One of the 2017 winter projects was to take a little closer look, and have a bit of fun, pulling crystals to get a wider range of frequency use. It turns out to be fairly easy to do – with the right techniques.
View the Paper here: CrystalPullArticle
Thanks Dan, AB7SQ, for writing up, guidance, and putting the final touches on this article!
*** HC49s Crystals are available in the store ***
Below is extra data, O-scope and other screen shots used in building the article.
Click on pictures for larger view.
After building the basic oscillator (no buffer or regulator at this point) some tests were performed with various inductance ranges similar to the article. Below is the full schematic, build, and initial o-scope capture of the waveform. See table at bottom for various values and their results.
Initially I used JT-65 hooking into my IC-725 as a visual aid for stability. In most cases the output was stable near the crystal while frequency and stability is worst farthest away. Same is true with the Theremin effect of our hands being near the circuit changing frequencies. Below are a few JT65 examples showing stability at the “far end”. Note you may see strange duplicate images of the waveform, please ignore them and only focus on the main red line. The interference is not from the oscillator but rather my ViewSonic 22″ monitor spits out tons of RF hash which is then mixing back into the receiver! You can even hear the tonal quality improve when disconnecting the monitor from A/C. Time for a new monitor!!
Stability with 27uH and 15uH Chokes
|JT-65 27uH||JT-65 2x27uH|
|JT-65 2x15uH||JT-65 3x15uH|
Next up was the toroid 55t on a T68-2 is about 17.24uH and ~46″ of 26ga wire. Note the difference in waveform with and without the extra 20pF capacitor. In a few cases the “blip” in the waveform was severe enough to cause the frequency counter to double. The 20pF value is critical, the type of capacitor will also greatly affect performance. I only had cheap computer grade disc and mono (MLCC) capacitors available and found the MLCC to be slightly more stable than disc. Just by changing 1-2pF can make the difference between stability and insanity.
During tests we tried several different toroids (of T50/68 2/6 mix) just to show to try what you have on hand. The table of toroids is at the bottom of this page. FT37-43 and FT23-43 was also tried using the same inductance ranges and DO NOT WORK.
For stability we found an even better program than JT-65 as a visual aid. From this point we used Spectrum Lab from DL4YHF. The link can be found here: http://www.qsl.net/dl4yhf/spectra1.html
* The “Blip” can get pretty large*
*No Blip *
Stability becomes worse farther away from the crystal
Just a few pF lower made a huge difference
A few more screen shots of various combinations
|Drifting on 30M with 17.25uH Toroid||More Stable 30M with 15uH and 3.3uH choke|
|20M 4-xtal 7.2uH Toroid||
A 4uH toroid gave a much more stable result
A quick look at the oscillator from a spectrum analyzer
4×7050 crystals, Cshunt=15pF L=18.24uH (T68-6)
Slight attenuation on the harmonnics
Last but not least we applied some of these techniques to a few of the Chinese QRP kits out on the market by replacing the crystal with a 4xcrystal/inductor/polycap combination.
|Pixie – switch replaced with pulling parts||
Closeup of Pixie Pulling board
Although we did provide more frequency range with these kits a few problems with RIT and chirping was noticed.
The RIT control was not consistent anymore and the amount of RIT was dependent on the amount of pulling applied to the crystal. I found placing the RIT control a hair above fully CCW would give an approximate 800-1000hz Rx/Tx difference across the pull range. BUT once adjusted leave the RIT pot alone! The Rx/Tx shift could be as much as 9KHZ with the RIT fully clockwise!
Chirping was noticed. The 49er had no real observable chirp. The Pixie has a chirp on the receive side – which would not be an issue to the receiving station. The Super RM Rockmite had a TERRIBLE chirp. At the time I have not looked at the schematics and determine if modifications would enhance operation.
