Need help with a FM radio transmitter project - Page 1
Forum section: electronics / projects

author: ezalb98
author_rank: Contributor
timestamp: July 25, 2024, 11:04:48 pm
content: helloi want to build this circuit. I found it on a website with no other informations about... i've modified it a bit cause aparently the original had some errors and i added the power switch.I want to know if it will work, and if there are some improvements i can do.I also tried simulating it with multisim, but i noticed, with an oscilloscope, that the output wave is linear and not sinusoidal... why?Thanks to anyone who will help me

author: xrunner
author_rank: Super Contributor
timestamp: July 25, 2024, 11:31:46 pm
content: Pretty generic parts list, simple to breadboard - why don't you try it and see?

author: moffy
author_rank: Super Contributor
timestamp: July 25, 2024, 11:37:31 pm
content: Just a few comments:1. R1 at 10R seems low.2. The 80% variable capacitor has no ID or value.The oscillator using Q2 can sometimes be difficult to simulate, but the fact that the variable cap has no visible value is a starting point.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 25, 2024, 11:44:46 pm
content: As mentioned above; R1 is most likely a 10K. Also, R6 and R2 bias the oscillator stage but they appear not to be connected - there is a missing dot!And, C1, C4 and C6 appear to be specified as 470 Farad!

author: ezalb98
author_rank: Contributor
timestamp: July 26, 2024, 12:12:24 am
content: Quote from: Andy Watson on July 25, 2024, 11:44:46 pmFixed, not i see the wave, but is a bit weirdthe first is the output, the second is the input

author: JoeyG
author_rank: Regular Contributor
timestamp: July 26, 2024, 12:25:49 am
content: What happens in the SIM if C1 does it oscillate in the FM Band?Also the SIM may not take in to account parts that are not fully characterised in the FM band (at 100MHz)Check this out

author: moffy
author_rank: Super Contributor
timestamp: July 26, 2024, 03:18:07 am
content: As an example of a very similar valued FM oscillator which I created in LTSpice that oscillates I have included the screen capture of the circuit and its oscillation. There are quite a few errors in your schematic making it easier to show something that works.P.S. And a slightly different version as well.

author: ezalb98
author_rank: Contributor
timestamp: July 26, 2024, 12:08:04 pm
content: Quote from: moffy on July 26, 2024, 03:18:07 amok thanks you!But i didn't undertand how i should modify my circuit, were are the errors that you mentioned?

author: moffy
author_rank: Super Contributor
timestamp: July 26, 2024, 12:18:20 pm
content: Quote from: ezalb98 on July 26, 2024, 12:08:04 pmJust compare your schematic to mine, the differences are fairly obvious e.g. your C6=470F whereas my equivalent C3=470pF etc.

author: ezalb98
author_rank: Contributor
timestamp: July 26, 2024, 12:41:48 pm
content: Quote from: moffy on July 26, 2024, 12:18:20 pmlike this? it dosn't seem right...i dont also understand why when i turn on the switch the wave becomes linear?

author: wasedadoc
author_rank: Super Contributor
timestamp: July 26, 2024, 01:06:32 pm
content: 1. Your scope is probably loading the oscillator so much that it isn't oscillating.2. Your scope timebase is 10ms/div. Waveform frequency is about 100Hz.

author: ezalb98
author_rank: Contributor
timestamp: July 26, 2024, 02:31:53 pm
content: Quote from: wasedadoc on July 26, 2024, 01:06:32 pmoh, i seewith the design that i have it should work on real hardware right?

author: TurboTom
author_rank: Super Contributor
timestamp: July 27, 2024, 01:49:23 pm
content: Decades ago, a dear friend of mine celebrated a birthday party in their "student home" which was a rather old, facework building with little ground area but spanning vertically over three floors. Just like me, he loved anciend "steam" radios and had quite a collection. To be able to play the same music on all the floors, he asked me to make a small FM transmitter for him that should be powerful enough to provide a good signal to the radios on all the floors, to connect to his "central" hifi system. Finally, while the party was going on, all the people living in his village could tune in to his music...Anyway, the design is proven to be working with a fairly good audio quality. It may be of slightly old-fashioned concept with the transistors in common-base circuit but it's simple and easy to build. I attached a screenshot of the schematic, and if you like, I can share the (single-layer) PCB layout as well.Cheers,ThomasP.S. I used the Sanyo varactors and oscillator transistor simply because I "inherited" an almost unlimited supply of those. Other, similar components will work just as well.

