PLL-based FM transmitter not locking - Page 1
Forum section: electronics / rf-microwave

author: Phaseseeker
author_rank: Regular Contributor
timestamp: February 03, 2021, 05:56:42 pm
content: I've built a FM trasmitter circuit using the schematic and the PCB layout I found on this website:https://pira.cz/entx4.htm(I know it's an old design, but it's one of the few I could find that used a PLL instead of the usual circuits that drift like crazy warming up); The problem I'm encountering is that no matter how i adjust the L1 inductor the TSA5512 does not lock. I modified the code to put the chip into "test mode", which exposes the output of the internal frequency divider, and the divided frequency is around 5.8 KHz instead of the 7.8125 KHz it should be when the circuit is in the locked state. I measured the voltage at the base of Q4, and it's around 0.4V; Since increasing the base voltage increases the oscillator frequency, it seems like for whatever reason that the VCO doesn't oscillate at the right frequency. My knowledge of RF is fairly limited, so I don't really know what could be causing the issues I'm having.The only modification I made to the circuit shown on the website is changing the reference crystal to a 4MHz one because I couldn't find a 3.2MHz one (this changes the step size, but I should still be able to have the loop lock), but I recalculated the divider values to match my crystal and, also using the test mode, I can see that the crystal oscillator is working properly and oscillating at the right frequency.Thanks in advance

author: CJay
author_rank: Super Contributor
timestamp: February 03, 2021, 07:21:39 pm
content: Why not take the PLL out of the loop and wire in a potentiometer in place of Q4?If you're able to measure the frequency of the oscillator itself at the base of Q2 then you should be able to work out the range of your VCO

author: Phaseseeker
author_rank: Regular Contributor
timestamp: February 03, 2021, 08:21:45 pm
content: I tried using a potentiometer, like you suggest (I tried with 10K pot), but it did almost nothing until the very end of the travel, and even then the frequency varied by around 100 Hz max (measured at the divider output, with the divider set to 1398).The thing that confuses me is that changing the divider seems to change the frequency of the oscillator even with the same voltage on the base of Q4. I suspect that this is related to the fact that even with Q4 disconnected, the charge pump of the chip still applies voltage on the two varicaps, varying their capacity. I say this because i found that, with Q4 connected, the frequency at the divider output remains somewhat constant even after changing the divider ratio by a lot, which seems to confirm this theory.Looks like I really need to measure the frequency directly at Q2 to be certain

author: CJay
author_rank: Super Contributor
timestamp: February 04, 2021, 10:56:13 am
content: If you don't remove the PLL then it will continue to try and control the VCO.If you remove it and then use the pot you should be able to work out the VCO range, that will tell you if it's capable of running at the desired/calculated frequency and divisor.The PLL will never lock if it can't.

author: Phaseseeker
author_rank: Regular Contributor
timestamp: February 07, 2021, 09:57:15 am
content: I repeated the potentiometer test removing C29 to disable the charge pump, and the results are pretty much the same: the VCO goes from about 60 MHz to about 65 MHz, so not only it doesn't have enough range, but it also doesn't reach nowhere near the right frequency; This is all pretty strange considering I used all the components specified in the schematic (it didn't specify the tolerance on the resistors, so I used standard carbon 5% ones, but I don't think it'd be enough to make the circuit not work). Adjusting L1 does have an effect but obviously it isn't enough. How can I change the VCO range?

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 07, 2021, 10:50:07 am
content: If your VCO frequency is too low then either your coil has too much inductance and/or the resonance capacitor values (C2,C3) are too high.That also explains why the range is too small, the lower the resonance frequency the more you have to change the capacitance for a certain change in output frequency. How did you measure the frequency, do you have a frequency counter or did you calculate it from the output of the divider? If you calculated it, it could be that the divider is not doing what you think it is doing.Check C2, C3 and C5 and see if there are no mistakes.Check L1 for the correct number of turns and the correct diameter. What kind of wire did you use?Can you post a clear picture of the oscillator part?

author: CJay
author_rank: Super Contributor
timestamp: February 07, 2021, 12:02:53 pm
content: Excellent, you've found the problem and learned from the experience, great fun.PA0PBZ has given pretty much the same advice I would have given and offered even more still so I've nothing to add except that I'd be interested in knowing how you get on with it.Please keep us updated**** Edit****The 5% resistors will be absolutely fine, they won't affect the frequency.It doesn't sound like you've got anything wrong as such but with RF the need for accuracy becomes more important as the frequencies rise.As wavelengths shorten dimensions become more critical and small errors make bigger changes to circuit operation.For a VHF circuit it could be as simple as you've left a component lead too long, you've miscounted the number of turns or not quite got the dimensions of the coil right.Plenty for you to explore and experiment with.

author: Phaseseeker
author_rank: Regular Contributor
timestamp: February 07, 2021, 02:24:12 pm
content: So, I checked the capacitor values, and they where right, but then I read the part about long component leads, so I went around and shortened all of them (and rewound the coil just in case)et voilà - now the frequency is right and the loop locks.Thank you both for your help!

