going beyond dozens of MHz - Page 1
Forum section: electronics / rf-microwave

author: cmrincon
author_rank: Newbie
timestamp: February 15, 2020, 08:18:27 pm
content: Hi!Im trying to design a very basic colpitts, ~40 Mhz oscillator ( or above), the problem is that i can't make a realizable air core inductor (i can't make a inductor with less than 1 loop) or maybe i choose the wrong architecture.Do you have any suggestion for me?

author: Yansi
author_rank: Super Contributor
timestamp: February 15, 2020, 08:26:17 pm
content: Different topology will be better.Good up to hundreds MHz, easily.

author: coppercone2
author_rank: Super Contributor
timestamp: February 16, 2020, 04:47:52 pm
content: if you need to make small coils, you need very thin wire. What I like to do is wind it around bobbins (like you get yourself a miniature machinists vise and put a drill bit in it and wind it around the drill bit). Stripping tiny wire precisely can be difficult because its so fragile, some people will actually dip it into carefully molten sodium hydroxide and then clean it in water/alcohol. you need good tweezers, good fixtures (weigh it down) and careful hand eye coordinationwith some insulations you can also dip it carefully into more hot then usual solder to melt it away sometimesyou can also get glass or ceramic rods that you can snap after prescoring after winding if you want a more real/tough part.nylon tweezers (the ones that come in first aid kits) might be helpful if you are worried about abrading the very thin wire

author: JDubU
author_rank: Frequent Contributor
timestamp: February 16, 2020, 05:05:48 pm
content: What about just buying an SMD inductor?https://www.digikey.com/products/en/inductors-coils-chokes/fixed-inductors/71?k=&pkeyword=&sv=0&pv2087=u250nH&sf=0&FV=-8%7C71&quantity=&ColumnSort=0&page=1&pageSize=25

author: Yansi
author_rank: Super Contributor
timestamp: February 16, 2020, 06:25:44 pm
content: And what about stopping nonsense? Even VHF oscillator can have a small air coil with like 6 or more turns.40MHz is a piece of cake. I have even posted 50MHz Colpitts for reference. Easy. Just need the right scale for the caps.

author: szoftveres
author_rank: Regular Contributor
timestamp: February 16, 2020, 06:41:50 pm
content: There circuit and the topology is just fine up till several hundreds of MHz, however there are serious problems with your component values.C1 and C2 (200p and 400p respectively), are HUGE, the resulting series capacitance (~160p) is in parallel with your LC tank and C4. I would reduce them to at least an order of magnitude lower (20p and 40p).Rule of thumb: The resulting series capacitance of C1 and C2 should be in the range of 1/3 of the tuning capacitor (C3 in your schematic).Here's a schematic of a local oscillator from an FM radio (~100MHz range), see the capacitances.

author: cmrincon
author_rank: Newbie
timestamp: February 17, 2020, 02:47:30 pm
content: For the coils i use 0.5 mm wire and drill bits of a few mm (3 - 5 mm), which, let me make coils of ~100 nF. Probably i will need to buy thinner wire if i want to go to the vhf upper limit.Yansi, i make your design but i made it in my breadboard and it was a mistake, parasitics of the breadboard reduced the oscillation frequency to ~14MHz. PRobably i will try to make a board with this oscillator and an class A amp, Probably it will be useful for another proyect i want to do (gilbert cell).Thank you allCesar.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 17, 2020, 03:30:31 pm
content: Quote from: Yansi on February 16, 2020, 06:25:44 pmYou took the words right out of my mouth!This seems to be a continuing problem with not only Noobs, but sadly, even among those who should know better.Because tne resonance formula works with many different values of L & C, they assume that circuits with huge capacitors & miniscule inductors (& vice versa) are viable ways of building resonant circuits.

author: Yansi
author_rank: Super Contributor
timestamp: February 17, 2020, 05:32:10 pm
content: I also suggest this video, as the basics for oscillator design.

