Spent some Quality Shop Time measuring an assortment of crystals, some data from which will make up a Circuit Cellar column.
And the raw numbers will come in handy one of these days, so here they are…
| 12 MHz Asst | HC-49/U | Co+Cc/2 | 2Cc | Fs | BW | Rs | |
| ECS | 1 | 4.85 | 1.47 | 12.000162 | 787.5 | 40.1 | |
| ECS | 2 | 4.50 | 1.42 | 12.000150 | 725.0 | 40.1 | |
| ECS | 3 | 4.70 | 1.42 | 12.000325 | 1100.0 | 50.0 | Rs out of range |
| HC1 | 4 | 4.34 | 1.11 | 11.999000 | 600.0 | 36.0 | |
| HC1 | 5 | 4.24 | 1.06 | 12.000137 | 537.5 | 36.9 | |
| Sentry | 6 | 5.14 | 0.96 | 12.000250 | 625.0 | 31.8 | many spurs |
| 11.0592 MHz | HC-49/U | ||||||
| 1 | 4.90 | 1.42 | 11.059275 | 562.5 | 9.3 | ||
| 2 | 4.99 | 1.46 | 11.059112 | 575.0 | 14.7 | ||
| 3 | 4.87 | 1.42 | 11.059275 | 512.5 | 9.6 | ||
| 4 | 4.87 | 1.41 | 11.059125 | 550.0 | 10.0 | ||
| 5 | 4.29 | 1.43 | 11.058935 | 750.0 | 18.1 | ||
| 6 | 4.93 | 1.47 | 11.059000 | 537.5 | 10.7 | ||
| 7 | 4.95 | 1.47 | 11.059200 | 525.0 | 8.1 | ||
| 8 | 5.03 | 1.45 | 11.059037 | 575.0 | 11.1 | ||
| 10 MHz | HC-49/U | spur +150 kHz | |||||
| 1 | 2.57 | 1.36 | 9.997888 | 200.0 | 14.2 | ||
| 2 | 2.61 | 1.30 | 9.997738 | 225.0 | 16.7 | ||
| 3 | 2.75 | 1.30 | 9.997788 | 225.0 | 20.0 | ||
| 4 | 2.67 | 1.30 | 9.997750 | 225.0 | 16.1 | ||
| 5 | 2.75 | 1.26 | 9.997725 | 250.0 | 22.7 | ||
| 6 | 2.69 | 1.27 | 9.997788 | 225.0 | 21.0 | ||
| 7 | 2.69 | 1.26 | 9.997825 | 212.5 | 16.5 | Circuit Cellar example | |
| 8 | 2.69 | 1.22 | 9.997832 | 212.5 | 18.4 | ||
| 9 | 2.72 | 1.24 | 9.997788 | 250.0 | 23.4 | ||
| 10 | 2.68 | 1.20 | 9.997738 | 225.0 | 18.0 | ||
| 18.43 MHz | HC-49/US | many spurs +10 +76 kHz | |||||
| 1 | 3.86 | 1.33 | 18.432425 | 1.56 | 24.1 | ||
| 2 | 3.79 | 1.22 | 18.432987 | 1.21 | 10.9 | ||
| 3 | 3.93 | 1.39 | 18.432050 | 2.44 | 46.3 | ||
| 4 | 3.97 | 1.40 | 18.431175 | 1.90 | 27.7 | ||
| 5 | 3.89 | 1.33 | 18.431888 | 2.11 | 32.2 | ||
| 6 | 3.92 | 1.39 | 18.430888 | 1.39 | 16.5 | ||
| 7 | 3.99 | 1.35 | 18.431500 | 1.36 | 11.8 | ||
| 8 | 3.97 | 1.35 | 18.431675 | 2.18 | 38.4 | ||
| 9 | 3.95 | 1.31 | 18.430512 | 1.30 | 10.1 | ||
| 10 | 4.04 | 1.50 | 18.431427 | 1.36 | 11.8 |
The 18.43 MHz crystals are in the short /US cans with surprisingly high stray capacitance. Their bandwidths are in kHz and all over the map, as are the series resistances. Weird. Bad crystals? Bad technique?
Capacitance measured with that fixture.
Frequency & bandwidth from HP8591 spectrum analyzer with a fixture similar to the K8IQY design; the bandwidths seem to come in 12.5 Hz increments despite a (very narrow) 2 kHz span. The general process is there. Resistance measured from a cermet trimpot using a multimeter good for 0.1 Ω around 10 Ω.
