LX711 - 200kHz Signal Generator

LX711 - 200kHz Signal Generator

The LX711 provides precision arbitrary waveform generation via direct digital synthesis for a wide variety of applications and measurements. Various high resolution distortion measurements can be performed, as well as very broad band frequency domain and time domain amplitude measurements.

The LX711 provides a wide range of low noise output amplitudes through passive constant impedance attenuator arrays spanning 45dB, with 3.01dB step size. Four different output impedance values are selectable: 40, 150, 600, and INF Ohms.

Moreover, four different output connection modes are preconfigured to enable easy testing of balance circuits, unbalanced circuits, common mode input rejection, and common mode output rejection.

Features
• Ultra High Resolution Waveform Generation
• Direct Digital Synthesis - DDS
• Arbitrary Waveform Generation
• Waveforms: Sine,IM,Multi,Chirp,MLS,Any
• DAC Resolution 32 Bit
• Frequency Bandwidth DC-200kHz
• Frequency Resolution 0.01Hz
• Maximum Sample Length 4M
• Multiple Sampling Rates
• Large Dynamic Range
• Very Low Distortion
• Output Modes: Bal, Unbal, CMI, CMO
• Output Impedance: 40, 150, 600, INF
• Stopband Attenuation 140dB
• Isolated/Floating Common
• 2 Groups of 4 LED Indicators
• Interface: XLR, BNC, Banana Jacks

Digital / Analog Conversion

Multiple high resolution multibit delta-sigma modulators are combined through custom multirate filters and interpolated to a 32 bit word size, at the desired sampling rate. Multiple sampling rates are provided. Data is transferred synchronously between memory and the digital signal conversion circuitry without interference. Sample data can be read/written to the controller host or PC asynchronously without affecting sampling timing or accuracy.

Filtering & Correction

Both digital and analog reconstruction filters are utilized to provide high performance and the elimination of all out-of-band frequency image components. Digital FIR filters attenuate most above band components to 160dB. The 6th order analog filters are specially designed to provide very low noise, very high linearity, and very wide dynamic range to attenuate all very high frequency components by 140dB. During the calibration process, the exact analog filter response is measured and a unique digital correction filter is synthesized to correct response deviations producing extremely flat response.

Output Driver Stage

A discrete output driver stage pair is employed to provide minimum distortion at all drive levels and output impedance. The output stage topology features ultra linear circuitry designed with special frequency compensation to improve linearity. Substantial bias in the output stage pair reduce nonlinearity at high frequencies. The output stages are protected for continuous short circuits being limited to 150mA peak with timer delay restart.

Precision Output Attenuator

To provide a wide variety of output levels at low noise, the LX711 utilizes a passive log-binary constant impedance attenuator block. The attenuators are constructed from 0.1% thin film low TC resistors with 3.01dB step size. Maximum attenuation for the network is 45dB. Finer grain attenuations are controlled by a digital scaler within the custom FPGA logic digital signal processing chain.

Embedded Controller

Each module contains a 32 bit softcore embedded controlled operating at 100MHz synthesized from FPGA logic. The controller is responsible for managing all LXBUS data and command communications, Flash memory, PSRAM sample memory, and module control functions. The firmware can be updated in the field through the LX700 mainframe controller.

Sample Memory

All sample data is stored locally within the module using multiple memory banks of PSRAM. The memory is dual ported allowing the computer interface to upload data asynchronously without affecting the synchronous performance and timing accuracy of the internal digital stream.

Isolated/Floating Common

An isolated DC/DC converter is used to float the generator circuitry and provide an isolated generator common. A switching frequency of 300kHz is utilized to raise any residual noise components beyond and outside the 200kHz measurement range. The transformer employs double Faraday shielding and produces only 15pF of coupling between the isolated common and system ground.

Automated Digital Calibration

The LX711 does not contain any analog trim pots. Instead all calibrations and trimming are performed by automated computer controlled routines in a GPIB test system, with adjustments provided by digital trim DACs or digital constants. All calibrations are stored within the non-volatile flash memory of each module, and peformed with traceable equipment and standards. Accuracy is measured to better than 0.01dB or 0.1%, as applicable to each parameter. A calibration chart is included with each module to document the actual performance, parameters, and calibration results.