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LMX2572LP: LMX2572LP vs LMX2571 what's the major difference if any ?

citizen
Expert 2840 points
Part Number: LMX2572LP
Other Parts Discussed in Thread: LMX2571

Hi,

Am looking at choosing between the LMX2572LP and LMX2571 synthesizers and at first glance there is not that much of a difference between them other than lower current consumption fir the LMX2571. This might be after all consideration in choosing one over the other …

One other difference is the uWire interface mentioned on the LMX2571 vs the full SPI bus interface on the LMX2572LP. The uWire bus is apparently a subset of SPI, half-duplex and runs at lower clock frequencies from what is written about it… Is this interface on the LMX2571 100% compatible with the standards SPI bus specifications and is a 10MHz clock a problem for it or should I not be too worried about this difference mentioned in the datasheets ?

The features of phase adjustment and ramp generation on the LMX2572LP are not crucial for our application, but otherwise the performance seem to be matched between them.

Have I possibly missed any other major differences ?

Many thanks in advance,

citizen

  • 0
    TI__Guru** 107030 points

    Hi Citizen,

    As you know, LMX2572LP burns more power than LMX2571. This extra power is used to improve the phase noise of VCO. LMX2572LP has much better VCO phase noise than LMX2571. If your application cares the phase noise or jitter, you should use LMX2572LP.

    The programming interface are same in both devices, speed is also similar.

    LMX2571 has two frequency register settings, you can do super fast switching between two frequencies.

    If you need FSK, LMX2572LP can support higher modulated frequency deviation. 

  • 0 in reply to Noel Fung
    Expert 2840 points

    Hi,

    Thanks for your quick reply. At first glance comparison the Phase Noise does not seem to be that much different :

    LMX2572LP

    LMX2571
    Output frequency (Min ) (MHz ) 12.5 10
    Output frequency (Max ) (MHz ) 2000 1344
    Normalized PLL phase noise (dBc/Hz ) -232 -231

    VCO phase noise is not given in the comparison list but I gues there must be a difference that I haven't checked ... On what pages in the datasheets is this information given ?

    Ok for the programming interface.

    The application will be for FHSS radio where frequency switching is required, however what do you consider "super fast switching" ?? We would be very content to switch frequencies within less than 75us but even 100us would probably be ok. Where is this minimum switching speed given ?

    Otherwise we have a dedicated modem for IQ modulation/demodulation. the LMX2572LP will serve as upconversion/downconversion L.O. and stability, phase noise, fast and easy programming would be important factors to consider.

    regards, citizen

  • 0 in reply to citizen
    TI__Guru** 107030 points

    Hi Citizen,

    The VCO phase noise spec is on page 6 for both devices. We need to "scale" the frequency in order to do an apple-to-apple comparison.

    The dual frequency register setting in LMX2571 does not fit for frequency hopping application.

    100µs switching time is doable with both devices as long as we do something to assist VCO calibration. 

    In LMX2571, we can assign which VCO core (we have three VCO cores) to start VCO calibration.

    In LMX2572LP, we can do full assist (datasheet section 8.1.4) to bypass VCO calibration, this method produces the shortest lock time.

    Both devices work well for the LO of a mod/demod.