Please help me design a narrowband UHF VCO (595 MHz) - Page 1
Forum section: electronics / rf-microwave

author: Yansi
author_rank: Super Contributor
timestamp: May 12, 2019, 05:15:26 pm
content: Hi!I haven't poked any RF for quite some time, now trying to make myself a bit of enjoyment with some devilish RF homebrewing.I am currently trying to build something (more on that later) out of the junk already accumulated. It is some sort of downconverter, that requires a 595 MHz LO.Now I am standing in front of the task of making the VCO. Unfortunately, I could not find any ready-made module capable of this frequency, so why not making a custom one and learn something? Over the time, I have built numerous VCOs for various frequencies from HF up to SHF, however these were just almost random groups of slapped-together components, that was somehow made to work. I could sure do this even this time, however I'd rather at least for once try doing it the "right way" or sort of.The requirements for the VCO are these:Frequency of interest: 594.55 MHz, so only narrow tuning range requiredSupply voltage 5VTransistor: NPN 2SC5773 (have loads of these),datasheete herevaricap: HVU202 (have loads of these),datasheet hereOutput level: 0dBm into 50ohm load(It will be later tuned by LMX2315 PLL)What oscillator topology will be best suited for achieving the above stated goal?Unfortunately, I lack even the basic understanding here, so I can only guess at best. This was my long time favorite VHF-UHF oscillator topology:Usually taking the signal from the emitter node (which, as I look at it now, is not probably the best to do?)Supposed I can stick using this topology for 595 MHz, the big questions then are:How to size the L and C for optimum performance? (of course, I know how the resonant frequency gets calculated, but it is all about theratio of C versus L and the ratio of the capacitances the emitter node connects to). Also, the varactor thing complicates this thing a bit.How to guesstimate collector bias current for required power output?I quite can't tell where to begin with this. (Other than making wild guesstimates and then wildly tweaking until it begins working)What I can only tell, is the following:The HVU202 capacitance diode will give a range from about 20 to 5 pF of range from 0 to 5 V tuning voltage. So we can state that the steady state tuning voltage will be ideally at half the range, about 2.5V, so we can expect something like 12pF from the varactor, according to its datasheet.Thank you for any hints how to proceed.Y.

author: bob91343
author_rank: Super Contributor
timestamp: May 12, 2019, 07:37:08 pm
content: That circuit is a Colpitts oscillator. It should have a bypass capacitor added across the power source.The square root of L/C represents the circuit impedance. The losses (usually mostly in the L) determine the Q of the resonant circuit, which directly influences the phase noise of the oscillator. Phase noise can be thought of as the unsteadiness of the oscillation frequency.The frequency stability can be a problem. If your result has to be close to the intended frequency you may not be able to work with such a simple scheme. A better way might be to obtain a crystal whose frequency is an integer fraction of the desired frequency, and multiply. Else your may find the frequency unmanageable.Taking the output from the emitter is probably the best place.There are ICs available for not much money that can generate stable signals. Do some Googling.

author: bd139
author_rank: Super Contributor
timestamp: May 12, 2019, 07:44:04 pm
content: I would see if you can build a varactor tuned crystal oscillator at a much lower frequency and multiply it up to 595 MHz. Much easier!

