Precision VXO for Crystal Characterization & Matching

Description: The Precision VXO (PVXO) and its corresponding Crystal Test Fixture (CTF) were developed to offer an economical method for evaluating the characteristics of crystals and measuring their series resonant frequency. This information enables the construction of low-cost, high-performance crystal filters for both receivers and transmitters. The PVXO was initially offered as a kit by the NJ QRP Club, but those units have since sold out. However, photographs of the unit remain available for reference. A schematic diagram of the PVXO is provided, along with a detailed explanation of the circuit's functionality, which can be found in the Atlanticon 2002 paper titled "Simplified Tools and Methods for Measuring Crystals," linked at the bottom of the page. The circuit is implemented using Manhattan-style construction. The front panel features a frequency control potentiometer, inductance switches, and an output BNC connector. In later developments, the Arizona QRP Club introduced the affordable "Stinger Singer" CW frequency readout designed by Dan Tayloe, N7VE, which was incorporated into the instrument, allowing it to display the set frequency. A single-pole, single-throw (SPST) push-button switch was added to the front panel to control this module. The NJ QRP Club sold 100 PVXO kits during 2002 and 2003. The finished kit included all components, a case, and templates for lettered covers. The production of this kit was relatively costly due to expensive components, particularly the precision 10-turn potentiometer, five switches, and the case, which affected sales. However, purchasers received a valuable device considering its capabilities. A spectrum plot of the PVXO with a 4.9152 MHz crystal illustrates that harmonics decrease as the crystal frequency increases, moving the output frequency closer to the cutoff frequency of the output low-pass filter. The schematic diagram of the Crystal Test Fixture (CTF) used alongside the PVXO provides the necessary drive level to the tested crystal and matches its impedance. Data obtained from using both instruments together allows for the determination of a crystal family's characteristics and the matching of units for future use as crystal filter elements. The Crystal Test Fixture was also constructed using Manhattan-style techniques. A photo depicts the 25 Ohm cermet potentiometer used to measure the equivalent series resistance (ESR) of a crystal. The PVXO drive connects to the left BNC connector, while the external detector connects to the right BNC connector. A side view of the Crystal Test Fixture shows a double-pole, double-throw (DPDT) switch that selects between an RF probe and direct output from the RF amplifier to accommodate various readout instruments. One notable measurement made with the PVXO and CTF was demonstrating the effects of grounding the case of a crystal under test. It was observed that while the series resonant frequency remained unchanged, the parallel resonant frequency increased by approximately 1.5 kHz when the case was grounded. This indicates that the performance of a crystal filter may also be affected, shifting the response slope further from the passband. The Atlanticon 2002 paper titled "Simplified Tools and Methods for Measuring Crystals" is available for download in PDF format, sized at 1 MB. It describes the use of the PVXO and CTF combination to characterize a set of crystals and match them for crystal filter applications, although it does not cover filter design, which is best accomplished using the free "Filter" program from the AADE website. Parameters for several common computer crystals are also provided, many of which have been utilized in homebrew rigs.

The Precision VXO (PVXO) is a versatile oscillator circuit designed to deliver a stable frequency output that is crucial for crystal testing applications. The circuit typically consists of a voltage-controlled oscillator (VCO) with a crystal as a frequency-determining element, allowing for precise frequency control. The inclusion of a frequency control potentiometer enables users to fine-tune the output frequency, while inductance switches allow for the adjustment of the oscillator's characteristics by changing the load on the crystal.

The output of the PVXO is connected to a BNC connector, facilitating easy integration with various measurement equipment. The addition of the "Stinger Singer" frequency readout module enhances the usability of the PVXO by providing real-time frequency monitoring, which is essential for evaluating crystal performance accurately. The front panel design incorporates user-friendly controls, ensuring that operators can easily navigate the functions of the device.

