18-04-2011, 08:10 AM
There was a posting earlier about crystal calibrators, so I've attached some pics of one that I built way back in 1981 from an article in the long since defunct 'ETI' magazine. It used a 1 MHz HC6U crystal, then four decade divider chips to give 100khz, 10khz, 1 khz and 100hz. For scope calibration, these frequencies equate to 1uSec, 10uSec, 100uSec, 1 mSec, and 10 mSec. It can also be used to calibrate (or check the calibration of) such things as radios, especially homebrew.
I've searched internet, and there are surprisingly few circuits for homebrew calibrators, and those that I have found, use either a 1 MHZ or 100kHz crystal, which are either not easy to find, or are expensive, compared to other frequencies. 5 MHZ HC49U wire ended crystals on the other hand are quite cheap, so a good solution would be to use a 5 MHz crystal, then a divide by five IC to give 1 MHz, then decade dividers to give 100KHz, 10 KHz and 1 KHz.
I'm toying with the idea of designing such a calibrator, with on-board switching using a standard (cheap!) 6 pole two-way Lorlin type switch mounted onto the PCB to avoid errors in switching. It would use high speed CMOS 74HC04 chip and a 5 MHz crystal in the oscillator, then four74LS390ICs in the divider chain, configured to give outputs outlined above (plus a 5 MHz output). The literally are ‘cheap as chips’ – the crystal and ICs should come to no more than £2.00 and the total cost probably not more than a tenner. The calibrator would be powered from a 9V PP3, with an on-board 5V regulator.
Would there be any interest in such a design from other forum members?
It isn't something I'd be able to put togther overnight, as it involves drawing up a circuit, designing a PCB, building, testing and de-bugging a prototype but if there's enough interest in such a project it's something I'd have a stab at over the next few weeks, exclusive to this forum, to stimulate interest in homebrew and forum participation.
Anyway, below are some pics of my 1981 ETI calibrator, including a pic of it displaying 1 MHz on my 1976 homebrew 200MHz digital frequency counter which used nixie tubes, and still gives a good account of itself. Commercial counters were way out of the pocket of amateurs back then, and I felt rather chuffed, if not a little smug, to be the first guy on the block to have one, especially given that I built it myself! (It was built on tagstrips and veroboard).
The scope trace is of a 1 kHz square wave from the calibrator, at 5V per division.
Hope that's of interest.
David
I've searched internet, and there are surprisingly few circuits for homebrew calibrators, and those that I have found, use either a 1 MHZ or 100kHz crystal, which are either not easy to find, or are expensive, compared to other frequencies. 5 MHZ HC49U wire ended crystals on the other hand are quite cheap, so a good solution would be to use a 5 MHz crystal, then a divide by five IC to give 1 MHz, then decade dividers to give 100KHz, 10 KHz and 1 KHz.
I'm toying with the idea of designing such a calibrator, with on-board switching using a standard (cheap!) 6 pole two-way Lorlin type switch mounted onto the PCB to avoid errors in switching. It would use high speed CMOS 74HC04 chip and a 5 MHz crystal in the oscillator, then four74LS390ICs in the divider chain, configured to give outputs outlined above (plus a 5 MHz output). The literally are ‘cheap as chips’ – the crystal and ICs should come to no more than £2.00 and the total cost probably not more than a tenner. The calibrator would be powered from a 9V PP3, with an on-board 5V regulator.
Would there be any interest in such a design from other forum members?
It isn't something I'd be able to put togther overnight, as it involves drawing up a circuit, designing a PCB, building, testing and de-bugging a prototype but if there's enough interest in such a project it's something I'd have a stab at over the next few weeks, exclusive to this forum, to stimulate interest in homebrew and forum participation.
Anyway, below are some pics of my 1981 ETI calibrator, including a pic of it displaying 1 MHz on my 1976 homebrew 200MHz digital frequency counter which used nixie tubes, and still gives a good account of itself. Commercial counters were way out of the pocket of amateurs back then, and I felt rather chuffed, if not a little smug, to be the first guy on the block to have one, especially given that I built it myself! (It was built on tagstrips and veroboard).
The scope trace is of a 1 kHz square wave from the calibrator, at 5V per division.
Hope that's of interest.
David