author: Yansi
author_rank: Super Contributor
timestamp: May 12, 2019, 08:43:03 pm
content: Lol... in no way that would be easier! Just way too many stages to care of, tune, tweak, and maybe after half year of development, it starts to work.From your conservative suggestion, guessing you are oldtimer, bd139! Nothing against that, all seasoned radio-amateurs I happen to know would do it this way.What indeed would be simple, is to slap there something like ADF4360-7 and voila, job done! But that would not be fun, would it?Well bob91343, I may not have written in bold, butit will be later tuned by LMX2315 PLL, so... I am not bothered about frequency stability, as decent TCXO will be available for reference. So of course, it will be reference locked by a PLL, but first, I have to make the VCO, so I have something to throw the PLL at.On the other hand, phase noise tends to be an issue. The less the better of course. Fortunately, my spectrum analyzer is an old junkie, that can't measure phase noise, has RBW of 1kHz minimum and probably has its own phase noise worse than my worst of VCOs.What I find an interesting practical piece of information, may be this article:https://www.qsl.net/va3iul/High_Frequency_VCO_Design_and_Schematics/High_Frequency_VCO_design_and_schematics.pdfI seem to like the parallel tuned colpitts VCO in the common collector configuration - as observed by me, this is the quite often used topology in the of-the-shelf canned VCOs.I have pretended to calculate a little bit, and came up with this modification of the circuit. It probably ain't pretty, ain't correct nor it will likely work as wished, but I have tried. I will also need to make a slight modification of the biasing, to allow for 5V supply voltage.I expect to prototype this later this week, will be busy doing other boring stuff couple of days now.

author: Yansi
author_rank: Super Contributor
timestamp: May 12, 2019, 09:02:41 pm
content: So here is the bastardized original VCO, scaled down to about 600MHz.

author: bd139
author_rank: Super Contributor
timestamp: May 12, 2019, 09:04:01 pm
content: Not that much of an old timerFor practical consideration I’d probably build a multiplier because it’s harder to build a discrete oscillator with a narrow tuning range that is stable at high frequencies. At lower frequencies it’s a piece of piss. I tried for the SA I have been putting together on and off for the last few years. I can throw a multiplier stage together in about five minutes under 200Mhz. Less fun above that (couple of hours). For me, easier. Yes it’s an old way of doing it but I’d rather be slapped 8 times than punched in the face once.Incidentally my Marconi TF2015 uses a colpitts up at 520Mhz. It’s drifts all over the place. Which is the problem.Honestly I would find a clock generator IC of some sort (Si labs?) and low pass it but someone said discrete.

author: Yansi
author_rank: Super Contributor
timestamp: May 12, 2019, 09:23:04 pm
content: The oscillator will not be free running after all. Will be phase locked to the only frequency of interest, and that is 594.550 MHz. I do not want it for anything else to do. Just to feed a LO port of a mixer, in another piece of junk I am trying to tackle and tame.Yes, I've said discrete. 3, 2, 1.. Fight!... to learn something. (I seem to be an unstoppable idiot, who always thinks it can win a fight with discrete RF)I could more than likely slap there ADF4360-7 or ADF4350 or whatever I reach first and be done first try, but that would not be fun, would it? Quite boring use for such nice and very expensive ICs.

author: bd139
author_rank: Super Contributor
timestamp: May 12, 2019, 09:24:12 pm
content: In that case, problem solvedI like to fight discrete RF as well but screw doing it at ~600Mhz

author: Yansi
author_rank: Super Contributor
timestamp: May 12, 2019, 09:32:57 pm
content: What would you fight with then? HF? or SHF? HF is DC... no fun there.

author: Yansi
author_rank: Super Contributor
timestamp: May 12, 2019, 09:52:45 pm
content: Prototype PCB ready for fight.

author: OwO
author_rank: Super Contributor
timestamp: May 13, 2019, 03:38:10 am
content: I've built a 900MHz VCO before, tuning range ~20MHz. It was a common collector colpitts oscillator using a 2SC3357. The circuit is similar to this:

author: bd139
author_rank: Super Contributor
timestamp: May 13, 2019, 06:44:29 am
content: Quote from: Yansi on May 12, 2019, 09:32:57 pmUp to VHF. HF is cheap!

author: OwO
author_rank: Super Contributor
timestamp: May 13, 2019, 08:11:31 am
content: If phase noise is important then automatic level control is probably required, but this adds a lot of circuitry and complexity. Better to integrate that onto a chip.

