Practical RF Filter Design

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.

References and additional reading:

Description Reference
Designing a high-Q VHF bandpass filter A VHF Bandpass Filter for the QST Spectrum Analyzer, Wes Hayward, W7ZOI
Enameled wire capacitors Capacitance of a Wire Above a Foil, Wes Hayward, W7ZOI
Tutorial on double tuned bandpass filters The Double-Tuned Filter: An Experimenter’s Tutorial, Wes Hayward, W7ZOI
Air core inductor Q The Elusive Q of Single-Layer Air-Core Coils, George Murphey

Crystal Motional Parameters

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 C0 capacitance for each crystal holder type

Average motional resistance vs frequency

Average motional resistance vs frequency

Average motional inductance vs frequency

Average motional inductance vs frequency

Average unloaded Q vs frequency

Average unloaded Q vs frequency

Average unloaded Q for each crystal holder type

Average unloaded Q for each crystal holder type

Unloaded Q of glass-sealed crystals vs average unloaded Q

Unloaded Q of glass-sealed crystals vs average unloaded Q

Overall statistical analysis of unloaded crystal Qs

Overall statistical analysis of unloaded crystal Qs

References and additional reading:

Description Reference
Crystal motional parameters and relevant equations Understanding Quartz Crystals and Oscillators, Ramon M. Cerda, Chapter 1.27
Measuring quartz crystal parameters Measurement of the equivalent circuit of a quartz crystal, Omicron Lab
Crystal measurements with a VNA Crystal Bandpass Filters, W0QE
Crystal measurements with a Spectrum Analyzer Crystal Parameters — Experiments with a Tracking Generator + Spectrum Analyzer, QRP HomeBuilder
Comparison of crystal measurement techniques Crystal Motional Parameters: A Comparison of Measurement Approaches, Jack R. Smith, K8ZOA
Crystal test fixture design Assembly and Usage Notes for K8ZOA Crystal Test Fixture Revised for Version 1.2 PCB, Jack R. Smith, K8ZOA

Building a Better RTL-SDR TCXO

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 quality temperature compensated oscillator, 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!

UPDATE: Elia has kindly designed a PCB for this circuit, using a commercially available TCXO. Now available from OSHPark!

KiCAD schematics and additional project files are available on github.

28.8MHz TCXO schematic diagram

28.8MHz TCXO schematic diagram

TCXO f-T curve

TCXO f-T curve

References and additional reading:

Description Reference
Oscillator temperature compensation techniques Design Technique for Analog Temperature Compensation of Crystal Oscillators, Mark A. Haney, Virginia Polytechnic Institute
TXCO tutorial Tutorial on TCXOs, Vectron International
R820T datasheet R820T: High Performance Low Power Advanced Digital TV Silicon Tuner, Rafael Microelectronics
Guide to proper toroid selection Iron Power Cores for High Q Inductors, Jim Cox, Micrometals, Inc.

Oscillator Simulation and Design

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.

References and additional reading:

Description Reference
Oscillator synthesis/simulation software Genesys, Keysight Technologies
Randall and Hock’s IEEE paper (no paywall) General oscillator characterization using linear open-loop S-parameters, Mitch Randall, Terry Hock
Application of the Randall-Hock correction formula in oscillator synthesis Discrete Oscillator Design, Chapter 1.2.1.5, Randall W. Rhea
Randall Rhea’s oscillator design webinar Discrete Oscillator Design Tools and Techniques, Randall W. Rhea, presented by Keysight Technologies
Effects of S11 and S22 on oscillator loop gain Practical RF Circuit Design for Modern Wireless Systems, Vol. 2, Chapter 6.2, Rowan Gilmore, Les Besser
Evaluating and optimizing oscillator performance using Genesys simulation Improving the Vackar Oscillator, QRP Quarterly, Volume 56 Number 1, January 2015, p.20, David White (WN5Y)

Colpitts Crystal Oscillator Design

A qualitative and quantitative analysis of the Colpitts crystal oscillator circuit, oscillation requirements, and practical circuit design considerations.

References and additional reading:

Description Reference
Quantitative analysis of Colpitts crystal oscillators Crystal Oscillator Design and Temperature Compensation, Marvin E. Frerking, Chapter 7.3, Appendix F
Oscillator phase vs frequency, and crystal loaded Q analysis Crystal Oscillator Circuits, Robert J. Matthys, Chapters 6, 7
Colpitts crystal oscillator phase vs gain analysis Oscillator Design and Computer Simulation, 2nd Edition, Randall W. Rhea, Chapter 11.2
Crystal drive level equations Intel application note AP-155 (Oscillators for Microcontrollers), Appendix A
Common collector gain equations Common collector, Wikipedia
Transistor biasing RF Circuit Design, Chris Bowick, Chapter 6

Quadrature Mixers

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?

Additional references:

Description Reference
Introduction to RF mixers Basics of RF Mixers in Radio Receivers, Alan Wolke (W2AEW)
Mixer theory and switching mixer operation Downconversion Mixers, Gino Giusi, Ph.D
Introduction to IQ signals and phasor diagrams Basics of IQ Signals and IQ modulation & demodulation – a tutorial, Alan Wolke (W2AEW)
IQ signal processing fundamentals, Euler’s identities A Quadrature Signals Tutorial: Complex, But Not Complicated, Richard Lyons
IQ phase relationships at positive and negative frequencies IQ Modulation, Keysight Technologies
The Tayloe quadrature detector Ultra Low Noise, High Performance, Zero IF Quadrature Product Detector and Preamplifier, Dan Tayloe

Crystal Oscillator Design

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!

Description Reference
Oscillator circuit design and crystal loaded Q analysis Crystal Oscillator Circuits, Robert J. Matthys, Chapters 5.8, 6.1, 10.9
Pierce oscillator negative resistance and gain margins Understanding Quartz Crystals and Oscillators, Ramon M. Cerda, Chapters 9.2, 9.4
Pierce oscillator crystal drive level equations Intel application note AP-155 (Oscillators for Microcontrollers), Appendix A

Building an RF Power Meter

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)

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.

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