Hi,?
Trying to get a JFet oscillator to work.? I'm using Ashar Farhans schematic for his spectrum analyzer.? (100 mhz 2nd Oscillator)
So far I've built two boards and burned out Q1 in each board even though I've checked all the traces/components with ohm meter/cap checker.? Any one had experience with this design?
I used the MPF 102 j fets, but have since sent for j310's (which is what Ashar originally used.
https://drive.google.com/file/d/0BzRNYeu10K6DdE1CSDVfZUdTdzg/view?resourcekey=0-m4_gR-o3xRUQfTF4m44fag

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On your first tries, did you get semiconductors from Tier 1 vendors, or off of eBay? ? On any case, did you test them first?

I haven't built that particular oscillator but I have designed MPF102 and J310 oscillators operating from 100 MHz to above 500 MHz.? (Mine didn't use quartz crystals, mostly they used LC, microstrip, or ceramic resonators.)? A quick glance at the schematic doesn't reveal anything that would lead me to believe that it won't oscillate, nor do I see anything that might burn out the FET.? The source bias resistor is 470 ohms, which is right in the normal range, and 12 volts is fine for drain voltage.? My best guess is that the devices are something other than MPF102s.? Other possibilities would be that the supply voltage is considerably higher than 12 volts, or the source bias resistor is something much lower than 470 ohms, but I assume you have verified those.? Bad parts.

It doesn't say to remove the 1/4-20 bolt for ALL of the inductors, but you should.? The bolt is only a form to wind the inductors on.

Hello Andrew / KM4TRT,
I built the SPECAN approx. 2 years ago after almost 40 years of absence from ham radio.
I also had problems with the 100MHz oscillator: I also used a MPF102 and I could tune it to 100MHz with a wire instead of the crystal. After I put the crystal in, it was not more stable than with the wire!
Due to that I decided for a different design, depicted in the Analog Devices AN-419 "A Discrete, Low Phase Noise, 125 MHz Crystal Oscillator". I only built the oscillator with the MPSH10 replaced by a BFW92 as far as I remember. Instead of the MC10ELT21 as an amplofier I used (1) an MMIC together with the ADE-1 mixer, (2) fed with a 6dB attenuator an AD831 mixer instead of the ADE-1.

Generally I have not been able, to get the SPECAN free of spurs, so that I finally abandonded the project and moved to the TinySA. The TinySA design was started by Erik Kaeshook also in this HBTE group. I built several of those following the evolution of the project.

--
Joerg, DB2OO

I would be very careful to properly identify the pin out of Q1/Q2 and also verify by testing that it is an N channel JFET. Mistakes in pinout could have 10 V forward biasing the gate to drain or gate to source, with no current limiting (since DC wise, the drain is connected to the supply voltage and the gate is directly grounded); instant smoke. These are old part numbers, Onsemi says the j310 is obsolete, don't know about MPF102. Ebay suppliers often substitute and remark other devices to supply demand for these obsolete parts, perhaps without knowing that their supplier has done that. You could get a bipolar transistor or a MOSFET. Even if its a JFET, it could be a low frequency switching device that isn't RF capable.

In reviewing the circuit design, it is my opinion that neither Q1 or Q2 are at risk of being burnt out if the circuit is properly assembled. It also won't be damaged if the xtal is bad and there is no oscillation or if there is strong oscillation at any frequency. This circuit will probably operate with either 3rd or 5th overtone 100 MHz xtals, the schematic gives no indication which to use. If you have doubts about your 100 MHz xtal, you could substitute a fundamental xtal in the 20 MHz range, which would operate on its 5th overtone or one around 33.3MHz, which would operate on its 3rd overtone. While shorting the xtal, as another commenter tried, probably wouldn't hurt anything, it would be a bit better to substitute a 0.01 uF capacitor to avoid upsetting the bias currents in the two transistor circuits, while still passing the RF signal. L10 and the 22 pF variable, plus stray circuit capacitance, forms a parallel resonant tank circuit that strongly favors the desired oscillation frequency and strongly rejects the fundamental and 5th overtone (in the case of a 3rd overtone xtal) or the fundamental, 3rd and 7th overtone in the case of a 5th overtone xtal. The xtal operates in series mode, which is low impedance at the desired frequency.

