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Crystal Searching[long]
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This is not an easy answer.? It is mathematically, but there are some
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variances between manufacturers and quality control.? Let me explain. The series resonant frequency of the crystal is determined by L_1 and C_1, the motional parameters due to the physics. In a VXO, we are interested in just how far we can "pull" the frequency of operation, say delta_f.? We can approximate this value by the formula:? (Colpitts oscillator is typical) delta_f ~= 0.5*(L_ext/L_1 - C_1/C_ext). L_1 and C_1 we can't do anything about.? It comes with the crystal as it is manufactured.? In both the SST and 1W transceivers, look at the schematics.? What do you see in series with the crystal.? Yep.? An inductor, let's call that L_ext and a variable cap C_ext.? L_ext is fixed.? As you vary C_ext, you shift the frequency. At first you say, well heck let's make L_ext large and C_ext large.? Several things happen.? You can't shift far (L is fixed) and L_ext swamps the capacitance change. A bad side effect is that the oscillator stability goes to heck in a hand basket. This leads up the answer for the question.? I have to take the different frequencies, different varactor diodes, different molded inductors and transistors (2N3904, 2N4401, or even a JFET) and see what gives the greatest range for each band, greatest output and if that is good at the output of the mixer, least drift, ....? You see the problem, but I have the fixture set up on a Manhattan layout. As soon as I get the crystals off the boat, I will do the experiments and report to the group.? In the meantime, maybe some of the group can also give their results from previous builds and lab work. Look at the frequency range of the specs of every rig that uses a VXO.? This may help you see just how much you can in tunable range of the VXO. Get out the G3UUR and measure the L and C values. R_1 of the crystal is also at play in the range of frequency pull you can get and a large R_1 will get you drift due to heating. Since we are getting these things from China, there may be a spec sheet that gives some ranges, but crystal axis cutting and accuracy causes some variance between batches.? That's why you buy crystals in bulk. They come from the same batch and setup and you are more likely to get a close match more easily. We used to see a date code, but with the HC-49/S, there is no room and the new crystals rarely show a batch code like transistors. BTW, the HC-49/S and the HC-49/U should have almost the same L_1 and C_1 values. Go to K8IQYs home page and go to the crystal page values he has measured. <> L_m and C_m are the L_1 and C_1 variables used in the formula.? You will see both sets of variables spread across the Internet.? L_m and C_m are the ones used by the industry.? I'll make up a table and put it on the web for my results to add to the body of knowledge to be found on crystals.? Maybe W7ZOI has some values also on the web. If the circuit capacitance across the crystal matches the C_o value, then the circuit should resonant at the frequency marked on the crystal in parallel mode. This is just the tip of the iceberg..... FYI On 5/23/25 14:29, Alan N8WQ via groups.io wrote:
Thanks for posting this info Chuck. --
chuck adams, aa7fo QRP WAS 17m started Apr 2, 2025 WAS-17 QRP --> TX,FL,SC,GA,OK,AR,KS,AL,AZ,MN PA,CA,CO,WI,NE,ID,IL,NC,NY,WV MA,OH,OR,RI,MT,WY,WA,MI DXCC W,VE,JA,PY,ZL,TX9 |
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Good stuff, yes the constraining math of quartz acoustics. We are blessed by its unique properties and having lots of quartz material. It's the scarcity of manufacturing.?
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Similar acoustic behavior lurks inside our smart phones. Not just for frequency stability but fitting all these waveforms into it. Not many know this, but bulk acoustic filters are its key. Not quartz so much but materials that are piezoelectric that can be deposited onto semiconductor wafers. Two such materials are known, ZnO and AlN. The latter is largely favored due to its better temperature stability. And those same formulas, along with AlN acoustic and electrical properties determines the bandwidth of these filters. They operate at GHz frequencies and are very tiny. I was blessed to work with 2 pioneer researchers who knew both the math and physics to make them. Meanwhile, I testify much more fun to work with quartz in our home labs (otherwise I might still be working (-:
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73 Curt wb8yyy?
Former EE etc.
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