December 2017 - PY2OHH Overtone Crystal Oscillator

Even simple items of test equipment are useful. This is a story of how a new one made its way onto my bench recently. It isn't groundbreaking or even new research but rather the description of a process of testing - using existing simple equipment to create another. I think it also shows what a sharing pastime ham radio can be. I used two items of test equipment, one created by a ham in India and another from the USA - shared freely with the community to help with the creation of another circuit from a ham from Brazil. What a great hobby this is.

The GQRP Club is a marvellous source of components for members. Often, our sales manager Graham G3MFJ will find absolute bargains to pass onto members. This time it was a large bundle of mixed crystals, including a quantity marked 40MHz. Graham tried them on his crystal tester and got nothing. The club goes to some length to check items that we sell to members, so as a challenge he sent a couple each to both myself and Nick G8INE check them out.

My crystal tester didn't want to know them either. A quick alternative test was to see what my Scalar Network Analysers made of them. First I tried the SNA Jr designed by DuWayne Schmidlkofer KV4QB.

There was definitely some activity at 40MHz.

I then scanned it using my "Sweeperino" designed by Ashar Farhan VU2ESE.

Clearly these were going to be overtone crystals - you can see that from the response at 13MHz - and then at 40MHz and it was going to need a tester/oscillator designed for overtone rather than for fundamental crystals.

Nick G8INE did some research and found a design by Miguel PY2OHH
[Miguel has a website very much worth visiting]

Miguel also provided a very helpful layout for a board using pads.

The numbers on the pads correspond to the numbers on the schematic.

Nick got his unit built first using MeSquares from QRP-Me. These are also available from GQRP club.

You can see that Miguel's layout was easily translated to "MeSquares".


Later in the day I finished my tester.

My unit was built on a small matrix board that I created on my mill. Then I sat it on top of a 9v battery holder.

and a nice healthy output seen on my 'scope.

At the end of the day, Nick and I each ended up with a simple piece of test equipment which in the future we can drag out and verify an overtone crystal. It doesn't even really need to go in a project box. We also confirmed that these crystals were OK to sell to GQRP club members. What they will do with them may be seen in a later issue of Sprat magazine.

Some observations about components used. Constructors shouldn't get too hung up about not having the exact component specified. I didn't have any 2N3904's. A quick rummage found some NPN transistors marked 2N3704. I tried it and it worked. I also didn't have any 33pF capacitors or .22uH inductors. I used 22pF and .47uH inductor and they were close enough. Nick didn't have any inductors that size so he wound 7 turns on a T37-6 core.

Thanks to Miguel PY2OHH we didn't even need to pore through text books to find a design so that we could test these club crystals. A design was just a click away, so thanks again to Miguel for creating his website.

Thanks also to DuWayne and Farhan for sharing their test equipment projects with us.




Articles (2017)

December 2017
Overtone Crystal tester
Simple and Cheap to build.

September 2017
My take on Earthing Rods
and a good excuse to buy an SDS drill.

September 2017
A font of knowledge
Some superb blogs to follow.

September 2017
Audio Amplifiers
and a great offer from GQRP club .

August 2017
Atmel Madness
Programming clever little chips .

June 2017
Auto Cut-Off Timer by Bob AE4IC
Saves on batteries.

May 2017
SNA Jr. by DuWayne KV4QB
Unbelievably useful test instrument. Link to DuWaynes Google +

April 2017
Aerial Phasing Noise Canceller
For battling that S9 local noise.

March 2017
QRP for the Tightwad
A Club Talk.

January 2017
A Comb Generator
Handy Wide Band Noise Source.