author: Wolfgang
author_rank: Super Contributor
timestamp: February 17, 2020, 08:15:32 pm
content: Quote from: cmrincon on February 15, 2020, 08:18:27 pmSome hints:- your capacitive divider is wrong. The upper cap must be larger (3-5 times) than the lower.- your LC is wrong, too. L should be a lot larger, and C accordingly smaller. Then making a reasonable coil is a cakewalk.- Try a Clapp (series resonator) instead of a Colpitts (parallel).- Buy a good book about oscillator design theory. IMHO, a Colpitts or Clapp at 40MHz is absolutely no rocket science.ARRL has some info, or Rohde, Oscillator Design, ...- If you want low output noise, decouple thru the resonator. You will need an isolation stage anyway.- The transistor fT should at least be 10x the oscillation frequency. Not true for your 2N2222A.Have fun experimentingWolfgang DL1DWG

author: Yansi
author_rank: Super Contributor
timestamp: February 17, 2020, 08:27:17 pm
content: Quote from: Wolfgang on February 17, 2020, 08:15:32 pmErrr... what?

author: Wolfgang
author_rank: Super Contributor
timestamp: February 17, 2020, 08:32:33 pm
content: The argument runs like this:- The base input represents a negative differential resistance defined by gm.- The base/emitter cap divider splits the AC voltage. The part along the BE junction should be small,and the part across the emitter resistor should be large. If you do it the other way, you will earn a strongly nonlinearbehaviour and large negative VBE swings, at a comparably low amplitude. Why do this ?- Rohde (N1UL) oscillator book has a derivation of this inside.regardsWolfgang

author: Yansi
author_rank: Super Contributor
timestamp: February 17, 2020, 08:37:33 pm
content: In most oscillators I have seen, the lower part of the divider is the part that is bigger and definitely not 5x. (maybe 1.5 to 2x)

author: Wolfgang
author_rank: Super Contributor
timestamp: February 17, 2020, 09:16:01 pm
content: I absolutely agree. Its still wrong, only most people did no calculations behind their designs.Suggestion: Try it out, and watch the signals.PS: I know a little about this because the PHD thesis I am working on right now is about USOs.

author: Yansi
author_rank: Super Contributor
timestamp: February 17, 2020, 10:41:26 pm
content: I'm not saying what's wrong and what's not. My oscillator-fu is still child level. Just noticing the obvious. However what you say makes sense. Sometimes I will sober up and read more about it. Designing meaningful oscillators and VCOs might be a handy skill to have.

author: cmrincon
author_rank: Newbie
timestamp: February 22, 2020, 08:48:10 pm
content: I built the common colector oscillator, it should be a 83 MHZ osc:I had some smt resistors and caps so i used them, now, i think that it was unnecessary and it made harder to test the circuit.here are the photos:There is a strange effect around 100 MHz i think that this is my oscillation. I cant clearly see it because my osciloscope bw is 100MHz. All voltages tested with my multimeter seems ok.Cesar.

author: cmrincon
author_rank: Newbie
timestamp: February 29, 2020, 01:56:25 pm
content: Today i have made the same circuit using leaded components, but i can't get an oscillation. You can view the output (transistor emitter) of the oscillator in the image belowI used 2n2222 because i don't think that the transistor is the problem becuse last week i replaced the mmtb2222 for the mmth10 and i get the same result. Also i used a shunt capacitor at transistor collector of 1n but nothing changes.The layout it's the same that the previus post.What can be wrong?

author: Wolfgang
author_rank: Super Contributor
timestamp: February 29, 2020, 02:08:17 pm
content: Quote from: cmrincon on February 29, 2020, 01:56:25 pmThe BE and E to GND caps are too small (C1,C2), IMHO. the negative resistance is gm*XC1*XC2 and this must exceed the circuit losses. gm is Vt/Ic, with Vt 25mV. Another thing is the L is a bit small. make it 1uH and C about 22pF.

author: iMo
author_rank: Super Contributor
timestamp: February 29, 2020, 02:36:32 pm
content: For 80MHz and up the oscillator shown by Yansi above is a better way to go. Also you would need a better transistor, imho.Btw what you want to achieve? What is your application?

author: chris_leyson
author_rank: Super Contributor
timestamp: February 29, 2020, 04:28:25 pm
content: There is a good discussion of the Colpits in "Crystal Oscillator Circuits" by Robert J. Matthays, page 31.https://bgaudioclub.org/uploads/docs/Crystal_Oscillator_Circuits_Krieger_Matthys.pdfPersonally the Colpits is not one of my favorite oscillators with regard to phase noise, much prefer Pierce.

author: Wolfgang
author_rank: Super Contributor
timestamp: March 01, 2020, 12:36:07 am
content: Quote from: chris_leyson on February 29, 2020, 04:28:25 pmI disagree. All world-class USOs are Colpitts/Clapp 5-10MHz SC cut crystal OCXOs., with PN down to -135dBc@1Hz.Try RAKON or Wenzel for product data.