LMX2572LP: Audio FM using I2S interface

Graeme Parker

Part Number: LMX2572LP
Other Parts Discussed in Thread: LMX2572

Just a preliminary question trying to select the right part.

We have a requirement to FM (modulate) a 100-500MHz carrier using a 20-20kHz analog audio input. The audio is passed through an ADC which then connects direct to the I2S port of the LMX2572LP.

Am I correct in thinking that I have to set the PLL loop filter bandwith to the lower frequency range of my audio input, i.e. 20Hz. Doing so, in the PLLatinum simulator, you obviously get HUGE capacitor values in the region of 3300uF.

If I increase the loop bandwidth to 1kHz, then the cap values are sensible, but does this reduce the audio input bandwith to 1-20kHz?

21 days ago

0 Graeme Parker 20 days ago

Expert 1275 points

I think I've been led up the garden path by Google AI, which tried to convince me that the loop filter bandwidth was a limiting factor for the bandwidth of the I2S data.

Another Google AI conversation said that this is not how the 2572LP works, and the loop filter is not a limiting factor for I2S data.

I would love to hear this confirmed by users of this device who are inputting I2S audio (or other) data.

0 Noel Fung 14 days ago in reply to Graeme Parker

TI__Guru** 121900 points

HI There,

ADC output is still representing the audio "voltage", while the data going to the I2S interface of the LMX2572 represents the corresponding "frequency deviation". That means, you need a logic device to convert voltage to frequency deviation.

See below appnote for details.

www.ti.com/.../snaa309.pdf

0 Graeme Parker 14 days ago in reply to Noel Fung

Expert 1275 points

I'm not sure what you mean. The ADC converts the analog voltage (audio) into a digital I2S stream which goes to the LMX. The higher the voltage, the larger the deviation. What type of logic device is needed?

0 Noel Fung 14 days ago in reply to Graeme Parker

TI__Guru** 121900 points

Hi There,

You need a FPGA or DSP to do voltage to frequency deviation conversion.

0 Graeme Parker 13 days ago in reply to Noel Fung

Expert 1275 points

Voltage to frequency deviation is an analog transformation (scaling - as shown in the diagram).

Sampling is by the ADC, which is then passed to the LMX.

I don't see the need for a DSP or FPGA.

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LMX2572LP: Audio FM using I2S interface