The K8IQY  "P-VXO"
Precision Variable Crystal Oscillator Kit



Master designer and craftsman Jim Kortge, K8IQY has developed a precision VXO (variable crystal oscillator) and the NJQRP Club is providing it as a kit including: all parts, a gorgeous red pc board, all board-mounted controls, a Ten-Tec enclosure, and a very comprehensive construction and operating manual. 

This kit is useful in crystal measurement and sorting, as an extremely precise crystal oscillator, as a way to bend a given crystal to place it directly on the desired frequency, as a local oscillator in a radio, or as a direct conversion transmitter unto itself.

The Precision VXO generator employs a Colpitts oscillator using a JFET.  The crystal is usually one from a set that will be used for building a filter or for Local Oscillator service.  However, if the Precision VXO is being used for receiver alignment, the crystal might be one whose frequency is inside one of the lower ham bands.  Controlling the frequency of oscillation is accomplished with a varicap diode and a set of 5 molded inductors configured as a binary weighted set.  Inductance needed to force the crystal to oscillate at its marked case frequency is experimentally determined by selecting the desired total inductance using five toggle switches mounted on the board.  When no inductors are selected the crystal will oscillate at its highest frequency.  As more inductance is added the crystal’s oscillating frequency moves lower.  Maximum inductance is obtained when all of the inductors are selected and connected in series.  Under these conditions many higher frequency crystals may cease to oscillate. 

Small frequency variations are effected by means of a 10-turn potentiometer that varies the voltage applied to the varicap diode. This fine tuning provides the overlap needed with most crystals to provide seamless coverage for the digitally selected inductance.  It can also provide sufficient frequency span so that the resonant frequency of another crystal under test can be determined.  Frequency precision is obtained by using the 10-turn pot and stability is achieved through the use of an internal regulated supply, and by minimizing the RF levels in the oscillator circuit.

A second transistor stage is used as a source follower to further isolate the oscillator from any changes reflected back from the output load.  A voltage divider is used to provide an output impedance of 50 ohms which properly drives the fixed input impedance of a noiseless 2N2222 Norton amplifier. This output stage provides a gain of +12 dB and amplifies the generated signal to a level of approximately –10 dBm, or 250 millivolts peak-to-peak.   Downstream of this amplifier, a low pass filter is used to reduce harmonic content above 20 MHz.  A spectrum plot of the generator’s output is shown in figure 2. 

Overall, this Precision VXO generator is designed to work with commonly available HC49 and HC49U style computer crystals, often used in IF filter service.  It will operate with crystals in the frequency range of 3.5 MHz through 13.5 MHz, but is optimized for crystals at 9 MHz and below.

Besides having a clean output spectrum, this low cost generator also has excellent frequency stability.  Its warm-up drift is a few Hertz and the stability over a 24-hour period is within 20 Hz.  Short-term stability is not measurable with the equipment in the K8IQY laboratory.
The typical tuning range with a 4.9152 MHz crystal is plus and minus 250 Hz - more than adequate for covering the series resonant frequency of crystals being characterized or matched. More importantly, with the wide range of inductance available, the oscillating frequency of the VXO crystal can be moved over a considerably wider range, thus ensuring that one can find the series resonant frequency of a crystal under test.

The parts supplied in the kit contain everything needed to assemble the unit except a power connector.  (This part was not supplied, since everyone seems to have his own favorite.A Ten Tec TP-17 case is included. The case is not drilled nor labeled - the builder must perform those operations and templates are provided to assist with those construction steps.

Assembly is broken up into logical steps, so that each section can be built and tested.  While that approach is recommended, it is not necessary, as the PC board is logically laid out and well marked with the component locations.  No alignment operations are required after assembly.

Note:  Enough interest was also shown in the companion "crystal test fixture" that Jim used to sort the crystals for the 2N2-40 Group, and the NJQRP will also be kitting that project in the near future in order to further enhance the usefulness of this K8IQY-designed test equipment.

A REVIEW ... de Chuck Adams, K7QO

Friday night at the Ramada Inn during the vendors display I purchased from the NJQRP Club the K8IQY 'Precision VXO' kit.  For those that have been hiding out in a cave, this is a VXO that allows to do a number of things with crystals.

If you are an experimenter you can use this with some other equipment to measure a number of crystal parameters or for a simple test application you can use it to match crystals for a crystal filter.  You'll see over the next few months a number of individuals post some applications that they have found for this kit.

I got mine built today and it worked from the start.  I skipped all the intermediate test stages, but you should not just in case.

The PC board is 10.2cm by 7.7cm and is neatly silk screened with a red board and white lettering and parts outlines.  Not only is the part number, e.g. C8, shown but also the value to be used as a double check.

The manual says that you can do up to 13.5MHz but I find that I was able to use crystals up to 18.0MHz.  Above that the crystals that I have did not resonant or show any activity at all, but 18.0MHz is good enough a it is rare to find an IF frequency higher than 13.5MHz.

There is a web site that will have application notes and features as they develop over time.

I think that the kit could even be built by a first timer even though the construction notes are not step-by-step. You just need some basic ability to tell the difference between a resistor and a molded inductor and a capacitor.

I do not know how many kits the NJQRP club plans to kit, but if you have not gotten one I seriously suggest that you get one ASAP or we'll be seeing you post to QRP-L later asking if anyone has one unbuilt.  :-)

It comes with PC board, parts, and case.  The only thing missing is the 12V power jack and that you have to get according to the standard you have settled on in your shack.  You do have to drill the case and label it.  No serious metal work involved and the instructions have a template for this work.

Thanks to Jim Kortge, K8IQY, for the design and the NJQRP club for the kit.  This puppy is going to be one of the top test equipment devices in my lab for sure.

I have no association with the club or this kit other than a satisfied and happy customer.  They do good work.

I will have an RF voltmeter circuit to be used with this puppy by Monday on my web site along with digital pictures showing my layout and jig for testing crystals. Previously I was using an expensive DDS VFO to do this work and for those on a limited budget this is a much better way to go.

For more serious work later you are going to need something like the old Heath IM-2410 Frequency Counter if you can find one for under $50 at a swap meet and it is in working condition.  I found one in good shape for $40 for Doug Hendricks, KI6DS, at Dayton years ago and Doug can tell you how valuable that puppy is......

FYI es enjoy,

    Chuck Adams, K7QO  CP-60


The PVXO Kit is no longer available.


The PVXO Kit is no longer available.

The PVXO Kit is no longer available.


Last Modified:  January 11, 2004