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TRF3722: TRF3722 Output

eSS
Prodigy 20 points
Part Number: TRF3722

Hello,

I'm seeing a strange output from my TRF3722 device. It's configured to generate a tone at 3GHz, using a reference of 50MHz. The output centered at the correct frequency (3GHz) but its generating about 30MHz of equally spaced tones. I've double-checked that all of the voltages are correct and that soldering of the part is good.

I am seeing the lock detect as high, but this is what the loop filter's VTUNE signal looks like:

I have this same design on a few other boards, but I haven't this issue. Have you seen anything like this before, or have an idea of what could be happening?

-ESS

  • 0
    Guru 35930 points
    Sounds like you have some signal leaking to the I/Q inputs.
  • 0
    TI__Intellectual 2245 points

    Hello,

    Something is definitely going on with the Vtune. This should be a fixed voltage, indicating that the modulator has locked on to a frequency. With the Vtune oscillating like the screenshot shows, the VCO will not be able to accurately lock onto a fixed frequency and will be sweeping across a range of frequencies.

    My first suggestion would be to adjust the "CAL_CLK_SEL" to 128. This will increase the time for calibration in order to obtain a more accurate result. If this does not work I would also suggest opening the external loop filter and apply a clean fixed voltage, 1V for example, to observe if you are able to achieve a stable output frequency. The VCO frequency may be a little random but the purpose here will be to observe if you are able to have a fixed output.

    You also stated that you have the same design on a few others boards, is there anything on this board with a frequency that matches the frequency at which the Vtune is osciallating? Something that my be coupling into the external loop filter?

    Regards,

    Oscar

  • 0 in reply to Oscar Guevara
    Prodigy 20 points

    Thank you for your suggestions! 

    1) I did end up injecting a 1V signal into VTUNE, and this did produce a single tone (close to where I excepted it, ~5MHz off). This tone drifted lower in frequency at about 1MHz/min, possibly because its not phase locked?

    2) I've tried playing around with different register settings; I saw some interesting things when changing the CAL_CLK_SEL to 128 and doubling the CP current...I was actually able to see a tone (exactly where I expected). But if I waited about a minute this tone would eventually move down in frequency, and I could see small spikes every 110kHz (same frequency of the VTUNE signal) start to appear.

    Something really curious happened when I started to play with this...if I touched the chip with my finger, I'd see the tone come back towards where I'd expect it, letting it go it would drift down again. I'm curious now if the part isn't connected to the ground pad underneath it, causing the part to possibly overheat. I'm not sure what this does to the charge pump, or how it would correlate with VTUNE oscillating.

    I'll update you as I find more...

    Regards,
    ess