author: bob91343
author_rank: Super Contributor
timestamp: May 13, 2019, 04:51:27 pm
content: The Clapp oscillator is a modification of the Colpitts and potentially can improve frequency stability and give better control over tuning range. However, it still requires careful constuction. Low phase noise goes hand in hand with frequency stability.In a closed loop, phase noise is also reduced but only by the open loop gain. Ultimately it depends on the reference characteristics and the bandwidth of the loop.This whole topic can be profound, depending on your focus. There are no simple answers and each application is unique.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2019, 11:48:52 am
content: I quite liked the behavior of this,https://www.seventransistorlabs.com/Images/100MHzOsc.png(ignore the bottom filter/buffer section), which operates at 100-130MHz. Presumably it could be scaled up, using slightly smaller (and faster) components. The best behavior is variable amplitude (by bias voltage) with no squegging.It is again a Colpitts, B-E feedback, as seems best suited to BJTs.Tim

author: Yansi
author_rank: Super Contributor
timestamp: May 20, 2019, 09:21:36 am
content: So a little update: PCB was prototyped, and works. The frequency range I can obtain from it was like 540 to 580MHz, ie the center frequency of 560MHz, instead of 595MHz. That is just 6% error, which as I can tell is quite good, considering how much I have neglected designing the circuit.Most likely the error comes from the parasitic inductances, I have neglected completely. I have calculated with 15nH inductor, but should have subtracted the parasitics from it.So okay, we have a working VCO that could be likely tweaked where I want it (595 MHz).Now the output signal properties! Using the schematic I have posted above, the output signal strength was like -6dBm. I then checked the bias (which was way lower than I thought it would be), so I hacked in 6k8 R1 resistor instead of the 10k R1 and 100ohm as R2. That boosted the collector bias to I think about 17mA. That boosted the output to about -3dBm.The output harmonic content didn't look that great, second harmonic way down (>-30dBc I think), third harmonic was maybe 15dBc below.What I need is however about 0dBm output. Why?I need to feed both a PLL chip and a level 7 mixer. So my thinking was:If I split resistively a 0dBm VCO into a PLL (-6dBm), I can amplify the other leg up to the required +7dBm for the mixer (requiring just a single about 15dBm MMIC, like MGA82563 for the job).But currently, the VCO provides about 6 to 3dB less than what I need. What can be done about it? What I can think of are these options. Which one of them will likely be the best?1) Boost the VCO bias even more to say 30mA. (I am not sure if that will not degrade the performance somehow?)2) The output capacitor is just 4.7pF, which if I calculate right represent an impedance of about 56 johms in series with the output. Wouldn't increasing the coupling capacitance to say 10-15pF increase the output level by quite a lot?3) Or shall I for some reason keep the VCO bias low (less distortion possibly?) and then add another amplifier stage, with smaller gain to also isolate the VCO from the load?

author: CJay
author_rank: Super Contributor
timestamp: May 20, 2019, 11:42:09 am
content: Gain is cheap and easy, MMIC.

author: Yansi
author_rank: Super Contributor
timestamp: May 20, 2019, 12:05:56 pm
content: SOT23 NPN is certainly cheaper, even when considering the RLC mess around. ... but, that was not the question asked.I hate to slap MMIC everywhere like some arduinist. I'd rather for once learn something. (Although, rarely do I).

author: EVS
author_rank: Contributor
timestamp: May 20, 2019, 01:02:13 pm
content: Capacitors C3 and C4 are too large. Try reducing both to about 2pF.

author: Yansi
author_rank: Super Contributor
timestamp: May 20, 2019, 01:09:36 pm
content: Quote from: EVS on May 20, 2019, 01:02:13 pmCould you please kindly add explanation? I do not agree on that. The original circuit I based my values on operated at 3x the frequency, yet used only slightly lower capacitance values. Why should I use half the original capacitance, when I need to operate at a almost a third of the original frequency? Does not make much sense.Original schematic is here, current one attached

author: CJay
author_rank: Super Contributor
timestamp: May 20, 2019, 02:47:04 pm
content: I'd assumed as you were deriding the age old tried and tested methods for creating the VCO you'd also be wanting to use something more modern for the buffer amp but yes, you can learn a lot from your own design and experiments..As for the Arduinist comment, sometimes the best thing you can learn is that just sometimes, the pre-packaged solution is by far the best (Arduino is *never* the best).