Hi Dave, I found that the MPF102's pin out didnt test properly, but the J301's did.? So replaced with J301.? Now I get 2.3-2.4 volts on the source with 12+V in on drain. with no overheating.? However, it will not oscillate.? I've tried changing caps and stretching coils, but no luck.? I'm thinking that the double sided board is changing the capacitance of the circuit.? Going to try a breadboard or a circle cut board.
Andy

I am hoping that you meant J310 instead of J301 for the transistor part number. And, to reconfirm you are getting 2.3 - 2.4 Volts on each of the two sources? What are you using to verify whether the oscillator is working? What is your source for the 100 MHz xtal? Have you tried substituting a 0.01 uF cap for the xtal? It probably won't operate on frequency and changing the 22 pF cap and stretching/compressing the coils will probably let you set it about on frequency.?

All your answers seem OK and I am out of ideas and suggestions. All I can say is good luck and let us know when you get it working, maybe we will all learn something.

Hi Dave,
I rebuilt the 100 Mhz oscillator on a breadboard and got exactly the same results-? 11-12 Volts going into the J310s and 2.35 on Q1 and 2.7 on Q2. .? I replaced the 22 pF with a variable all to no avail.
Also tried that on the original board.? Nothing..? I'm beginning to think that the J310's are low, but dont have a curve tracer, so I'm going to build a simple one stage oscillator to see if they work.
Andy KM4TRT

Hi All,?
After doing some more research, It appears that the crystal I'm using is basically good only for the fundamental.? The article says that you may not get the crystal to work at all and if it does it may be unstable. Does anyone know where to get crystals of a 5th overtone nature?? I found one company - Vectronics-? but don't know a source for them.
Andy KM4TRT

Is there some reason why you have to build it completely according to the diagram?? Why not use a 100 MHz TCXO such as:





Or one of the SiTime programmable oscillators available from both Mouser and Digikey?

Or one of the many Si5351 programmable modules available on ebay?? Or the Si5351 programmable module available from QRP-Labs, also available with .5 ppm TCXO:



Or a standard 2 dollar clock oscillator, if you don't need extreme stability:



https://www.mouser.com/ProductDetail/Microchip-Technology/DSC1121CM1-100.0000T?qs=1mbolxNpo8cmMM1u4KiJ4A%3D%3D

There are a million other solutions available, don't get stuck on a particular one.? Throw the box away and think outside the box!

Thanks Jim, for your insight and references, and you are right, but other people have made this fairly simple oscillator and made it work.? It's supposed to oscillate with only a cap and a resistor in place of the crystal and I can't get it to do that.
So obviously I'm missing something very basic.? Ashar made his out of the junk box.? Maybe just a cold solder joint some where.? ?Still reading.
Andy KM4TRT

On Fri, Jun 24, 2022 at 02:28 PM, KM4TRT wrote:

other people have made this fairly simple oscillator

That's not a fairly simple oscillator.? This is a fairly simple oscillator:

Source:?

Well, I'll say this for you: you've got a heck of a lot more patience than I have!

I'm still reading also, and confused as to which is the real 100 MHz oscillator.? There are two versions of the same topology, one in the large schematic (and in the Google drive clearer image), and another in the article (scroll way down) section 7 description of the 100 MHz oscillator:

10 pf or .01 source follower output loading may or may not matter.? But the reactance difference loads the Q2 source and sure affects the mixer drive level.

But it seems to me that more importantly the two coils should not couple so that the crystal path determines the feedback, not an accidental transformer.? If you enlarge the photo of his construction (the overall photo with shields removed) in the upper left hand section you can see the layout of his 100 MHz oscillator and crystal, with the two coils close but perpendicular forming a T geometry.?

I speculate that minimizing and phasing the coupling of L10 and L11 could be make or break.

Wally