The PCB files package includes:
- Readme IG-18SL Osc.txt - a list of the files.
- IG-18SL Osc.brd - Complete Eagle v.6 layout file.
- IG-18SL Osc.sts - Bottom solder mask
- IG-18SL Osc.sol - Bottom copper
- IG-18SL Osc.plc - Silkscreen
- IG-18SL Osc.drd - Excellon drill data
- IG-18SL Osc.dri - Readable ASCII summary of drill information.
- IG-18SL Osc.gpi - Readable ASCII summary of photoplotter information.
Formats: txt, dri, gpi are ASCII. The drd file is excellon drill data. Others are Gerber. Note that this layout is intended for a single sided board. The topside traces shown in the BRD file are only to guide topside jumper installation. They are not suitable for fabricating a topside copper layer on the PCB.
Intriguing New Developments from Dick Moore
In addition to creating the PCB for the IG-18SL, Larry Burk also contacted Dick Moore, who has a fascinating website here. Dick has done extensive work on various approaches to enhancing the IG-18. With one of Larry’s PCBs, Dick constructed the circuit and did some advanced measurements which showed that the distortion at lower frequencies could be far lower than what I was able to measure, with the limitations of the HP8903A analyzer. His write up on it is here. I must say that the distortion figures seem almost too good to be true (under 0.0001% at 1kHz, 0.0003% at 10kHz). However, his measurement methods, while indirect, certainly make sense. Note that Dick calls this build the “IG-339A.” As mentioned above, Dick also found that an LT1468 opamp works very well, in place of the hard-to-find HA3-2625.
He also experimented with using the original Heathkit tuning network with the new PCB circuitry, calling that build, the “IG-339B.” His write up on that version is here. The Heathkit tuning network uses a 10:1 ratio of capacitors in the bridged-T, versus HP’s 100:1 ratio. It originally seemed likely that the higher ratio was responsible for the low distortion performance of the HP-339 oscillator. However, he reports measured distortion figures under 0.0005% over 100Hz to 10kHz. While I find this very surprising, Dick obviously has done a lot of excellent work on this, so I would hardly question his results. I really wish that I had time right now to get back into this project but alas, there are so many others that I’m juggling, now. (Maybe you know the feeling—so many toys, so little time :)