author: wasedadoc
author_rank: Super Contributor
timestamp: July 27, 2024, 04:19:55 pm
content: Quote from: TurboTom on July 27, 2024, 01:49:23 pmOperating an FM transmitter with such range is illegal in many countries.

author: TurboTom
author_rank: Super Contributor
timestamp: July 27, 2024, 07:04:44 pm
content: Quote from: wasedadoc on July 27, 2024, 04:19:55 pmYes, of course. I was well aware of that, also back then when I built this gadget. So what? We made sure the channel that we used wasn't busy with a "non-pirate" program that we could possibly interfere with. And we operated our "Pirate Radio" for exactly one Saturday evening. What's the risk of getting sued for this? As we say "Wo kein Kläger ist, ist auch kein Richter" (No plaintiff, no judge). And today, this "sin from the youth" is long time expired.And if someone really would like to copy this design, it's simple to reduce its power (which isn't much more than 100mW anyway) to a level really necessary for "the job" by increasing R6.

author: moffy
author_rank: Super Contributor
timestamp: July 28, 2024, 12:08:46 am
content: Quote from: TurboTom on July 27, 2024, 01:49:23 pmNice interesting circuit, I don't think I have seen a BD139 used at such frequencies before.It took a little while to see how the modulated FM got from T1 to T2, but if I am correct, the PCB shows that there would be magnetic coupling feeding from L1 to L2 which feeds the emitter of T2, the schematic doesn't make that clear, though it subtly implies it by L2 being shown underneath L1. Thanks for the circuit.

author: TurboTom
author_rank: Super Contributor
timestamp: July 28, 2024, 02:23:24 pm
content: Okay, the power estimation might have been a little optimistic, I measured power output of my "spare specimen" to be 25mW after a little tweaking. Anyway, with a nice λ/4 wire attached, that's sufficient to cover some 100m radius with radio program. It's also possible that the other unit worked a little better due to component tolerances (especially of the BD139). Nevertheless, this "old faithful" performs better than expected at these frequencies, but a common base configuration is required for this.Correctly adjusted, the output is fairly clean for the simplicity of the circuit with the first harmonic at round about -30dB. I attached a scope screenshot and a few photos of my spare sample...@moffy: Yes, you are correct, the signal coupling is managed by magnetic influence between the adjacent coils.

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2024, 03:00:23 pm
content: The BD139 datasheet gives a GBWP of 190MHz, so it seems reasonable.https://www.kitsandparts.com/BD139-16.pdfAlso note that this figure purely relates to the current gain. A BJT can still provide voltage and therefore power gain, even when the hFEis below 1 which means it can oscillate a higher frequency than fT.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 20, 2024, 04:54:53 pm
content: L1 should be 47nH give or take. ~3.5 turns on a pencil.Not 470nH. BTW the best variable cap for the job is a beehive type, eghttps://www.ebay.co.uk/itm/312970361696Connect the outer to V+. The 547 will get very hot. I used to use 56R for R7 and 2N2219A's with a clip on heat sink for Q2.C4 and C6 set the base of Q2 an AC ground. The value is not critical but it does set the LF roll of the signal. ie. f=C4*R2The real thing makes good use of parasitics in the circuit. That makes it tricky to simulate.These bug transmitters are very dirty and generate plenty of harmonics.

author: A.Z.
author_rank: Super Contributor
timestamp: August 21, 2024, 03:09:34 pm
content: Folks, wasn't multipost "forbidden" on these forums, I'm asking since...https://www.eevblog.com/forum/rf-microwave/can-some-one-recommend-me-a-low-range-fm-transmitter-design/?all