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.
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.
I’s and Q’s and negative frequencies, oh my! Today we discuss mixers and frequency conversion, in particular, quadrature mixers and Tayloe detectors: what they are, how they work, why you might want one, and what do we use all this I and Q stuff for anyhow?
Today we look at how to generate a pair of quadrature signals from the output of a crystal oscillator using a pair of d-type flip-flops, as well as discussing the advantages and disadvantages of this technique.
Ever wonder what goes in to the design of a crystal oscillator? We’ll examine the operational theory of crystal oscillators, and design a discrete Pierce crystal oscillator suitable for use as a local oscillator in an HF receiver.
My discrete Pierce oscillator design tool can be found here; references and additional reading are listed below!
Oscillator circuit design and crystal loaded Q analysis
Here is yet another AD8307 based RF power meter which adds an opamp to produce a DC output voltage of 1mV/dBm (e.g., 0mV = 0dBm, -10mV = -10dBm, etc). The RF power in dBm can be read directly with a multimeter, and I’ve found it particularly useful for measuring RF filter response when combined with a sweep generator and an oscilloscope:
Visualizing a 6MHz Low Pass Filter Response over 1-11MHz (10dBm/div)
The AD8307’s high dynamic range and good accuracy over a wide bandwidth make it especially useful if, like me, you don’t own a spectrum analyzer which would otherwise be ideal for filter measurements.