Build digitally controlled FM transmitter (PLL)

[Zipdox]

Yeap, with the pico the resolution would be quite bad for FM.
But what about slapping a varicap on the crystal capacitors ?

What? The entire point of a crystal is to have a stable constant frequency.

[fourfathom]

Yeap, with the pico the resolution would be quite bad for FM.
But what about slapping a varicap on the crystal capacitors ?

What? The entire point of a crystal is to have a stable constant frequency.

If you are using a wide-bandwidth PLL to multiply a FM-modulated reference, then the VCXO is a perfectly good way to generate the reference.  People have been pulling XTALs with varicaps for essentially forever and the stability is *much* better than an LC oscillator can deliver.  You can always add temperature compensation to the VCXO circuit.  Of course you can always buy a pre-made VCXO and the spec sheet will tell you what you need to know (usually).

[BrianHG]

Seriously, if the OP doesn't wan to over complicate things, just buy one of these IC's:

https://www.ti.com/product/CDCE913

Tie a 10.7Mhz FM radio IF filter to the crystal inputs (may need to add a tuneable inductor) and feed his modulation to the Vctr input pin.  TV audio 4.5mhz filters will also allow a narrower band.  Also try SAW filters in the 10 to 30 mhz range.

You cannot use a crystal as they cannot be pulled far enough for FM radio's 200khz width.

Use the TI clock software to program it as it is I2C control.
TX from 10mhz up to 230mHz.

You will need to bandpass filter the output into a sine wave unless you don't care.

I still say make a proper 10.7mhz modulator and use a freq synth with mixer to transmit properly.

[fourfathom]

You cannot use a crystal as they cannot be pulled far enough for FM radio's 200khz width.

True, but that's why I mentioned the PLL multiplication.  You can reasonably pull a 5 MHz xtal +/- 5 KHz, which gives you +/- 100 KHz deviation at 100 MHz.  But yes, there are easier ways to do all this.

[Zipdox]

I think I'll just wing it and design a circuit around the LMX2541. Modulating the the VCO input using the AC coupled audio signal and tuning it using the filtered charge pump output. If I'm not mistaken, band limiting the PLL by filtering the charge pump output is how this circuit does it. I'll have to add a circuit to manipulate the modulation depth, aka the audio amplitude.

[fourfathom]

I'll have to add a circuit to manipulate the modulation depth, aka the audio amplitude.

FM transmitter, right?  Then it's deviation.  Modulation depth is an AM thing.

[BrianHG]

You cannot use a crystal as they cannot be pulled far enough for FM radio's 200khz width.

True, but that's why I mentioned the PLL multiplication.  You can reasonably pull a 5 MHz xtal +/- 5 KHz, which gives you +/- 100 KHz deviation at 100 MHz.  But yes, there are easier ways to do all this.

In other words, tie a 5Mhz crystal to the TI PLL IC I listed.  Set the PLL to multiply by 20 to get 100MHz out.  I'm still not sure you can push and pull a 5MHz crystal quick enough to get that full band broadcast.  You would probably need some serious pre-emphasis.

I think in this topic, we are just going all over the place as the OP hasn't described the quality and adjust ability he needs.  PLL pulling a 5MHz crystal will need gain an pre-emphasis re-adjustmend if he goes from 88Mhz, then selects 107MHz.  It's the same with tuning a fundamental oscillator directly.  That is why I say make a 10.7Mhz modulator, then use an off the shelf PLL osc/mixer running at + or - 10.7 Mhz the output freq you want to make a quality tunable transmitter.

[Zipdox]

I think in this topic, we are just going all over the place as the OP hasn't described the quality and adjust ability he needs.  PLL pulling a 5MHz crystal will need gain an pre-emphasis re-adjustmend if he goes from 88Mhz, then selects 107MHz.  It's the same with tuning a fundamental oscillator directly.  That is why I say make a 10.7Mhz modulator, then use an off the shelf PLL osc/mixer running at + or - 10.7 Mhz the output freq you want to make a quality tunable transmitter.

I need only about 60 kHz bandwidth to include RDS. Maybe round that up to 80.