Simple, cheap exciter

[Albert H]

Here's one from a few years ago. It gives 5 clean Watts (if it's built sensibly) but MUST have a PLL to keep it stable. We used to use the TDA1057 and a PIC with a 4 MHz crystal, which tunes nicely in 100 kHz steps down Band II. The PLL output coil is pushed into the main 5-turn VCO coil. The output is clean enough to connect straight to an aerial, but the output filter coils mustn't "see" one another, and should be screened from the rest of the circuit.

Cheapo Exciter.png

The trimmers at the output stage can be a couple of the yellow 65p type. There are lots of these in Eastern Europe.... It's a bit like an updated "Stentor" transmitter (an old Dutch favourite we all cut our teeth on).

[BLF278 unlimited]

Nice one.
But watch out: There many fakes of 2SC1971.

[nrgkits.nz]

Nice one.
But watch out: There many fakes of 2SC1971.

It’s an obsolete part, however you can get some very good fakes out of China if you know where to look, these work just like the original - I bought a few bags of them from AliExpress. Just beware of the ones which have the collector and base swapped around and occasionally you might get the odd one which doesn’t have as much gain. You might actually have better luck with second hand ones, I bought a bag of 100 which were pulled out of old equipment (probably RT’s) and I haven’t had any problem with these as they’re all original.

[thewisepranker]

Nice one.
But watch out: There many fakes of 2SC1971.

It’s an obsolete part, however you can get some very good fakes out of China if you know where to look, these work just like the original - I bought a few bags of them from AliExpress. Just beware of the ones which have the collector and base swapped around and occasionally you might get the odd one which doesn’t have as much gain. You might actually have better luck with second hand ones, I bought a bag of 100 which were pulled out of old equipment (probably RT’s) and I haven’t had any problem with these as they’re all original.

That's not how fakes work. They don't copy the die, they fit something in there that looks roughly like the correct device when you conduct a very basic test (i.e. the pins are in the right order and work as expected, i.e. give the device a bit of gate voltage with respect to source and D-S goes low impedance) but is much cheaper to buy than the original device, hence it being worthwhile to do. Sometimes they'll even be quite close on a curve tracer, but the counterfeit device will never be able to dissipate anywhere near the rated power. This is for the simple reason that surface area is the name of the game, and more silicon costs more money.

[BLF278 unlimited]

Thats what i meant.
There are CLONES out there that will may perfectly fit.
But there are defenetly some real fake ones out there that will instantly go up in smoke if you try to get Power out of them.
They may put the label of RF transistors on on 7805 .

[Bton-FM]

Is the take off from the dual-gate FET 1/2f?

[Albert H]

No. The whole thing runs at the output frequency.

[Bton-FM]

What's the approx power output after BFR96?I want to know whether or not I can run it into my analyser.

[rigmo]

96s 300mv

[Bton-FM]

Do you mean milliwatts?

[radium98]

yes look at datasheet 300 mw for a genuine one not the copy that sprone harmonics

[Bton-FM]

I finished this today (not including the 2sc1947 stage) and put it on my analyser.I got about 100mw with a clean carrier and no spurious.The harmonics were quite high though.

The only problem is the Q of the oscillator isn’t very high which means the carrier is about 100khz wide.How can I make the signal more narrow so I’m not transmitting on the adjacent frequencies ?

[nrgkits.nz]

The Q of the oscillator will be high if you’re using a wire wound inductor. The noise around the carrier is most likely from your PLL, make sure you have a good RC low pass filter on the PLL control voltage to the oscillator otherwise you’ll get the phase pulses and noise from the PLL being modulated.

[Bton-FM]

I haven’t attached a pll yet I’m just testing the oscillator.I’ve put a trim pot from the 9v rail and a 100k resistor to the varicap instead of the pll voltage.

When I say the signal is wide I’m mean the audio sound crystal clear on a 2 different frequencies 100khz apart.

I could send some pictures of the analyser readings?

[teckniqs]

I think it usually just depends on your receiver. Some with more narrow filters will receive it more narrow on the dial and be less spread out either side.

....Try it outside on a car radio down the end of the road, they usually have a lot more narrow filters than domestic house tuners/radios so you should be able to see if there is a problem with it being too wide if it still is on a car radio.

[Bton-FM]

I think it usually just depends on your receiver. Some with more narrow filters will receive it more narrow on the dial and be less spread out either side.

....Try it outside on a car radio down the end of the road, they usually have a lot more narrow filters than domestic house tuners/radios so you should be able to see if there is a problem with it being too wide if it still is on a car radio.

OK thanks I'll give that a go, I was testing it in very close proximity before.

[teckniqs]

Yeah the signal is usually a lot wider when you are very close to it.

[Albert H]

The BFR96 stage normally gives around 350mW, and the device needs to be firmly against the groundplane to get rid of the heat! The 2SC1971s we used to have would develop around 4 Watts with that drive, and a particularly good one would go as much as 5 Watts. A friend of mine on Germany etched some really pretty boards for this circuit, and included the SAA1057, PIC and the switches along the bigger edge of the board. He also included screens soldered to the topside groundplane areas, screening off the output filter, the digital bit and the VCO coil. His version was designed to fit into a standard tinplate box. If I can get in touch with him, I'll try to get his artwork.

[Marcel]

Albert can also share the diameter and wire thickness about the coil with us?

[Bton-FM]

I used 18 swg wire 4 turns around 5mm.