RF filter design is a piece of cake these days thanks to computer design and simulation tools. But actually realizing the simulated filter response in the real world can be a completely different matter! This video provides an introduction to practical RF filter design by building, testing, and tweaking a 137MHz bandpass filter suitable for NOAA APT satellite reception.
Build fun circuits! Impress your friends! (Or at least the ones who aren’t in-the-know 😉 )
With some inspiration from The Fifth Element and Iron Man, here’s a voice-activated light switch that provides the illusion of a more advanced artificial intelligence, with the simplicity of “the clapper”.
I’ve gotten a lot of questions on the blog about the new version of the MHS5200A function generators available on eBay. Viewer Tolga was kind enough to send one in to me to review and tear down. Although some improvements have been made over the older models, there are some concerning issues with these new models too!
I recently designed an infrared sensor board (dubbed “IRis”) for my friend’sDefcon talk. This video walks through the circuit design of the photodiode amplifier, and discusses some of the pitfalls associated with photodiode amplifier design.
Schematics, BOM, and KiCAD design files for the described IRis board are available on github.
Ever tried searching through your datasheets for the motional parameters of that quartz crystal you just bought? Good luck! Vendors simply don’t specify these parameters to general end users, and for most applications that’s OK. But for high Q oscillator and filter design, measuring and matching crystals can be important.
This video discusses crystal motional parameters, how to measure them with a crystal impedance meter, and finally examines the measured values of 150+ real world crystals.
Below are some interesting correlations/statistics gathered from the measured data; raw measurement data is available here.
Average C0 capacitance for each crystal holder type
Average motional resistance vs frequency
Average motional inductance vs frequency
Average unloaded Q vs frequency
Average unloaded Q for each crystal holder type
Unloaded Q of glass-sealed crystals vs average unloaded Q
Overall statistical analysis of unloaded crystal Qs
References and additional reading:
Crystal motional parameters and relevant equations
Its hard to beat the cost and versatility of the ubiquitous RTL-SDR dongles, but the temperature stability of their reference oscillators isn’t sufficient for some applications. While the internal 28.8MHz quartz crystal in these units can be replaced by a high qualitytemperature compensatedoscillator, these tend to be relatively expensive and/or difficult to source.
Here’s a scratch-built 28.8MHz TCXO capable of +-1ppm stability from 0C-55C; best of all, it’s not only easy to build, but is designed entirely from readily available and inexpensive components. For improved temperature stability, the main oscillator can even be replaced with one of many commercially available TCXOs!
Today we explore the use of oscillator synthesis software (Genesys) for practical crystal oscillator design, and the impact of the Randall-Hock correction formula on linear open loop analysis accuracy.
Here’s an inexpensive precision peak detector circuit that accurately tracks the peak voltage of input signals at frequencies up to 100kHz and has zero voltage droop over an indefinite period of time…no microcontrollers required!
The following circuit uses a dual comparator, three op amps, and a digital potentiometer to provide two peak detection outputs: one “real-time” peak output, accurate to within 2% for input signals up to 100kHz, and one maximum peak output which outputs the maximum peak voltage seen since the last reset: