This FM oscillator schematic works with BC850C but not with BC847C, why?

[William Edwards]

Hello everyone!

Ok, I have made a very small FM circuit after this simple schematic:

http://www.talkingelectronics.com/projects/Spy%20Circuits/images/1TrBug.gif

With the difference that I added a 10k resistor over the 1n capacitor and changed the 4.7k out to another 10k to make the base at about the right working voltage regime of 1.5V as its all fed with 3V, the 330 resistor changed to 1k and no antenna and no mic.


The circuit built with a BC850C SOT-23 and a 0805 coil inductor oscillates just fine but the same circuit built with a BC847C SOT-323 and a 0402 inductor does not.

I cant figure out why, it should?

Might it have something to do with the 0402 inductor?

It´s a type with no seen winding as in contrary to the other 0805 one where you can see the very thin copper windings. But it shouldn't really matter, I mean 0.1µF is 0.1µF no matter the size?


Im to 99.9% sure its all connected right with the right components and all since I soldered one component at a time.

And due to the very small size that I have made I can´t really take any components off to test without having to do it all over again. And I really don`t want to do that until I really know what is the cause since its a major pain to do and take a lot of time, took 3 days to put it all together in the first place.

Thanks!

Regards William

[wraper]

Most likely because of the inductor. Both inductors must have a ferrite core unlike in schematic. Inductance and losses are highly dependent on frequency for such inductors, read/show the datasheets.

But it shouldn't really matter, I mean 0.1µF is 0.1µF no matter the size?

It's not a capacitor.
Secondly none of both transistors are really suited for this (their gain drops drastically at such frequency) and you would better use something higher frequency.
EDIT: actually they are worse than I thought. Transition frequency only 100 MHz min. So both of them are barely functional here (and with condition you get a unit with high f_T)

[G0HZU]

It's many years since I messed with the classic FM bug circuit but this circuit generally relies on having a fairly slow transistor that has parasitics that give some extra phase shift to get the right conditions for oscillation. So often you will see them designed with BC107, BC547 etc etc. Faster devices might not work as well here because they don't have the parasitics required to get the right phase shift around the loop.

You also really need an inductor with a reasonable unloaded Q (= Qu). Most people make these with hand wound inductors and the unloaded Q of this inductor type will be very good. But I think the circuit can tolerate an inductor with much lower Qu. Your 0805 100nH inductor could have a Qu of just 40 at 100MHz which should still be OK but the 0402 inductor could have a much lower Qu. It could easily be much less than 20 (could be <10) and this can damp the circuit enough to stop it oscillating.

Do you know the Qu of your 100nH 0402 inductor (at 100MHz) from the datasheet? Coilcraft do some 100nH 0402 inductors that have Qu just over 20 at 100MHz. This is still very marginal IMO and I think you might need to stick with 0805 unless you start looking for a specific transistor that might still work here with the highest Qu (@100MHz) 0402 inductor you can find. Normally I'd expect any of the jellybean BC107, 2N3904, BC547 (BC847?) transistors to work in this circuit but the Qu of the 0402 inductor could be the problem here.

I've seen these bugs work at 3V before and your biasing mods to the circuit are still probably OK but you could also try changing your bias circuit to bias it harder again. But I would be most suspicious of the 100nH 0402 inductor if the datasheet shows it has a very low Qu at 100MHz.

[wraper]

Normally I'd expect any of the jellybean BC107, 2N3904, BC547 (BC847?)

Transistors he uses have much lower f_T than those.

[G0HZU]

I'm out of touch with the various specs of the jellybean transistor types but the ones we are discussing tend to have Ft 'min' 100MHz and/or Ft 'typical' 250MHz on the datasheet. The oscillator circuit uses the transistor as a common base amplifier and I think the circuit relies on having a transistor that is in this speed class in order to get the phase shift required for oscillation with such a crude circuit.

It could be that the particular BC847 he has used is a duff/fake/slow one but I'd be keen to find out the spec of the 0402 packaged inductor. It's easy to find examples of these where the Qu is less than 10 at 100MHz and I think this would be enough to cause problems. Or it could be a combination of the transistor and the inductor? But I'd expect a typical BC847 to oscillate in a typical FM bug circuit even though I've not tried making one with a BC847. I last made an FM bug when I was a student and that was decades ago

Put another way, I expect it would be quite difficult to find a genuine/healthy BC847C that will not oscillate in a typical healthy FM bug circuit. But if you replaced the coil with an 0402 chip inductor with a Qu less than 10 at 100MHz I think it could be difficult to get it to oscillate at all (unless the circuit values were changed, eg increasing the 4p7 cap may help here). So it would be useful to know the part number of the 0402 chip inductor

I suppose you could try swapping across the BC850 to the circuit board to see if that particular BC847 is a duff/slow example? But even if this fixes the oscillation issue I'd still advise checking the datasheet for the Qu of the 0402 inductor in case it is lossy and this could affect the range of the bug to some degree. There may be better 0402 inductors than the one that is currently being used.

[vk6zgo]

It's many years since I messed with the classic FM bug circuit but this circuit generally relies on having a fairly slow transistor that has parasitics that give some extra phase shift to get the right conditions for oscillation. So often you will see them designed with BC107, BC547 etc etc. Faster devices might not work as well here because they don't have the parasitics required to get the right phase shift around the loop.

You also really need an inductor with a reasonable unloaded Q (= Qu). Most people make these with hand wound inductors and the unloaded Q of this inductor type will be very good. But I think the circuit can tolerate an inductor with much lower Qu. Your 0805 100nH inductor could have a Qu of just 40 at 100MHz which should still be OK but the 0402 inductor could have a much lower Qu. It could easily be much less than 20 (could be <10) and this can damp the circuit enough to stop it oscillating.

Do you know the Qu of your 100nH 0402 inductor (at 100MHz) from the datasheet? Coilcraft do some 100nH 0402 inductors that have Qu just over 20 at 100MHz. This is still very marginal IMO and I think you might need to stick with 0805 unless you start looking for a specific transistor that might still work here with the highest Qu (@100MHz) 0402 inductor you can find. Normally I'd expect any of the jellybean BC107, 2N3904, BC547 (BC847?) transistors to work in this circuit but the Qu of the 0402 inductor could be the problem here.

I've seen these bugs work at 3V before and your biasing mods to the circuit are still probably OK but you could also try changing your bias circuit to bias it harder again. But I would be most suspicious of the 100nH 0402 inductor if the datasheet shows it has a very low Qu at 100MHz.

I really hate these type of circuits,but 1980s people seemed to have "cut their teeth" on them.
Beginners don't "get" that to miniaturise an inductor, you have to sacrifice something---in this case, "Q".

Another common mistake made by "noobs" is thinking all LC values that satisfy the resonance formula will result in a  circuit which works in the real world.

We had one beginner recently who was trying to make a MF receiver,using a  10nH chip inductor in parallel with a 1uF cap.
He said "It worked alright when simulated with LT Spice"!

"Greybeards " like us can cheat from time to time & use "off the shelf" inductors, (as witness some of VK3YE's Receiver designs), but beginners get "all bent out of shape" when random circuits off the Net "don't work like the website says it will"-----even though they have made modifications to them.

[William Edwards]

Thanks for the answers guys.

Ok, Im not for example some 15 year old kid that want to spy on someone or whatever, I´m way past that age.

I only make this circuit as a challenge for my self to see how small I could get the circuit and I use the BC847 sot-323 as the PCB for the components so its a little bid fiddly to change anything to say the least since everything is as one sort of speak.


The page where I bought the inductor I remember it showed a Q of 16 for this which I decided would be borderline ok for this schematic, so that is the reason I bought it.

But since this now doesnt work I downloaded the datasheet and yes it show it only have a pitty Q of 8 att 100Mhz.. I had my thoughts.

Its Wurtz:

http://katalog.we-online.de/pbs/datasheet/744784210A.pdf

But I bought it from a Wurth reseller, I think either I bought wrong type or they had the wrong info about it since it show that is sure doesn't have a Q of 16, which of course explanes it all.

They don´t have any 0402 with higher Q as it seem within a reasonable value I guess so I have to stick with this one atm and hope for the best.

I will try to give it some HLR with another 4.7pF and see since its the smallest value I have right now but a combined value of 9.4pF feels a little to big, but I will see.


This circuit is far faar from the best, we all very well know that, but I choose it since I saw it to have a have good potential to make it very small since the components would fit just fine on the transistor, nothing else.

The BC847 and BC850 was what I had at home at the time I build it and since sot-23 is bigger then sot-323 thus the latter was the one I used. I was worried about the Ft of 100Mhz on both builds, be sure about that, but as mentioned it was a minimum so I kinda saw at least that part should work and the frequency used is just below about 90Mhz and even it is a borderline so is it lower then 100Mhz.

The transistors were bought from Digi-Key so I expect them to be ok, but sure you never know but I still would have my doubts.

The inductor must of course be the problem and I really hope it could work with little more capacitance because it would be a real pain to change anything out, but I have to do that anyway later on if this would work but only then.

[Z80]

With a Q of 8, calling it a tuned circuit is being kind, it will be as wide as a barn door  .  If you really want to use tiny inductors, perhaps they make other values with a better Q (adjust C to keep LC)?  May be worth a try but if it's not easy to swap parts out then possibly not.

[William Edwards]

No these circuits are not even close to tuned that would be dam stretch to say they are all over the place.

I remember I checked one up that I build once many many years ago that I found some year ago. It was the ordinary one with own coil and such and I remember that it said hi even at around 500Mhz or so where I found some life. Extremely weak of course but still, and that was with a ordinary coil with way better Q. This 0402 would be worse no doubt.

Yea if this doesnt work I have to go for some other values and hope, if there exist that is for 0402 I will see.

