how do i make this FM Transmitter work without Varicaps? - Page 1
Forum section: electronics / beginners

author: Fionaa
author_rank: Contributor
timestamp: February 23, 2025, 07:12:06 pm
content: i just want an normal Carrier for now in the normal FM Broadcast Band but without those varicaps in the circuit diagram, how would i do that?

author: Benta
author_rank: Super Contributor
timestamp: February 23, 2025, 07:22:36 pm
content: You won't.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 23, 2025, 08:36:00 pm
content: If you mean an unmodulated signal, then it will work without the varicaps. You can substitute small fixed capacitors.

author: Zero999
author_rank: Super Contributor
timestamp: February 23, 2025, 08:49:30 pm
content: Try replacing them with a rectifier diode such as the 1N4001, which might just be good enough,

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2025, 09:01:25 pm
content: Quote from: Fionaa on February 23, 2025, 07:12:06 pmFM modulation needs to change frequency depends on the message signal. How you're thinking to control frequency with voltage with no varicaps?Technically you can use digital frequency synthesizer and control frequency by writing digital words calculated from current sample of audio signal. But as I understand you're want simple analog circuit...Another way - you can use VCXO oscillator, it already has varicaps inside, so all what you need is just to put audio signal on its frequency control pin.If you want just a carrier with no FM modulation, then just replace varicaps with capacitors. You can use variable capacitors if you want to be able fine frequency tune,

author: Xena E
author_rank: Frequent Contributor
timestamp: February 23, 2025, 11:55:07 pm
content: Quote from: Zero999 on February 23, 2025, 08:49:30 pm+1A 1N4007 may be better, (bigger variation of capacitance).To the TS:If you have a VHF capable N channel JFET available, (like a J310), I'll share a really simple circuit if you just want to build a 'blocking oscillator' to create a carrier signal.X

author: Fionaa
author_rank: Contributor
timestamp: February 24, 2025, 08:37:41 am
content: Sure, would be awesome

author: Xena E
author_rank: Frequent Contributor
timestamp: February 24, 2025, 11:19:48 am
content: Quote from: Fionaa on February 24, 2025, 08:37:41 amSimple circuit, put it together this morning just to check I remembered correctly!FET is J112 (what I had on hand) range is a few metres with a 30 cm aerial.

author: Fionaa
author_rank: Contributor
timestamp: February 24, 2025, 02:41:33 pm
content: Awesome, Thank You so much!is there also an Common Drain Version of this?

author: PGPG
author_rank: Frequent Contributor
timestamp: February 24, 2025, 03:02:46 pm
content: Quote from: Fionaa on February 23, 2025, 07:12:06 pmIf you want only carrier (without FM modulation) you can replace varicaps by capacitors having the capacity as varicaps at specific voltage.If you assume varicap (at specific DC voltage provided to it) will have for example 20pF (I was not searching its datasheet so my shot can be not precise) than you replace each of them with 20pF.But they are connected in serie so instead of two 20pF capacitors you can use one 10pF connected in paralel to L1.Anything at your schematic to the left of it is not needed if you don't want to modulate frequency.

author: shabaz
author_rank: Frequent Contributor
timestamp: February 24, 2025, 06:35:29 pm
content: There are some transistor-based FM transmitter circuits, that do not use varicaps. See this:https://www.talkingelectronics.com/projects/MicroBug/MicroBug-P1.htmlI have not tried it, but from everything I've seen on the talkingelectronics website, I think you can have fair confidence that all their circuits function to a good extent. It's a really good resource.Once built, you could (if you have access to it) connect a 'scope to the output, and you'll probably notice the output looks visually better at some range of frequencies, and it may well look poor or even collapse at a different range, You can use that to approximately tune the output frequency to something suitable.Bear in mind that if your transmission travels any significant distance, it will cause annoyance, not just at the transmitted frequency (there will be transmission at other frequencies, too, multiples of your intended frequency) and is likely not legal in most countries unless it is a very low level.Finally, it would still be worthwhile purchasing some varicap diodes (and perhaps some RF transistors, trimmer capacitors, and other bits and pieces) if you're interested in exploring more, so you can build additional circuits later because many such varying oscillator circuits (also known as a voltage controlled oscillator or VCO circuits) will use varicap diodes, so it could be worth having a few at hand for future experimentation.(image source: talkingelectronics.com website)

author: BrianHG
author_rank: Super Contributor
timestamp: February 24, 2025, 06:45:33 pm
content: For the capacitance range of the BB105, you may use a ~75v zener (maybe as low as 50v) diode in its place.Use the small ones, like 250-350mw.The lower the zener voltage, the higher the capacitance.Don't go below ~35v, they typically just stay stuck at a higher capacitance.

author: Xena E
author_rank: Frequent Contributor
timestamp: February 24, 2025, 07:05:40 pm
content: Quote from: Fionaa on February 24, 2025, 02:41:33 pmSorry, don't quite understand your requirement of "common drain". Please explain.The circuit I posted, the topology is called "common gate" the gate is grounded and the positive feed back from drain to source via the 10pF capacitor causes it to oscillate.Regards,X

author: Xena E
author_rank: Frequent Contributor
timestamp: February 24, 2025, 07:10:55 pm
content: Quote from: BrianHG on February 24, 2025, 06:45:33 pmNever knew that! Every day a school day, thanks. I guess you would connect as a normal zener, ie., cathode to positive?X

author: Benta
author_rank: Super Contributor
timestamp: February 25, 2025, 12:09:29 am
content: This little SO-8/TSSOP-8 IC will do it: MC100EL1648:https://www.onsemi.com/pdf/datasheet/mc100el1648-d.pdfCheck Fig. 15. It doesn't get much better.

