In this episode Shahriar investigates the noise generated from the DPH3205 Buck-Boost DC/DC Converter Module. This module is part of a family of affordable devices which can be purchased online. The particular model (DPH3205) shown in this video can be purchased from here. The Keysight S-Scope in conjunction with the N7020 power probe is used to measure the noise behavior of the circuit. It can be observed that the module produces many harmonics of noise spanning up to and beyond 1MHz.
A power operational amplifier is used in conjunction with a 4V Zener diode to produce a crude voltage follower. The PSRR of the operational amplifier can filter the noise from the DC/DC converter. This is verified by measuring the noise coming from the operational amplifier. The noise is significantly reduced. This circuit is not entirely practical and is used for demonstration purposes only.
In this episode Shahriar & Rosanah investigate an Agilent power supply which does not appear to power on. It can be quickly observed that the fuse has failed on the unit. Using an isolation transformer a small amount of AC voltage is applied to the unit after the fuse replacement. It is clear that a short is present somewhere in the instrument since even at 10V AC the instrument consumes more than 1A.
After separating the internal transformer from the main PCB, the high current consumption is eliminated. The short is traced to a damaged main rectifier in the instrument. A replacement rectifier is used which allows the instrument to power on.
The VFD display is however extremely dim. After a failed attempt at restoring the VFD brightness, a new display is purchased from eBay. The replacement produces a bright vibrant VFD display. The performance of the power supply is verified under load.
In this episode Shahriar goes over the operation and principle theory behind Lock-in Amplifiers. The SRS SR530 is one of the most iconic lock-in amplifiers ever made and since it offers two channels it can be used to perform very interesting experiments across many domains. After reviewing the block diagram and equations governing the theory of operation, a brief instrument teardown is presented.
Two unique and interesting experiments are also presented. In the first experiment the instrument is used to measure the speed of light. This is accomplished by measuring the wavelength of sound at 20kHz using a pair of speakers and a function generator. The distance between the speakers can be carefully adjusted and the relative signal strength from each lock-in channel is measured and thus the wavelength can also be measured.
In the second experiment the sensitivity of a red LED to blue laser light is measured. Due to the semiconductor composition of the red LED as well as its red plastic casing, the responsibility of the LED to blue light is extremely low. A chopper is therefore used to lock the light to the lock-in amplifier’s reference input. The measured induced current is measured down to very low optical level in the order of hundreds of fempto (10^-15) amps.
In this episode Shahriar repair a Fluke PM6685R Rubidium Frequency Counter. The instrument shows highly unstable results and produces high-pitch noise when powered on. The high-frequency noises point to a problem with the DC-DC converter circuit in the power supply module. A close examination points to degraded capacitors. After all capacitors have been replaced, the noise issue is resolved. The instrument’s performance is then verified and calibrated against an external Rubidium reference. The agreement between the units is better than 10ppb.
In this episode Shahriar investigates a peculiar problem with a SRS DS345 function generator. The instrument does not appear to produce the correct output signal frequency. After some investigation it becomes clear that a few of the display digits are not active which hide the actual frequency settings. The problem is traced to a broken resistor on the display driver. The schematic of the instrument shows that the resistor is responsible for strobing the affected seven-segment digit. After the repair, the instrument’s performance is verified including the OCXO accuracy.
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.