displays.

FFT Spectral analysis
In order to analyze the noise component in signal, we have developed FFT high precision spectral analysis function and integrate it in OE1022, which can ensure high precision frequency measurement between 1kHz to 100kHz, while displaying the spectrum distribution of signal in real time.

Simultaneous Multiple-harmonic Measurement

The traditional phase-locked amplifier can only measure fundamental frequency signal or one certain harmonic ware at the same time, so it can not meet the requirement in some occasions when measuring multiple frequency components simultaneously is needed. In digital end, OE1022 combined FPGA and ARM technology, which has wider processing bandwidth and more flexible digital framework. The digital processing precision could reach 48 bits and could measure 3-channal harmonic components simultaneously, which makes one OE1022 is equal to three traditional phase-locked amplifiers.

Internal Oscillator
The internal oscillator helps to generate a very low distortion (–80 dBc) reference signal. The single frequency of sine waves can be extended from 1 mHz to 100 kHz while the resolution generated is 0.1 mHz. Both frequency and amplitude can be set from the front panel or a computer. When choosing external reference mode, the internal oscillator is phase-locked to the external reference signal.

Signal Generator
OE1022 generates one sinusoidal signal in range from 1 mHz to 100 kHz, using 20-bit D/A converter. The frequency and phase shift of this sinusoidal signal are corresponding to that of the internal reference signal, and its amplitude could be set with control menu. The maximum amplitude provided by signal generator is 5Vrms.

Manual Operation
In addition to displaying data, the color TFT display panel is also used to display the control menu. In conjunction with keys grouped around the screen and numeric keypad, customers could adjust instrument's parameters effectively, such as oscillator frequency, which can be set to a fixed value
and to a high precision.

Auto Function
OE1022 has the ability to adjust measurement parameters to optimum setting for input signal. Gain, phase, dynamic reserve and display scaling could all be set with different independent key presses from the control menu. With this function, the input signals could be measured easily and automatically.

Interface
OE1022 regards RS-232 to USB interfaces as standard. And, all instrument functions can be controlled and read through USB interfaces. To debug conveniently, all of the interfaces could be viewed on front panel. For convenient debugging, characters received and sent via interfaces can be viewed on front panel.

Remote Operation
The built-in RS232 to USB interfaces on OE1022 allow full manual operation from a controlling computer, including setting or interrogating control and reading out data. Free LabVIEW program is provided. It makes it easy to set up and run complex experiments, such as remote control of every instrument function. The display menu offered by the LabVIEW program allows customers observe all command received and responses generated by instrument.