author: Fionaa
author_rank: Contributor
timestamp: February 25, 2025, 08:47:58 am
content: Like this

author: Zero999
author_rank: Super Contributor
timestamp: February 25, 2025, 09:45:34 am
content: Quote from: shabaz on February 24, 2025, 06:35:29 pmThat circuit works by the fact the capacitance of the PN junctions changes, as the bias point is altered by the audio signal.The downsides with these simple circuits are the there's not pre-emphasis (FM stations boost treble, to minimise the effect of high frequency noise, which is reduced at the receiving end), the frequency drifts with the power supply voltage and loading. Audioguru designed a circuit which overcomes these issues. The audio amplifier includes pre-emphasis, the power to the oscillator is regulated and a buffer is added to the output.

author: shabaz
author_rank: Frequent Contributor
timestamp: February 25, 2025, 02:08:57 pm
content: Quote from: Zero999 on February 25, 2025, 09:45:34 amYou're right, but I merely picked a circuit of proportionate size compared to what the OP was building.You get a lot of bang-for-your-buck with that circuit; I've not constructed that particular one, but did assemble a very similar one as a kid - the speech quality is pretty good as-is (I'm assuming this isn't intended for real broadcast purposes, just a play-around circuit at very low output for a few feet distance).But as you say, it can be improved a lot (I hinted the OP should try to observe the output if they have a 'scope) and all manner of things can be observed with it, including drift, tapping the desk, etc. The non-stable voltage from the cells doesn't help, of course, either.I've always thought a VCO makes an excellent beginner "opportunity to excel" project - for instance, to see how stable one can make it (a good way to learn physical construction skills, too) or to observe the harmonics and see how to reduce them.With the low-cost 'scopes, especially the 12-bit ones nowadays that I believe have decent FFT from what I've heard, it would be interesting for someone (perhaps the OP if they have one) to demonstrate what they can measure and how they improve it, e.g., with the circuit you mention.The attached screenshot shows the output from a simple similar-ish frequency BJT-based VCO I built a while back (the spectrogram shows the output for 30 seconds; the spectrum at the top shows a skirt that is phase noise from the spectrum analyzer's internal synthesizer revealing itself; another learning opportunity if the OP wants it, to see how to remove that from the display!) but the circuit will drift without stable voltage, and any knock to the desk, and so on. It was just a fun experiment for a hobby project while I was waiting for a PCB to be made, so I made no effort to make that coil more stable, for instance.

author: BrianHG
author_rank: Super Contributor
timestamp: February 25, 2025, 05:30:32 pm
content: Quote from: Xena E on February 24, 2025, 07:10:55 pmYup, wire it the same way illustrated in with the varicaps.I used to use 75v Zener diodes to voltage tune 20Mhz crystals for precise VCO control, directly tied to the pins of a PIC microcontroller where I had the MCU self tune/lock onto an external signal.They cost me a fraction of the MV209 tuning diodes at the time and by selecting the upper voltages, I got the tuning range I wanted combined with a varicap for bulk tuning since I only had a single 5v supply with a 5v IO pin doing the tuning.On a >50 zener, tuning should be close to linear from around 2v to 30v. The capacitance raises dramatically below 2v.

author: Xena E
author_rank: Frequent Contributor
timestamp: February 25, 2025, 08:11:39 pm
content: Thanks for that Brianwell worth knowing!Regards,X

author: Fionaa
author_rank: Contributor
timestamp: February 26, 2025, 08:24:21 am
content: I made this now, not sure if i need an coupling capacitor for L1

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2025, 09:28:34 am
content: Quote from: Fionaa on February 26, 2025, 08:24:21 amWhere's the input signal?

author: Fionaa
author_rank: Contributor
timestamp: February 26, 2025, 11:10:06 am
content: it "should" just do an carrier

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2025, 11:25:55 am
content: Quote from: Fionaa on February 26, 2025, 11:10:06 amThen it isn't FM (Frequency Modulated). It's an oscillator, or continuous wave transmitter. Putting FM in the thread title, has confused people.

author: iMo
author_rank: Super Contributor
timestamp: February 26, 2025, 11:37:35 am
content: QuoteYou should place an LC filter there to filter out the harmonics (a lot of them in your circuit), and match the antenna as well.Btw., there is a regulation in your country on the output power and spectral purity for such devices you are messing with - better doublecheck..

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2025, 12:01:36 pm
content: Quote from: iMo on February 26, 2025, 11:37:35 amBroadcasting over a certain power output requires a license.I also believe it's against the rules in most places to broadcast CW i.e. silence on the FM broadcast band. You'll probably get away with it at very low power levels though.

author: shabaz
author_rank: Frequent Contributor
timestamp: February 26, 2025, 12:32:46 pm
content: Quote from: Fionaa on February 26, 2025, 11:10:06 amGoing to ask this another way; what's your actual goal? What's the use-case, and what problem are you trying to solve, that requires the carrier that you mention?For instance, is the aim to (say) jam an FM station? Or to align an FM receiver? Or what..I'm asking this, in case you actually didn't need an FM transmitter circuit in the first place, but assumed that was the thing you needed. There may be a completely different circuit that better meets your needs.So; what's the actual scenario you've got, and actual end goal?

author: 807
author_rank: Frequent Contributor
timestamp: February 26, 2025, 02:30:51 pm
content: Quote from: Zero999 on February 26, 2025, 11:25:55 am...but the OP said in his first post "i just want an normal Carrier for now in the normal FM Broadcast Band...", so he's talking about the FM broadcast band.

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2025, 06:13:35 pm
content: Quote from: 807 on February 26, 2025, 02:30:51 pmBut the title says." how do i make this FM Transmitter work without Varicaps?"