Quote:
Originally Posted by G0HZU_JMR
I quickly tested each 4.032MHz crystal in a makeshift socket in the test fixture and on the HP8405A four of them were within 25Hz of the first crystal.
But two were at about -80Hz and one was +80Hz. This is just outside of the manufacturer's original/initial 15ppm calibration spec for this crystal when it was made 20 years ago. Note that I'm just measuring relative differences at the self resonant frequency. Next I need to try them with my first attempt at an accurate 30pF load capacitor to see how close they are to 4.032MHz.
|
This re-remembers for me the time spent late-one-night using the facilities at Rutherford Labs to quantify the series- and parallel-resonant frequencies of a 'job-lot' of around a hundred 11.177MHz HC6U crystals I 'acquired' back in the mid-1980s.
From the job-lot I was able to build half a dozen rather good SSB-bandwidth 8-crystal ladder filters' and point a CW-oriented friend in the right direction to use up the remaining rocks to build a filter or five for his particular predilection.
The biggest problem with building 'ladder' filters is that they need to work with consistent source- and sink-impedances, where 'consistent' extends from a few hundred KHz to around 5x the frequency of the crystals themselves. But Wahey! done properly [and with good screening/shielding to prevent IF 'hop-over'] by hand-picking the crystals you could get a great 6:80dB shape-factor - much better than the at-the-time amateur 'gold-standard' KVG XF9- series.