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?The 100MHz crystal oscillator? is a very bad idea. A 20MHZ fundamental mode crystal is used in a 100MHz 5th overtone circuit. It is a lottery if the 20MHz crystal shows activity at it's 5th overtone, many fundamental crystals are cut to avoid such activity.

Even if you get it to oscillate it won't be at 100MHz, if you were lucky enough to get some activity at? around 100MHz it would not be that close to 100MHz as the overtone frequency is NEVER an exact multiple of the fundamental frequency, this is a mechanical oscillation inside the quartz crystal slice that has different "end effect" distances for different modes.

A crystal is specifically cut for 3rd, 5th or 7th mode oscillation, normally tuned circuits are needed to suppress unwanted modes. The pulling range of an overtone oscillator is much smaller than a fundamental, an inverse square law is involved, 3rd pulls about 11%, 5th about 4%, 7th about 2% of the fundamental pulling range. Most overtone oscillators have serious high Q resonances HF of the nominal frequency, these need suppression.? Quartz crystal & ceramic? filters can also? have serious resonances HF of the nominal passband too, the effect diminishes with the pole count but is serious in 2 and 4 pole monolithic filters.?

The oscillator is a Butler series mode circuit which is a very predictable and reliable oscillator using bipolar transistors. JFETS are not so suitable as the gm is very variable from device to device. As the crystal is driven in series mode any additional series resistance greatly reduces the resonance width. A series mode overtone crystal should have an internal equivalent resistance of a few ohms, 20 ohms would be very high yet the suggested circuit puts nearly 100 ohms in series with the crystal so loosing a lot of gain ( from the 100/470 potential divider with a drain follower that probably has around 20 ohms of output resistance ( 1/gm).

So it's a bad circuit in that:-

• The designers did not produce a "reproducible with average parts"? circuit
• They assumed that a fundamental mode ( common or garden type) crystal would have overtone activity
• They put excessive resistance in series with the crystal which would seriously degrade phase noise and inhibit oscillation
• They used wide tolerance JFETS where bipolars would work much more reliably
• It was a bodge to get around using an expensive 5th overtone crystal that is not that commonly listed
• They got lucky with the original, others had less luck
  

In the years since the price of? mass market crystals has fallen remarkably low, improvements in mass manufacturing has resulted in generally higher activity, higher Q and sometimes better suppression of unwanted modes. Also packaged oscillators are now very low cost and very reliable. Today it would be very straightforward to use a ready made 100 MHz crystal oscillator package.

Regards, Alan G8LCO.

Wally, It appears that I used the .01 on the source follower.? That's a long way from 10pf.? Could that stop the oscillation?
see attached files.? Even though I originally put both oscillators on one board, I built them up separately.
Andy KM4 TRT

My schematic had a print error, but the board was built with J310's .? Sorry for the error.
ANG

I had started the Specan project back in 2019, and have just now got back into the project. ?My last activity had been to build the 100 MHz oscillator. ?I used ugly style construction with Me Pads. ?The oscillator works fine for me. ?I built the module using a 9 volt battery as its supply, and MPF102 transistors. ?I will check my notes to recall if I had any issues. ?I just checked the signal output on a spectrum analyzer and it looks good. ?At the time I may have tuned it using the FM radio method that Ashar Farhan described. I am about 60% completed on this project. ?I had issues with the Sweeperino software not working as well as it was described, and the documentation on the Arduino wiring to the LCD display was wrong.
regards, Nigel, VA3NMA.

Nigel, Did you use just any 20 MHz crystal, or did you purchase special "overtone" crystals.
I emailed Wes Hayward who has written books on the subject and he said that regular crystals will work,?
but so far I haven't been able to get Ashar's design to work, with the crystal or using the resistor and capacitors.
Hayward sent me another butler design that he uses and that I could get to oscillate at 100 mhz without the crystal
(just a resistor), but no luck with the xtal.
I built Ashar's design using (1)a printed circuit board, (2)A perfboard with eyelets and neither will oscillate.

As far as the Sweeperino, a friend designed a gui that addressed the Si570 and we were closing in on it, but I somehow burnt out the 570s that I had .? I found another supply, but so far haven't go back to that.

