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

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