Useful equations, with column headings in boldface:
- Lead-to-can capacitance for each lead: Cc = 2Cc / 2
- Lead-to-lead capacitance: Co = Co+Cc/2 – Cc/2
- Circuit Q: Q = Fs/BW
- Circuit resistance: R = Rs + 25 (assuming 4:1 transformers)
- Reactance XL = XC at series resonance: X = Q R
- Motional inductance: Lm = X / (2 π Fs)
- Motional capacitance: Cm = 1 / (2 π Fs X)
- Parallel resonance Fp = Fs √(1 + (Cm / Co))
More equations there.
Memo to Self: Zero the capacitance fixture before critical measurements!
The Smell of Molten Projects in the Morning 
Did you see the recent hackaday about tuning crystals using a Sharpie? It wouldn’t fix the ESR but I thought it was a great idea. You, being already an expert at this, probably knew about the technique but it was new to me.
tuning crystals using a Sharpie
Sharpies FTW. But on a crystal? Ick!
I sort of knew about tweaking crystals like that, but was under the impression that contemporary crystals buzzed in an inert atmosphere, if not a mild vacuum. Thus, carving the can off should cause instability as the electrodes oxidized: if you wanted a particular frequency, just wait until it drifted into view.
I cut some crystals out of their cans for a Circuit Cellar picture and one of them now has obviously tarnished silver electrodes. I’ll do a quick-and-dirty comparison with a nominally identical crystal still in its can to see whether exposure to air does anything obvious.
For what it’s worth, I used a Dremel cutoff wheel to good effect. I think if I was extracting a crystal for tweaking, I’d cobble up a CNC routine to run an ultra-thin slitting saw around the base of the can.
Dang, another project… I’ll get you for this! [grin]
Yeah well *I* am currently starting down a project to chill air to the point where I can separate out liquid nitrogen and argon (and uh other bits) so I can reconstitute that inert environment! All you have to do is put the Sherline in a glovebox, hook up a tiny tig welder to the spindle and put the crystal on the rotary table…
(I actually saw one of these once. It was a handheld automated micro-tig welder for butt-welding stainless steel tubing gastight, for fluorine lines. It took about four seconds and the joint was hard to see if you didn’t know it was there. One of the coolest tools I’ve ever seen.)
handheld automated micro-tig welder
A long time ago, I took a course in aircraft restoration taught by the Smithsonian guys who restore the birds for their museums. Their master welder gave us a tutorial on welding (“Stop waving the torch like a drunkard, dammit!”) and let us play with the toys. He was the guy NASA called to weld up magnetic enclosures for a satellite: 5-mil nickel sheet welded in a chilled fixture that exposed only the edges, producing hermetic joints that looked like they grew there. I suppose the right fixture made the job trivially easy for anyone with just a few decades of experience…
They turned us loose in the vast Silver Hill storage warehouses on our own. After a long crawl that involved not slitting my throat on an all-wing German Wonder Weapon, I stood up inside a large empty box that I eventually figured out was the forward bomb bay of the Enola Gay. Talk about chills up your spine…
Ed, IMHO you are not going to have much frequency accuracy with an analyzer. If accurate freq. is needed then a good counter with a Rubidium 10Mhz standard will give you good results in accuracy.
a good counter with a Rubidium 10Mhz standard
I actually have one of those HP Z3801A GPS-disciplined double-oven 10 MHz oscillators lying around here, but I had to give up keeping it turned on all the time because the power bill was getting absurd. If I were to fire it up again (and give it a week to stabilize down to a few ns/day), I could feed the 10 MHz signal into the Ref In jack in the back of the HP8591 and get superlative accuracy & stability…
But, eh, for a first pass I think the bare SA gets close enough to the goal.
Oh, you were being ironic. Around here, it’s tough to distinguish irony from thoughtful technical suggestions… after all, who doesn’t feel the need for a rubidium oscillator? [grin]
I was wondering why the 10mhz was off by so much. I guess its the K8IQY OSC circuit.?
My analyzer does not have a 10Mhz ref. jack at the back :-( I need to find one that does but such equipment in India is hard to come by.
I keep the TBolt GPSDO always on but fire up the hot plate Rb OSC when I need it. I found that some of the 5$ OCXOs from China *bay gives me good results in my counters.
The GPSDO is a great source of entertainment these days. One use was to map the sky 24h with Lady Heather v3beta and looking at the map I got a very good picture of the building that shadow me. This is vital info for VHF 6M enthusiasts like me.
the K8IQY OSC circuit
I built only the attenuators & impedance matching part of the fixture. The 8591 supplied the generator & frequency measurement. It’s probably more accurate than (some of) the crystals, although the measurement resolution depends on the span. Really narrow spans make the tracking adjustment very touchy (as you’d expect), so I don’t have much confidence in the last few digits.