In this episode Shahriar examines a faulty Fluke PM6303A Automatic RCL Meter. This unit power on however reports the incorrect measured value for all components. The instrument also does not provide the built-in 2V bias. After a quick teardown of the unit, physical damage to several input resistors can be observed. It becomes clear that a high-voltage discharge has caused a cascade failure of several components on the input signal path. These faults are traced to a chain of protection Zener diodes, resistors and general purpose diodes. After all the components have been replaced, the unit’s functionality is restored.
The schematic and block diagram of the unit is also presented and the principle of operation is explained. The repaired instrument is then used to measure several components to verify is functionality.
In this episode Shahriar repairs a Fluke PM6680B 225MHz High Resolution Frequency Counter. This unit which is equipped with a soft power button, does not power on. The standby LED is dimly lit and a high-frequency noise is emitted from the unit. This points to a switching power supply failure. The power supply is a simple fly-back converter with three outputs (5V, 18V, -8V). The 5V output is regulated while the other two outputs are single diode rectified.
The fault is traced to a bad precision reference IC which is replaced. The performance of the power supply is verified and the instrument powers on. An OCXO is used to calibrate the 10MHz reference crystal of the unit after the repair is completed.
In this episode Shahriar examines a faulty Valhalla 2701C Programmable Precision DC Reference instrument. Valhalla Scientific’s 2701C Programmable Precision DC Voltage Calibrator delivers ultra-precision, ultra-stable DC voltage from 100 nanovolts to 1200 volts. This unit does not have a working front-panel LCD screen. Luckily, that problem is resolved very quickly.
The internal build and operation of the unit is briefly examined and the instrument is calibrated using a Keithley DMM7510 7.5-digit multimeter. It is possible to upgrade this unit to a more stable and accurate reference voltage generator or to add the reference current capability to it.
In this episode, Shahriar and Shayan discuss the design and characterization of a deceptively simple CMOS inverter-based transimpedance amplifier. The the large and small signal behavior of the CMOS inverter is discussed and measured using the Keithley 2450 and 2460 source meters. The transient response is also measured using a Keysight MSO-S series oscilloscope.
The small signal gain of the circuit is calculated from small signal parameters which are extracted directly by measuring the devices I/V characteristics. The NMOS/PMOS devices used are from an ADL1105 quad-discrete transistor IC. Through the use of a shunt-shunt feedback, the CMOS amplifiers is converted to a transimpedance amplifier which is capable of amplifying the current from a photo-detector diode by a gain of 30kV/A. The feedback theory is used to calculate the gain of the amplifier. The slides for this tutorial can be downloaded here.
In this episode Shahriar explores the world of Delta-Sigma modulators with emphasis on a Delta-Sigma Analog to Digital Converter (ADC). The basic concepts of analog to digital conversion is presented, particularly with respect to quantization noise spectral shape and power density. Next, oversampling ADCs are presented to demonstrate the possibility of increasing SQNR (ENOB) through manipulation of quantization noise spectrum.
Due to the practical limitations of high oversampling ratios, delta-sigma modulations is explored. The principle operation behind delta-sigma ADCs is presented with detailed explanation on noise shaping, filtering and decimation. The signal and noise transfer functions for a 1st order and 2nd order delta-sigma ADC are derived. Finally, as a practical example, a 2nd order delta-sigma ADC based on a 1-bit quantizer is presented. The ADC uses two Miller integrator op-amps, one comparator and a D-Type flip-flop. The complete measurement of this delta-sigma ADC is presented. The impact of over sampling ration, op-amp linearity and input signal bandwidth is presented. The slides for this video can be downloaded here.
The Signal Path (TSP) is an electrical engineering video blog for industry professionals, students and hobbyists. TSP is a non-for-profit website dedicated to provide free education spanning a wide range of electrical engineering topics. Equipment reviews, tutorials and repair videos are posted regularly.