Are you still trying to make that work with a 20 MHz fundamental crystal?? It can probably be made to oscillate in an overtone oscillator circuit, but it won't be on 100 MHz.? It will probably be near 100 MHz, but not close enough to be usable.? One reference suggested frequency errors in the range of (overtone number times 25 kHz), so for 5th overtone operation, expect it to be off by 125 kHz.? That's a "rule of thumb" and highly approximate, of course, but with the potential for that kind of frequency error, why even bother with a crystal?? Stop beating your head against the wall: either buy a 100 MHz 5th overtone crystal, or come up with another solution.? Use your 20 MHz crystal in a fundamental oscillator with a multiplier circuit, or use a packaged 100 MHz oscillator, or something else.? Plenty of other ways to reliably generate a 100 MHz signal.

Jim, Thanks for the reply.? I understand your reasoning, but I'm just trying to understand why it doesnt work at all. Then I could worry about whether it was on frequency.? I already have a tiny SA, so I don't really need to build this, I'm just trying to get components to work that others have done before me. When Ashar Farhan builds one out of a junk box and makes it work, I wonder why I can't.? Also Wes Hayward sent me a schematic for his (similar) butler oscillator (5th overtone) with junkbox crystals and showed how the oscillator worked slightly off with just a resistor. (I built that and acheived 100 mhz).? Then he inserted the 20 MHz crystal and forced it into the correct frequency SHAPE as shown on his spectrum analyzer with the variable cap.? Again a fairly simple circuit.? However I could not get mine to oscillate with the crystals I have even though the thing will oscillate at 100 Mhz with a resistor.
Andy

Maybe the series resistances of the crystals you have is just too high to oscillate in that circuit.? Crystals have much higher equivalent series resistance at their overtones than they do at their fundamentals, and greater gain will be required to overcome that.? What transistor are you using in the circuit?? Have you tried more than one crystal in the circuit?? Same results with all of them?? How high a resistance value can you use in place of the crystal and still get it to oscillate?? Have you tried putting an actual overtone crystal in the circuit and see if it will oscillate with the intended device in the circuit?? Are the crystals you have tried of recent manufacture or older parts?? What case size are they?? There is a big difference in, for example, the tall case HC49 and short case HC49s crystals.

Hi,

I used 20.000 MHz ?20 pF crystals from Digikey.
part number 631-1091-ND
Purchased in January 2019z
tbey are made in China, Fox Electronics.

the first one I used worked in the oscillator.
i tuned the oscillator as Ashar Farhan described, by listening to an FM radio tuned around 100 MHz.

then I used a frequency counter to get it tuned closer to 100 MHz.

regards,
nigel va3nma

?

For what it's worth, I had some difficulty getting the oscillator to work. I started swapping out my jfets in sheer desperation and it started working suddenly! At this point I figured it was best to stop fiddling with it and leave well alone. I'm glad that my experience wasn't uncommon because I was at my wit's end.

Sanjay

Hi,

c13 on the 100 MHz oscillator should be 10 pF not 0.01 uFd. ? The Ashar Farhan Specan schematic on his website shows a full schematic and further down a smaller schematic that focuses just on the 100 MHz oscillator. ? The full schematic shows a 0.01 uFd cap, but the smaller schematic shows a 10 pF value. ?I tried both and it oscillated only with the 10 pF. ?I am still trying to confirm that I have the right power out to deliver +4 dBm to the Mixer ports.

Ok, more progress. ?I swapped the MPF-102 transistors with SMT J310’s (soldered carefully into ME Pad SOIC boards) and added some leads to create a leaded J310 equivalent. ? This gave me higher power output from the 100 MHz crystal oscillator. ? I then experimented. ?Again - use the schematic on page 10/12 of Ashar Farhan’s blog (Vu2ese’s radio experiments : Specan). ?I found using the 10 pF in parallel with 16 pF NPO cap going to the 6dB pad gave me the most stable 100 MHz oscillator. ?I can easily tune L11 and get a 3 dBm output level into the mixer, and the oscillator is very stable. ?Success. ? Again - don’t build the circuit as drawn in the large schematic (it didn’t work for me using the 22 pF trimmer cap in series with L11, and the 0.01 cap fro Q2 Source killed all oscillation). ?