The Crystal Test Fixture (CTF) complements the PVXO by supplying the necessary drive level to the crystal under test. The design of the CTF focuses on impedance matching, which is vital for obtaining accurate measurements of the crystal's characteristics. A cermet potentiometer is employed to measure the equivalent series resistance (ESR), providing insight into the crystal's performance and quality. The DPDT switch allows users to select between different output modes, making the CTF adaptable to various testing scenarios.

The combination of the PVXO and CTF provides a comprehensive solution for crystal characterization, enabling users to gather critical data on crystal families, which is essential for designing effective crystal filters. This synergy between the two devices allows for detailed analysis and matching of crystals, ensuring optimal performance in communication systems. The methodologies and results obtained from using these instruments contribute significantly to the field of electronics, particularly in the development of high-performance RF components.The Precision VXO (PVXO) and its matching Crystal Test Fixture (CTF) were created to provide a low cost means of evaluating the characteristics of crystals and a means of measuring their series resonant frequency. With this information, one can then build very low cost, high performance, crystal filters for receivers and transmitters.

The PVXO was kitted by the NJ QRP Club, but has since sold out those units. However, photos of that unit are available on this page for completeness. The schematic diagram for the PVXO. A detailed description of what this circuit is and how it works can be found in the Atlanticon 2002 paper entitled "Simplified Tools and Methods for Measuring Crystals" whose link is at the bottom of this page. The circuitry shown above is transformed to a working instrument using Manhattan-style construction. The front panel contains the frequency control potentiometer, inductance switches, and the output BNC connector.

Later on in the development of the instrument, the Arizona QRP Club offered their inexpensive "Stinger Singer" CW frequeny readout designed by Dan Tayloe, N7VE. This module was added and the instrument could now read out the frequency to which it was set. A SPST push button switch was added to the front panel to control the module. The NJ QRP Club sold 100 of the PVXO kits during 2002 and 2003. This picture shows what the finished kit looked like. Included were all components, case, and templates for doing the lettered covers. This kit was relatively expensive to produce due to the cost of the components, especially the precision 10 turn pot, five switches, and case, and therefore didn`t sell very well.

However, those that bought them got a real deal considering the capabilities of the unit. This is a spectrum plot of the PVXO with a 4. 9152 MHz crystal running in the unit. The harmonics decrease as the frequency of the crystal being used increases, and the output frequency moves closer to the cutoff frequency of the output low pass filter. The schematic diagram of the Crystal Test Fixture (CTF) which is used in conjunction with the PVXO. This circuit provides the required drive level to the crystal being tested, as well as approximately matching the impedance the crystal under test.

Data produced when using the two instruments together let one determine the characteristics of a crystal family, and then match units for subsequent use as crystal filter elements. The Crystal Test Fixture was also built using Manhattan-style construction methods. Shown in the photo is the 25 Ohm cermet potentiometer used to determine the equivalent series resistance (ESR) of a crystal.

The PVXO drive connects to the left BNC connector, and the external detector to the right BNC connector. Crystal Test Fixtureside view. The DPDT switch selects either an RF probe, or output directly from the RF amplifier to accomodate differing readout instruments.

One of the more interesting (at least to me) measurements made with the PVXO and CTF was to demonstrate the effects of grounding the case of a crystal under test. As can be plainly seen, the series resonant frequency is unchanged, but the parallel resonant frequency changes about 1.

5 KHz (higher) when the case is grounded. The obvious implication is that the performance of a crystal filter is also changed on its high side by moving that slope farther out from the passband. My Atlanticon 2002 paper entitled "Simplified Tools and Methods for Measuring Crystals" can be downloaded here.

The file is in. PDF format, is 1 Mb in size, and describes how to use the PVXO and CTF combo to characterize a set of crystals, and then match them for use in a crystal filter. It does not tell you how to design the filter. That task is best done using the "Filter" program, free from the AADE web site. Parameters of several common computer crystals are here. Many have been used in my homebrew rigs. These parameters were obtained usi