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 07, 2021, 02:42:23 pm
content: Good news! Thanks for letting us know.

author: CJay
author_rank: Super Contributor
timestamp: February 07, 2021, 04:32:20 pm
content: Quote from: Phaseseeker on February 07, 2021, 02:24:12 pmVery welcome and I'm glad you got it workingPictures of the project would be nice if you can?

author: Phaseseeker
author_rank: Regular Contributor
timestamp: February 22, 2021, 04:25:19 pm
content: Sorry if I reopen this topic after two weeks (I wasn't sure if it was better to create a new one since the problem is still with the same transmitter), but now I'm having problems with the power amplifier section; I had disconnected the transformer when testing the oscillator circuit, because with no oscillation the primary coil of the transformer/R7 were getting hot. Some days ago I reconnected it, and now the 2SC1971 is pulling a lot of current (about 2A @ 12V) and warming up the heatsink without producing any output (the incandescent bulb used as a load doesn't glow). There aren't many components in that section of the circuit so I'm really not sure what could be causing this; I read online about fake 2SC1971 transistors, could this be the cause? I got mine from ebay from a local seller (not from China).

author: thinkfat
author_rank: Unknown
timestamp: February 22, 2021, 06:41:38 pm
content: Q3 is not biased in this circuit, so, there must be RF at the secondary side of the transformer. If the transistor gets hot but you have nothing at the output jack, the output filters must be incorrect or the frequency is wrong.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 22, 2021, 07:10:47 pm
content: If you short R9 is the 2SC1971 still pulling current? Then it has died. If not you probably have oscillation in the PA stage, can you post a picture of the assembled PCB?Maybe you still have components with too long legs acting as a capacitor and creating a feedback path -> oscillation.Welcome to the wonderful world of RF where amplifiers oscillate and oscillators don't

author: Phaseseeker
author_rank: Regular Contributor
timestamp: February 23, 2021, 10:00:20 am
content: Shorting R9 makes almost no difference (and pulling the transistor makes the current draw go back to normal), so it looks like my 2SC1971 is blown. I had tried testing it by checking the B-C and B-E junctions with the diode function, like one would do for a regular transistor, and it checked fine, but evidently it is not.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 23, 2021, 10:41:24 am
content: Quote from: Phaseseeker on February 23, 2021, 10:00:20 amEither that or - depending how you built the thing - it is still oscillating. Can you make a direct short between B and E on the transistor and measure again?Still requesting pictures...

author: Phaseseeker
author_rank: Regular Contributor
timestamp: February 23, 2021, 11:18:56 am
content: I tried shorting B-E at the terminals of the transistor, still no change.Also, sorry for the pictures, I'll attach them at the end of this post.I attached the lamp the way I did because I couldn't find the exact PCB mount BNC connector, I hope that's not what's causing my issues (again, I'm pretty much a newbie in RF circuits)

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 23, 2021, 11:53:39 am
content: Apart from the orientation of L5 I don't see any problems yet, and L5 does not cause the problem you face. If you remove Q2 does Q3 still pull a lot of current?

author: thinkfat
author_rank: Unknown
timestamp: February 23, 2021, 04:21:45 pm
content: What's the tab of Q3 connected to? C, or E? What's the heatsink connected to? Do you have an isolation pad between the heatsink and Q3? I'm just checking...

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 23, 2021, 04:40:57 pm
content: Fortunately the tab is connected to the emitter, it would be a bad designed RF device if the tab was on the collector, even with isolation between the tab and the heatsink.

author: CJay
author_rank: Super Contributor
timestamp: February 23, 2021, 05:17:08 pm
content: Unfortunately the 2SC1971 is really hard to get so there are a *lot* of fakes on the market.2SC1971 is rather easy to prove as fake because it's one of a small number of TO-220 parts which have the BEC pinout so running it past a transistor checker which can identify pinout is a rapid way to know if you've got a fake.

author: Phaseseeker
author_rank: Regular Contributor
timestamp: February 24, 2021, 08:55:02 pm
content: Quote from: CJay on February 23, 2021, 05:17:08 pmI tested it and sure enough, my transistor is a fake! It's a NPN transistor, but it has the much more common BCE pinout. It also seems to have "1818" as a datecode printed on the part, which again is impossible.

author: CJay
author_rank: Super Contributor
timestamp: February 25, 2021, 10:22:18 am
content: Well that sucks but at least you know it's probably that which caused your latest problem and not something you've made a mistake on.

author: Phaseseeker
author_rank: Regular Contributor
timestamp: March 05, 2021, 07:21:57 pm
content: So, I ordered a new transistor, and this time the one that arrived seems to at least have the right pinout (I'm still suspicious about the package, because it's smaller than a standard TO-220); I replaced the old one and turned L5 the right way, and now the power draw is normal, but there's still no output.I finally was able to get an half-decent probe for my scope (I'm using the oscilloscope mode on an HP 16500A, which it's not exactly the right tool but it's the only one I have that goes up to 100MHz), so I looked both at the signal on the base of the base of the 2SC1971 and at the output (directly across the lamp terminals), and here's the results:(Base)(Output)It looks like the transistor is putting out something, but the output level is way too low