[G0HZU]

Yea if this doesnt work I have to go for some other values and hope, if there exist that is for 0402 I will see.

If you want 100nH in 0402 package size then I doubt you will be able to get something with a Q much above 25 at 100MHz. Coilcraft make several 0402 types that have a Qu of 20 and you can ask for a few free samples if you have a company address.

http://www.coilcraft.com/apps/finder/hiqfinder.cfm?indInput=100&freqInput=100

If I throw some numbers for a 100nH inductor into my RF coil design program with the dimensions of an 0402 package then I end up with a Qu of just over 20 at 100MHz. I'm not sure you can dodge the physics here because you need very thin wire wound on a tiny diameter.

With a Qu of just 8 at 100MHz you would be effectively putting a 500 ohm damping resistor across the main inductor. I'd expect a Qu of 20 to be a bit marginal and even a Qu of 25 will sap energy from your bug and will have some impact on the range even if you do get it to oscillate.

Can you try operating at a higher collector bias current? Your bias resistor changes mean that the initial Ic is going to be very low. It might help with oscillator startup/reliability if you increased this a bit if you also want to try a new 0402 inductor that might have a Qu of 20?

[William Edwards]

Ah yea Coilcraft always forget about them, thanks.

Found this one:

http://www.coilcraft.com/pdfs/0402hp.pdf

And yes its not much over 20 in Q at 100MHz even for this, but sure better then 8. Question is if it would be enough.

They are not expensive at all a piece so I would just buy them. Impressed though that these seem to be physical wire wound, don´t ask me how they achieve 0.1µH and higher in a 0402, impressive.

With a Qu of just 8 at 100MHz you would be effectively putting a 500 ohm damping resistor across the main inductor.

Dam, that much? It says a lot then.

Higher Ic is a good idea, but it will be major pain to do. But I could parallell the same value over them to double the Ic would be slightly easier, but would that be enough or would more say a 5 time higher value be better? Would this be something to test also for a Q of 8 as well or that part is a no go no matter what?

---

Change inductance to get better Q. Well the best of these, 11nH, have 30 at 100MHz but that would require like 230pF and that doesnt feel like it would work, but sure I could be wrong.

Change frequency. It has to be in the 100MHz range to be able to pick it up on a ordinary radio, its just how it is sort of speak, even if have receivers for GHz band where the Q on this would go up to around 50 which would work.

But to change frequency to the higher regime would also mean that I need to change the transistor for a HF one and that would be the same as to rebuild absolutely everything, which Im not that keen to say the least since it took days to built it in the first place.

[G0HZU]

Ultimately, you need >unity starting gain around the loop at the zero degree phase frequency (100MHz target here?)

The transconductance is a function of the collector current so I assume that a higher Ic will help here. It probably can still oscillate with a Qu of just 8 but I think you would need to play about with different transistor bias settings and components and so I think you would be much better off trying the Coilcraft part with a Qu of 20. Just fill in the automated sample request for (say) 2 x 68nH, 2 x 82nH 2 x 100nH and 2 x 130nH and as long as you use a business address and don't get greedy with the qty they will send them to you next day completely free including free postage. It's cheaper for them this way compared to you buying them formally. However, I think you would be better off trying to squeeze a larger package part in somehow. eg 0805.

Why not leave alone the one you have now and build something slightly bigger and easier to work on? The layout shouldn't be critical as it's only 100MHz.

I'd expect to be able to get a typical BC847C to oscillate up to 300MHz or more when run at 5V and with the right parts around it because I think the BC847 BC547 and BC107 are essentially the same part. Back in my youth everybody used a BC107 to make these things and I think they all have about the same Ft. It helps that the amplifier is common base here too.

[William Edwards]

Target is somewhere between 90 to 100MHz but better close to 90 the 100, but just as long as it works I´m happy.

It would be easier to change the inductor out, but still a pain, then to mess with the bias resistors but if this is what it takes to get it running I will absolutely do so.

Thanks for the tip about Coilcraft, have sent an order to them and hope for the best.

A 0805 or even a 0603 size inductor would be to big and also don´t fit generally or with with the "sleek line" I have going with this.

Why not leave alone the one you have now and build something slightly bigger and easier to work on?

Where is the fun in that? The previous one was just a warm up.


I made the first one only as a fun challenge to my self one day to se how small I could built this schematic but I used 0603 for that one since I had the components already at home.

Now I challenged my self (severe case of self-torture) to do the same but with 0402 and I succeeded but this one didn't work due to, I know now, a inductor with lower Q then I was sure that I bought.


The layout is pretty much as good as it can be anyway since the "as close as possible and as short trace as possible" at RF can´t be any smaller then the way I have built this since the components cant physical get any closer to the transistor and its legs.

But I see this schematic to be quite forgiving anyway, as long as you have a inductor with good Q that is.

300MHz with fex a BC547? Dam, didn't thought they would go that high.