author: Yansi
author_rank: Super Contributor
timestamp: May 30, 2019, 12:00:05 pm
content: How to analyze output impedance of the VCO? I have naively copied the schematic into RFsim99, but getting strange readings of a reflection coefficient - S11 even becomes positive.What is the proper way to analyze output impedance of the VCO (so one could match it to a succeeding stage)?Here's the model schematic:and here's the sim result, getting this 3dB S11 hump on a S11 plot (presumably, at the resonance point where it would oscillate)and here's the smith chart... clearly, something ain't right there, I'm getting some negative resistance there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 30, 2019, 07:17:49 pm
content: Well, yeah. You're tapped right on the resonator, it's going to see what the resonator sees.You need a cycle-averaged nonlinear analysis to show what you're after -- not small signal AC / steady state. The key is, the oscillator's amplitude saturates due to a nonlinear process, and it is the excess gain "burned off" against this nonlinearity that provides the output amplitude stability, and available power before quenching.But still for small signal purposes (which includes startup conditions), the relevant fact would be: what impedance can be connected in parallel, which retains a negative impedance and therefore is able to start up?I guess it's around 25 ohms.I don't think you want to put an oscillator in a position where it needs to handle reflected power. That's how you make a pulled or injection locked oscillator! If you need isolation, buffer it. Also lets you use less power in the oscillator itself, perhaps giving better thermal stability.Tim

author: Yansi
author_rank: Super Contributor
timestamp: May 30, 2019, 08:07:14 pm
content: Well, I kinda expected that it won't work this way. Was just experiment to see what it'll get.I could probably pull a non-linear sim of the circuit in QUCS Studio, but this opensource thing always returned garbage data when I have attempted a non-linear (harmonic balanced) simulation.Buffering the oscillator! Of course! That is what I am trying here to achieve with one more discrete amplifier stage.However I quite can't tell how to do it properly. What I think I should do is to lower the bias of the VCO to a minimum, then rather loosely couple to the output and use a buffer/amplifier stage.However, to design an UHF band amplifier, I quite can't get it right unless knowing how to match the input of the succeeding amplifier stage to the oscillators output? (the goal still sits, I need at least 0dBm out to drive other stuff with it).

author: Yansi
author_rank: Super Contributor
timestamp: June 15, 2019, 11:32:40 am
content: After some more f*ing around and trying to think, this is what I came up with.I have studied an off-the shelf UHF VCO (designed for 800-1000MHz I think), and came to these results:The profi-made VCO uses a parallel tuned colpitts. However, they do couple the output from the collector (not any clue as to why, as emitter port output provides less load to the resonant tank - or at least that is what I think)Then the coupled output goes through an attenuator, then to a standard common emitter amplifier (with a bridge bias, decoupled resistor in emitter). You can see here, this is my photo of the actual VCO:After thinking a bit about the attenuator - it can clearly serve a purpose to make the amplifier more stable and less disturbed by the output impedance of the oscillator. So I thought well, I could use that too! After all, I can adjust the output power with it.The output is matched pretty nicely to 50ohms at about those 600MHz. I have tried to include all parasitics in the circuit, that I thought I new their values reasonably precisely (ie. the emitter circuit parasitic inductances) and the capacitance of the transistor pads (nodes) to ground.The input seems to give also quite reasonable match (-15dB S11) at the frequency of interest. Also, adding the input pad made the amplifier calm and unconditionally stable.Overall gain is about 9dB, which should help me a lot. The gain is quite poor, but I can't make it better, at least not with my knowledge and the garbage in the emitter circuit. I will now try prototyping it. Quite interested in the result.EDIT: Jeesus crist, why does the forum fuck up now even IMGUR links?

author: bd139
author_rank: Super Contributor
timestamp: June 15, 2019, 12:24:11 pm
content: Imgur changes something. It’s giving me a ball ache too