Tx/Rx offset frequencies with various Chinese QRP radios
|RIT Fully CCW||RIT Fully CW|
Rx side chirping
This is with just a crystal
Significant Chirping with a VXO
Poly-tuner knob attachment examples
|Using 1/2″ compression insert|
|This works but had a wobble to it
|Replaced the flared end with a 1/4″ dowel and a drop of glue.||
|1||Kbob of your choice – Standard 1/4″ shaft style with set screw|
|1||1/4″ compression insert|
|1||Fuse Bead 5mmx5mm (Craft Store)|
|2||Alternative to Fuse Bead: Plastic Washer .2523″ODx.139″IDx.07″Thick|
|1||Cheap Ball point pen cut to width and a 1/16″ hole drilled at the end|
|1||2.6mmdia x .46mm pitch x 6mm long screw|
|1||1/4″ Dowell ~1-1/8″ long|
Toroids used in testing
Table of results
|Device||XTAL||#Xtals||Coil1||Coil2||Coil3||Cshunt||Cp||Low Khz||High Khz||Delta Khz||Stability Hz||Notes|
|Oscillator||3560||4||100uH Choke||27uH Choke||27uH Choke||470pF||3,552.32||3,560.65||8.33||18|
|Oscillator||3560||4||100uH Choke||27uH Choke||27uH Choke||470pF||3,533.22||3,560.50||27.28||117|
|Oscillator||7050||1||17.25uH Toroid||22pF||270pF||7,039.46||7,049.47||10.01||50||Cheap Disc Cap|
|Oscillator||7050||4||15uH Choke||15uH Choke||15uH Choke||270pF||7,001.09||7,052.18||51.09||60||Theremin effect within 3”, but stable|
|Oscillator||7050||4||15uH Choke||15uH Choke||270pF||7,038.82||7,053.94||15.12||40|
|Oscillator||7050||4||17.25uH Toroid||17pF||270pF||7,025.02||7,051.21||26.19||10||15pF+2pF Gimmick – Be careful of gimick placement|
|Oscillator||7050||4||17.25uH Toroid||270pF||7,046.29||7,053.52||7.23||1||T68-2 (Red) 55t 28ga|
|Oscillator||7050||4||18.24uH Toroid||15pF||270pF||7,029.23||7,051.66||22.43||3||T68-6 (Yellow) 60t 30ga|
|Oscillator||7050||4||18.2uH Toroid||15pF||270pF||7,030.65||7,051.68||21.03||5||T50-2 (Red) 55t 30ga|
|Oscillator||7050||4||27uH Choke||15uH Choke||270pF||6,897.52||7,051.65||154.13||450||Theremin effect within 2”, highly unstable|
|Oscillator||7050||4||27uH Choke||27uH Choke||270pF||6,277.15||7,050.16||773.01||>500||Theremin effect > 6”, highly unstable|
|Oscillator||7050||4||27uH Choke||6.8uH Choke||270pF||7,016.75||7,052.24||35.49||60|
|Oscillator||7050||4||7.2uH Toroid||15pF||270pF||7,048.91||7,051.80||2.89||1||T37-6 (Yellow) 48t 30ga|
|Oscillator||10120||2||12uH Toroid||3.3uH Choke||33-68pF||10,106.23||10,121.66||15.43||3|
|Oscillator||10120||3||12uH Toroid||3.3uH Choke||33-68pF||10,091.78||10,122.99||31.21||8|
|Oscillator||10140||4||15uH Choke||3.3uH Choke||33-68pF||10,063.12||10,144.35||81.23||35|
|Oscillator||10140||4||17.25uH Toroid||33-68pF||9,956.86||10,143.79||186.93||140||T68-6 Severe drifting|
|Oscillator||14060||1||7.2uH Toroid||33pF||14,051.31||14,060.49||9.18||2||T37-6 (Yellow) Cp=33-68pF|
|Oscillator||14060||2||4.7uH Choke||3.3uH Choke||33pF||14,041.42||14,063.49||22.07||11|
|Oscillator||14060||3||4.7uH Choke||3.3uH Choke||33pF||14,039.85||14,065.82||25.97||11|
|Oscillator||14060||4||4.7uH Choke||3.3uH Choke||33pF||14,030.26||14,068.68||38.42||4|
|Oscillator||14140||4||12uH Toroid||33-68pF||10,120.02||10,144.91||24.89||15||T50-6 (Yellow)55t 30ga|
|Oscillator||21060||1||3.3uH Choke||22-33pF||21,052.26||21,061.55||9.29||14||6.5Vpp Cp=22-33pF|
|Oscillator||21060||1||4uH Toroid||22-33pF||21,047.39||21,060.59||13.2||9||T37-6 (Yellow) 37t 30ga|
|Oscillator||21060||4||4.7uH Choke||22-33pF||20,994.28||21,067.12||72.84||74||“Wild Random Shifting”|
|Oscillator||N/A||0||27uH Choke||270pF||3,975.00||8,974.00||4,999.00||1400000||Free Running – Theremin effect within 2”, highly unstable|
|Pixie||7050||1||18.24uH Toroid||1uh Choke||17pF||7,048.50||7,050.00||1.5||5||Chinese Pixie – Good RxTx Offset with rit fully CCW|
|Pixie||7050||4||18.24uH Toroid||1uh Choke||15pF||7,044.20||7,052.30||8.1||10||Chinese Pixie – See RxTx Offset Notes|
|Pixie||7050||4||18.24uH Toroid||1uh Choke||17pF||7,042.00||7,052.30||10.3||11||Chinese Pixie – See RxTx Offset Notes|
|Pixie||7050||4||18.24uH Toroid||1uh Choke||20pF||7,032.90||7,052.20||19.3||11||Chinese Pixie – See RxTx Offset Notes|
|Pixie||7050||4||18.24uH Toroid||20pF||7,040.00||7,052.40||12.4||13||Chinese Pixie – See RxTx Offset Notes|
|Rockmite||7050||4||18.24uH Toroid||1uh Choke||17pF||7,045.80||7,053.10||7.3||22||Chinese Super RM Rockmite – Chirps bad on Tx|
|49er||7050||4||18.24uH Toroid||1uh Choke||17pF||7,047.10||7,053.50||6.4||6||Chinese 49er – See RxTx Offset Notes|
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