Colpitts selective xtal oscillator - Page 1
Forum section: electronics / rf-microwave

author: Teknow
author_rank: Regular Contributor
timestamp: April 30, 2017, 09:35:19 pm
content: Considering the following circuitI am confused and can't understand how the tank circuit, which is open at resonance, and the transistor pass the 2nd or any other harmonic of a crystal of a fundamental frequency of, say, 48 MHz.What frequency is at the output if the tank is open at resonance 48MHz for example?1. How does the tank circuit contribute to or govern the choice of the desired Harmonic at the output if a parallel LC is "open circuit" at resonance?2. How can I calculate L1 and C4 to have the desired frequency at the output?Do I have to include the transistors' output capacitance in calculating the tank circuit? What is formula for that?Many thanks in advance!

author: yada
author_rank: Frequent Contributor
timestamp: May 01, 2017, 07:51:47 am
content: That looks just like a circuit I was tying to build but with crystals. Mine sucked because it didn't have crystals. What site is that from?Why does the writing look likes its floating?

author: Teknow
author_rank: Regular Contributor
timestamp: May 01, 2017, 10:50:27 am
content: @MagicSmoker,Thanks for the reply. I understand that all sorts of odd and even harmonics are on the loose. From this circuit and a 198023-4 transistor, over 500 MHz harmonic peaks were present in the spektrum! But what to do to boost the one desired harmonic while attenuating the others, or at least only boosting the frequency of concern depending only on the tank circuit or this transistor circuit only without extending it?Can I simply calculate only the tank resonance?Or what?

author: voltz
author_rank: Frequent Contributor
timestamp: May 01, 2017, 12:25:10 pm
content: The collector tuned circuit is a 'rejector'. So the current is low - voltage across it is high, at resonance. That is correct, so your output voltage is maximum when tuned to the frequency (or harmonic) you want.Standard formula for tuned circuits is f=1/ 2pi root (LC)

author: yada
author_rank: Frequent Contributor
timestamp: May 01, 2017, 12:59:03 pm
content: Quote from: Teknow on April 30, 2017, 09:35:19 pmCan you list the values of the circuits parts? I want to build that.

author: Teknow
author_rank: Regular Contributor
timestamp: May 01, 2017, 07:09:58 pm
content: Thank you very much that summarizes it!As for yada I tried the ciruit with 2N3904 as the 198023-4 are a rarity and obselete.

author: Teknow
author_rank: Regular Contributor
timestamp: May 01, 2017, 07:13:15 pm
content: And here's another one with mor amplitude on nth harmonics. But with 12 volts VCC!

author: yada
author_rank: Frequent Contributor
timestamp: May 02, 2017, 04:58:51 am
content: Quote from: Teknow on May 01, 2017, 07:09:58 pmCould I use one of my 37 or 13.56MHz crystals or does that change all the parts. Do you know the NH of the inductors or just turns? If so what is the dia and wire size? MY last transmit, sucked so I need something with a crystal I think. Going to use my ghetto coffee can ground too.

author: Teknow
author_rank: Regular Contributor
timestamp: May 02, 2017, 07:22:23 am
content: Please have look herehttps://sites.google.com/site/linuxdigitallab/system/app/pages/subPages?path=/low-noise-crystal-experimentThere lots of interesting and tried VCOs etc. with 9018.The coils in my circuits are experimental.You can use the crystals you have without changes! But you only get their harmonics or multiplications by 1,2,3,4,5...etc and nothing in between.Coils are 'rule of hobby trial and error'Wire: copper or silver coated 0.8 mm air core 5mm. That's why I have been asking my questions.I'll be back later with more suggestions!

author: Damianos
author_rank: Frequent Contributor
timestamp: May 04, 2017, 09:41:09 am
content: Quote from: Teknow on April 30, 2017, 09:35:19 pmThe oscillator, in the presented circuit, is built in a common collector configuration (the input is the base and the output the emitter). Due to the positive feedback, the circuit tends to produce a non sinusoidal signal. Using the collector current we produce a signal voltage and with the tuned circuit we favor whatever harmonic frequency we want.Some more notes:- the 48MHz is also a harmonic of the crystal and not the fundamental frequency.- an ideal tuned circuit does not exist. All have a "quality factor". There is not open or short but higher and lower impedance, phase shift and losses.- if we had to produce a clean sinusoidal signal, on the fundamental frequency of the crystal, then we had to adjust the capacitors of the divider between base and emitter to match the respective impedances. In that case a mechanism to control the amplitude would be a necessity, to avoid clipping.That is the way that I understand how this circuit operates.

author: Teknow
author_rank: Regular Contributor
timestamp: May 07, 2017, 09:40:32 am
content: Thanks Damianos for more interesting information. I'll experiment on extracting a better sinusoidal wave tweaking these capacitors.