Microwave frequency VCO. HELP - Page 1
Forum section: electronics / rf-microwave

author: Cacao
author_rank: Contributor
timestamp: June 25, 2017, 06:00:08 am
content: Im an Electronics engineering student. Can someone help me check whether there is a mistake on this oscillator that supposed to have 900MHz - 1.2GHz output. The simulation is working pretty fine but the prototype doesn't.Simulation:https://pictub.club/image/7QdvWkPCB design:https://ibb.co/e6cMa5

author: kridri
author_rank: Contributor
timestamp: June 25, 2017, 07:44:13 am
content: Maybe you can add also a picture of you're prototype?That can be handy!

author: w2aew
author_rank: Super Contributor
timestamp: June 25, 2017, 01:28:32 pm
content: Quote from: Cacao on June 25, 2017, 06:00:08 amA picture of your prototype would help a lot. Analog circuits operating around 1GHz require *VERY* careful physical design & layout to work properly. This will NOT work on plug-in breadboard.

author: Cacao
author_rank: Contributor
timestamp: June 25, 2017, 04:31:21 pm
content: I don't have the prototype right now so i just added the pcb design. Hope you can help me. Thanks in advance

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 25, 2017, 09:26:42 pm
content: I'm not really into pcb design, especially not in the microwave range, but it seems like there's an awful lot of space between everything, with awfully long traces to follow.

author: Cacao
author_rank: Contributor
timestamp: June 26, 2017, 04:02:55 am
content: The size of the board is only 2x2 inch. It just looks like big thus making the traces look long

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 26, 2017, 06:26:16 pm
content: Nope, I stand by my words. considering lambda/2 at 1200 MHz is roughly 5 inches, I'd still say they're fairly long from an RF point of view.I can't say if that's why it's not working. But it's definitely worth thinking about. There can be a lot of inductance in a snaking trace like that.

author: DaJMasta
author_rank: Super Contributor
timestamp: June 26, 2017, 06:37:04 pm
content: While I can't comment on the specific design and how it's effected, the usual advice for high frequency designs is to minimize trace length within an element as much as possible - people use SMD components because the lengths of the leads on through hole parts are enough extra length do destabilize designs (inductance and phase), and when through hole stuff is used, it's often clustered together in a ball to absolutely minimize length as much as possible.For example, at 1GHz, the wavelength is about 30 cm. That means your N$3 trace, which is probably 6-8cm long, could easily be close to a quarter of the wavelength, so the same signal at the RF amp will be about 90 degrees out of phase at R1. Not critical at every component in the design, but if there was any chance of the stability of it being phase related, then 90 degrees is no small problem.

author: Twoflower
author_rank: Frequent Contributor
timestamp: June 26, 2017, 07:32:06 pm
content: Not sure if any of the following points will fix the problem but in the world of HF you might improve the performance:Far to long traces for the few parts. I think the placing in the three rows looks nice but it will not end up in the best routing results.Placing of the oscillator related parts as close together as possible to avoid parasitics.Try to avoid traces beneath coils (L2). GND might be acceptable as long as there is no oscillating current is flowing through the GND-trace (e.g. L1).Try to fill the open areas with ground.Place R1 closer to the base of the transistor to avoid picking up noise.Adding a 10uF and a 100nF cap over the supply. Not sure if the 2pF will be sufficient (which is not present in the layout). Eventually also a inductor in series?Avoid studs like you did on the N$3 trace. It's probably OK as it is relatively short related to the frequency. But it's easy to avoid here.Just noticed why is R5 connected to VCC in the PCB? So your PCB is not identical to the simulation. Can you check/provide your schematics from you created in the PCB Tool?

