Small FM transmitter question

[MiXiN]

I'm getting a good 300 > 400m range on this transmitter, but is there anything that can be done to the circuit to increase the range? Can any components be altered in the final stage to increase power output?

I'm already using it on the highest voltage of 9V.

Also, I can only get it working from 81 > 103 Mhz, so which Capacitor(s) needs to be changed to get it operating upto around 107 Mhz?






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[sinus trouble]

Greetings Mr Mixin!
Looking at the circuit diagram, the 9014 is just an audio/modulation amplifier
The first 9018 is your oscillator, the two 30p caps provide a center tap for feedback! You could adjust these for extra range but id recommend keeping them equal in value otherwise you could introduce distortion!
The second 9018 would normally be some kind of buffer amplifier to match the higher output impedance of the oscillator to something lower Eg 50ohm depending on what follows?
To get significant gain from this thing? youd probably have to add another 9018 stage before even thinking of any power transistors!
The most important thing to take into account is temperature will have you drifting up or down the dial so keep gain to a moderate level on each stage and pumping up the voltage will just make the situation worse lol!
Anyhow It will be as stable as a cat with one leg!! lol

[Albert H]

That thing is really a toy. It's using an unregulated supply for the oscillator - as the battery discharges, the frequency will change. It will also suffer from "hand capacity" problems: as you approach the board, your hand will detune it. There's little that can be done to improve this effort. If you want to build something better, look at the kits from the Netherlands or from NRG in New Zealand.

A good starting point is a small 1 Watt PLL exciter. It will be frequency stable (that's what the PLL is about) and you won't be able to do much damage with such low power. It'll cost you a bit more, but anything worthwhile will require quite a lot of parts, and will take careful construction. You'll also need some basic tools and instruments.....

[yellowbeard]

When you say it stops working do you mean it stops moving frequency or it stops oscillating? According to its own blahblah it should go 76-108 MHz. To get it to go up the band it'd be best to take a half or even a full turn off the the red coil with the tuning slug, or change the the two 30pF capacitors for two 27pF or 22pF ones like sinus says. The oscillator is a Colpitts around the second 9018, the details of it are in the wikipaedia including the equation for the frequency. For more range the obvious solution is a better aerial or one higher up. It's not a good plan to amplify a yoke like that, it will only increase the likelyhood it'll interfere with something. Have fun now!

Code: Select all

https://en.wikipedia.org/wiki/Colpitts_oscillator

[Albert H]

A quicker way to get it higher in frequency would be to - carefully - remove the slug from the tuning coil, and replace it with a brass screw with the same thread. As you screw the brass in, the frequency will go up rather than down.

You might find it interesting to try different metal and ferrite cores in that coil and see how it responds. Different materials will respond in differing ways. A brass or aluminium slug (for example) will raise the frequency of the oscillator and will be more frequency stable than the original ferrite core!

For best stability, the capacitors in the oscillator stage should be NPO types - these usually have a little black dot dipped onto the top of them which denotes a zero temperature coefficient.

I'd like to point out that it is possible to make a very stable free-running oscillator, but it would probably be well beyond the abilities of most of the people on this site:

You have to carefully select the circuit you're going to use - I favour either the Colpitts or the Vackar type - and then carefully select the components you're going to use. I'd use a dual-gate FET as the active component, with the second gate used for amplitude stabilisation, by means of a diode-pump feedback control loop. I'd use a very rigid air-cored coil, and select the capacitors that determine frequency for either NPO/COG types or for carefully selected temperature sensitivity to cancel the thermal drift in other components. With great care (and a lot of temperature testing, using a deep freeze and my bench lamp to get a range of temperatures) I can build an oscillator that remains on the same frequency (to within about 10 kHz) at ~50MHz. I'd then buffer it with a simple current amplifier stage and then drive a frequency doubler stage to get to Band II. As you can see, it's not trivial to get a stable, free-running oscillator.

This is why we use the PLL - the frequency of the oscillator is (loosely - to allow modulation) controlled by a digital circuit that uses a crystal as its reference. A crystal is several orders of magnitude more stable than you can achieve with a simple oscillator, and if you put the crystal into a temperature-controlled environment (because even crystals are slightly temperature sensitive), you can get really stable and accurate output frequencies with little hassle.

Another option - that really freaks out the authorities - is to use an off-air frequency standard to drive your PLL. For medium wave, I often use the BBC 198kHz Radio 4 signal as the reference, as this is sourced from the National Physical Laboratory, and is accurate to about 14 digits! On an RSL, I was able to demonstrate the inaccuracy of the DTI bloke's frequency counter! I've also used WWV on 10MHz as a reference for Band II synthesisers, and again can get spectacular accuracy. This is usually only done where I've had hassle from a licensing authority, who claim that my crystal-control isn't precise enough!

For pirate purposes, a crystal is good enough. Your Band II frequency will be within a couple of kHz if you load the crystal correctly (see the manufacturer's specification for loading capacitors) and use NPO capacitors in the crystal circuit. If you want to be even more precise, build a box around your crystal, packed with polystyrene foam and with a transistor heater in there (loads of circuits on the 'net) so that it runs at a constant 50°C - hotter than the hottest summer day.

[nrgkits.nz]

I have almost finished prototyping the NRG Pro5. This is an at-frequency design with a minimum of 5 watts across the band, the driver stage is a 4427 and the output is a 1971. The PLL uses a TSA5511 and PIC16F628. There is also an LCD which attaches to the PCB using a ribbon cable and is designed to mount on the front of the box. The PIC uses proprietary source code which I have just finished writing and debugging, I plan to add more features at a later stage which may require upgrading the PIC. Currently i'm working on the final PCB layout which is almost done. The PCB is double sided.

Varicap tuning voltage runs upto 30v using a hex inverter to get a good stable lock over the entire frequency range with less noise.

I have also gone for a longer lock time (although the 5511 does support two different charge pump current settings) to eliminate problems with bass frequencies.

There is also a trimmer paralleled with the PLL Crystal loading capacitors to allow fine tuning of the reference frequency, although without this I already have it within +/-1khz. Here in NZ the standard is +/-5Khz tolerance for LPFM use.

Harmonic content is -70dBc and another trimmer has also been included in the modulation input to adjust the audio response and phase angle between pilot and RDS.

I hope to have boards ready within the next few weeks. I have just finished ordering another batch of components from Digikey.

[sinus trouble]

Very interesting Mr NRGkitz!! Cant wait to see the finished product!
With a mild feeling of sorrow, I got the image of an RDVV in my head for a moment!
Have you sunk the flagship oscillator in this project??

[sinus trouble]

Getting back on topic for a sec lol
Thankyou Mr Mixin for bringing back memories of when I built my first Colpitts oscillator many many years back!
I found the schematic in an old electronics handbook (before the birth of the world wide web) When I heard that silent carrier on my receiver for the first time, the buzz was better than any party pills!!! I felt like a real pirate! HaHa!!

[MiXiN]

Sorry for the late reply necks.

I don't plan on using this for broadcasting, and am only using it as a bug.

It's actually surprisingly stable for what it is, and barely even drifts when I go to pick the thing up.

I'll try & find some aluminium or brass slugs, but failing that, I'll change the 2 capacitor's mentioned.

On the subject of range again - if I was to replace the 15, 47, & 47pF Capacitors at the pi output stage with 60pF SMT trimmer caps, could that increase range any?

[Albert H]

You might be able to get slightly more out of it by critically peaking those trimmers, but the improvement will be miniscule. Put it in a plastic box with a battery holder and sell it on EBay for twice what you paid for it! Put the money towards a better kit!