• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 10 subscribers
  • Views 249 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2664: Inverter switching noise on Local Oscillator

Leonard Cordingley
Prodigy 40 points
Part Number: LM2664

Tool/software:

LM2664 is being applied in a Local Oscillator Circuit; +5.5VDC input, -5.5VDC output. The -5.5VDC feeds circuitry which is not directly related to the PLL.

The LM2664 is operated using ceramic chip capacitors for the switching. CGA3E1X5R1A335K080AC (3.3uF). At the switching frequency, 80kHz, these caps have an ESR of about  20mOhm.

The input  +5.5VDC voltage to the LM2664 also feeds a few LDOs. The LDO outputs are applied to a PLL device for generating the LO/2 frequency for the Local Oscillator.

During Testing of the board, it has been noticed that the Local Oscillator has spurs which are +/- 80kHz offset to the carrier. Some disconnecting and isolating of circuits demonstrated that the source of the spurs is the LM2664.

At the moment, it appears that the main path for the 80kHz signal into the PLL is through the input of the LM2664, going "upstream" to the LDOs and corrupting the low noise outputs for the PLL. 

The sideband level of the spurs is approximately -57dBc. The product spec requires maximum -65dBc spur level.

Through troubleshooting, it is fairly clear that the signal path leading to the LO is conductive.

Please provide some strategies for reducing the switching noise getting to into the sensitive LO system. So far a small series resistor (12ohm) and shunt to ground 10uF tantalum prior to the resistor has been added to the input pin 5 for the LM2664 which reduces the spurs by a few dB, but more is needed. The LM2664 supplies the -5.5VDC load with up to 25mA.

In the attached screenshot, R34 is not populated. R33 is the series resistor at the input pin referred to above.

Thank you

Leonard Cordingley

  • 0
    TI__Genius 10015 points

    Hi Leonard,

    I am in a training, could I take careful look tomorrow?

  • 0 in reply to Nini Zhong
    Prodigy 40 points

    Hi Nini,

    Thank you. Have you had a chance to review this? If I mistakenly sent it to the incorrect forum, please let me know. The issue is a combination of technology areas, RF and power management. 

    Regards,

    Leonard

  • 0 in reply to Leonard Cordingley
    TI__Genius 10015 points

    Hi Leonard,

    Thanks for the patience. The ccap could be more closer to pin5 and 6. Also recommend to add small size capacitance to Pin 6 and Gnd, the then place a lager size cap. That's the way I could put forward now.

  • 0 in reply to Nini Zhong
    Prodigy 40 points

    Hi Nini,

    I tried these things with no improvement.

    Perhaps TI could suggest a way to reduce the pulsed energy in the DC signal which feeds the LM2664, even if it is not entirely eliminated I would like to bring it down by just a few dB. Are there other valid operation conditions for the device which could help in this, such as alternative capacitor values? It might also be helpful if the switching frequency can be increased. Is this possible?

    Thank you,

    Leonard

  • 0 in reply to Leonard Cordingley
    TI__Genius 10015 points

    Hi Leonard,

    Increasing the switching frequency will help but this device now couldn't support high efficiency. So up to now I have no better idea. By the way, could you add a LDO  afterwards?