author: Cacao
author_rank: Contributor
timestamp: June 27, 2017, 12:22:54 pm
content: Thanks guys. I'll upload my new design as soon as I finish it

author: Cacao
author_rank: Contributor
timestamp: June 27, 2017, 12:32:53 pm
content: This is the new pcb design guys hope I followed all your advicehttps://ibb.co/hg8JOQNote: the pcb size is on inchIs the distance of each components enough or should I compress it more? and does the trace width of 16 mils enough on a low current ckt?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 27, 2017, 07:10:52 pm
content: Is there a specific reason you aren't flush filling the ground plane? Making every area not a trace be ground? also, what are you going to do with output connector? it looks like you're just going to solder some wire on there, that may not be super RF-friendly. I'd consider a pcb mounted SMA connector , either vertical or horizontal, I'm sure there are standard pcb pads for those. Either that, or a pad close to the edge, with some un-masked grounded copper infront so you could solder a coaxial pigtail directly on there.

author: Cacao
author_rank: Contributor
timestamp: June 28, 2017, 01:19:00 am
content: I just want to isolate the output trace so I did not filled that part with a ground plane. As for the output we were still on experimenting phase so the wire will suffice but I plan to change it later

author: DaJMasta
author_rank: Super Contributor
timestamp: June 28, 2017, 01:53:06 am
content: I wouldn't say the ground plane is required at this frequency range, with normal thickness FR4 the grounding plane of the whole board would be ballpark 2-4pF or so I think, which while it probably wouldn't significantly inhibit operation as it does with some very high frequency designs, it may not be ideal and it probably doesn't have a particular benefit except for shielding.Though I would also second the idea to use coax. Any wire you use should be considered an antenna, both for transmitting and receiving, and when your working with even a couple hundred MHz if you want a clean signal that isn't radiating noise, you want it in coax. a PCB mount SMA is the standard (and is extremely cheap), but a little ground pad next to a wire for the output would suffice if you were just going to solder coax directly to the board - minimizing the unshielded conductor and keeping the grounded shield as close to the output line should be prioritized.Not that it really counts, but when you're packing together parts like that, a lot of times it's easier just to put the component designator instead of the value (R1, R2, C4, L2, etc.). You'll probably be assembling it piece by piece with the schematic open, so it shouldn't be too hard to keep track of what value things are, and you're unlikely to need to know the value from just glancing at the board, so it cuts out some clutter on the silkscreen. Many manufactured board get rid of the silkscreen altogether, but I think that would actually hurt putting it together and experimenting with the design.

author: BigBoss
author_rank: Regular Contributor
timestamp: June 28, 2017, 10:34:09 pm
content: Oscillator design is a really serious task."Any kind of simulator" cannot be used as desired, instead professional grade Harmonic Balance equipped simulator must be used.Especially 500MHz and beyond, spice like simulators, simple testbenches are useless.Also, your VCO configuration is wrong.A typical configuration for a 1GHz oscillator should be Collpits ,Clapp or Clapp Gouriet.If you want really to learn something, I recommend you to read that book carefully and find a professional grade simulator such as AWR.AWR can be free for students for a limited period and I highly recommend it."The design of modern microwave oscillators for wireless applications theory and optimization / by Ulrich L. Rohde, Ajay Kumar Poddar, Georg Bock"

author: KJDS
author_rank: Super Contributor
timestamp: July 01, 2017, 04:08:21 pm
content: Almost every professional oscillator requirement jsut buys something suitablehttp://www.qorvo.com/products/frequency-converters/oscillators

author: tec5c
author_rank: Frequent Contributor
timestamp: July 18, 2017, 12:27:16 pm
content: Quote from: BigBoss on June 28, 2017, 10:34:09 pmIMO, this was the most useful post for the OP.AWR or ADS would be your best bet for getting accurate simulation results. Not only do you have HB simulators at your disposal, once you have done your PCB layout you can run an EM simulation to see if your layout is going to work as intended and find any traps that are hidden in your PCB design.I understand that while all of this is useful information, it may not be helpful to you until you can get a copy of AWR. Not much I can do about that I'm sorry, however, the book that BigBoss recommended by U.L. Rohde is a great book and would definitely be of assistance to you.