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TRF3705 operation in frequency bands

SimonPack
Expert 1630 points
Other Parts Discussed in Thread: TRF3705, TRF3765

I am using a TRF3705 in my wideband design.

I am sometimes seeing that after changing frequency the device doesn't work too well (not very good quadrature balance). However, we have found that it can be fixed by momentarily taking the power down pin (PD) high then low. This seems to 'reset' the TRF3705.

Does the quadrature generator within the TRF3705 work in bands (e.g. below and above around 1.8GHz) requiring a power cycle if moving between these bands ? The graphs in the datasheet seem to indicate a sudden change in behaviour when crossing the 1.8GHz boundary.

Cheers,

Simon.

  • 0
    TI__Mastermind 18065 points
    Simon:
    Interesting. The quadrature generation is accomplished with a polyphase circuit; there are no “bands” to switch between. Performance plots of sideband suppression found in the datasheet show that the performance is smooth across the band; there are no sharp transitions.
    I can not think of any reason the SBS would degrade with changing LO frequency requiring a reset. I have not seen that issue in the past with that device. I assume the source is consistent and the drive level is sufficient before and after the switch. You can check to verify the voltage to the device remains constant during the frequency transition; if there is loading on the supply that drops the ’05 voltage it may come back in a strange state and require a reset.
    --RJH
  • 0 in reply to RJ Hopper
    Expert 1630 points

    Many thanks for your reply.

    The sharp transitions I refer to can be seen around 1.8GHz in datasheet figures 12 to 16. Also the notes for figure 67 seems to indicate a band switch.

    The LO source is coming from a TRF3765. The LO will momentarily disappear when moving frequency (while the TRF3765 calibrates). Can the momentary loss of the LO cause the TRF3705 to get into an invalid state?

    We are definitely seeing a short pulse on the TRF3705 PD pin is clearing some sort of fault. Is there any form of logic within the device that can get into an illegal state?

    Is there a minimum pulse duration allowed on the 'PD' pin?

  • 0 in reply to SimonPack
    TI__Mastermind 18065 points
    Ah, yes there is a transition seen in OIP3 performance that is due to the interaction with the internal balun.
    The TRF3765 does have specific bands and may be the root cause of the issue. The fundamental frequency of the VCO operates from 2.4 GHz to 4.8 GHz. For lower frequency operation dividers are employed. There can be a phase jump when the dividers are employed and if the LO is driven differentially there may be an issue that distorts the imbalance. As a quick experiment you can disable the LON and drive the LO single-ended to see if there is a performance difference.
    I have not experimented with pulsing the LO to see if there is an SBS impact. I would not expect this to cause an issue; however, this is an interesting theory.
    There is no minimum pulse defined for the PD and no anticipated "illegal states".
    --RJH