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MSk on 1120

Andrew Goedhart
Prodigy 180 points
Other Parts Discussed in Thread: CC1121, CC1120

The datasheet for CC1121 and CC1120 state that the receiver can do MSK, but RF studio has no option for this.

Can the receiver be set up to as  FSK with frequency deviation of 0.25 bit rate to get MSK ?

Will the settings be correctly optimised ?

IN that case can I get the receiver to run GMSK using GFSK as well ?

Normally MSK requires a coherent demodulator with the quadrature modulator of the CC1121 and CC1120 will we get good results ?

When running MSK what receiver sensitivity can I expect if I am running it with data rate of 80ks/s and deviation of 20Khz and receiver bandwidth of 100kHz 

  • 0
    TI__Guru**** 317180 points

    Q: Can the receiver be set up to as  FSK with frequency deviation of 0.25 bit rate to get MSK ?

    A: Yes. MSK is just another name for FSK when the frequency deviation is 0.25 bit rate.

  • 0 in reply to TER
    Prodigy 180 points

    The first of the questions was the easy one and almost rhetorical.

    The real question is how well the transmitter and receiver sections deal with the narrow frequency deviations of MSK.

    When increasing the bit rate and narrowing the frequency deviation I noticed that the receiver C1120 becomes more and more dependent on the length and auto correlation properties of the Sync work and the length of the preamble to get reasonable error free output.

    4-GFSK with 15 KHz deviation and 40ks/s if the sync Word is  (set with 3 byte preamble):

    <= 2 byte: I get CRC's when the devices are on the same desk.

    = 3 bytes the RSSI still jumps around 20-40 dB

    = 4 bytes the RSSI is rock solid.

    I get similar issues with the length of the preamble

    <= 1 byte CRC errors

    = 2 bytes the RSSI has 40db jumps

    = 3 bytes it is stable 3-4db variations

    >= 4 bytes or more its rock solid.

    The question really is if I take 2-GFSK or 4-GFSK and narrow the frequency deviation to close to MSK levels do I get the performance I would expect from a receiver with dedicated settings optimised for MSK.

  • 0 in reply to Andrew Goedhart
    TI__Guru**** 317180 points

    From some other posts you have posted I see that you are operating with arounf 100kbps. For this setting the radio uses zero IF and it needs longer time than for low IF to settle the DC filter. Is using a 4 byte preamble/ 4 byte sync a problem in your application?

    You could also look at this post if you need short preamble/ sync: http://e2e.ti.com/support/low_power_rf/f/155/t/228639.aspx