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LMX2592: it is found that there are 5/5.2/5.4/5.6/5.8/6.2/6.4/6.6/6.8/7 GHz spurious around the 6GHz frequency of LMX2592 output.

Lian Wu
Genius 5250 points
Part Number: LMX2592

Dears:

Could you kindly help to check the below schematic.

LMX2592 on board outputs 3GHz and 6GHz for mixer local oscillation.

However, it is found that there are 5/5.2/5.4/5.6/5.8/6.2/6.4/6.6/6.8/7 GHz spurious around the 6GHz frequency of LMX2592 output, and the spurious amplitude is about - 70dBc.

This kind of spurious is very similar to phase discrimination spurious, because our phase discrimination frequency is 200 MHz. Our charge pump current is about 12 mA and the loop bandwidth is about 100 kHz. The loop filter is shown in the red box below.

These spurious and 6GHz main signals in the mixer intermodulate 200/400/600/800/1000MHz low-frequency signals just into our intermediate frequency bandwidth. So how to reduce the phase discrimination spurious, whether it can pass through modify the loop filter parameters or LMX2592 configuration parameters to obtain, mainly reducing 10 dB is enough.

Pls. kindly help to give some advice!

  • 0
    TI__Guru* 81580 points

    Hi Lian,

    Assume fosc=100MHz, fpd=200MHz, I verified the performance with an EVM, I also got spurs at multiple of 200MHz offset but the worst one is better than 82dBc. 

    Charge pump current is 12.1875mA; loop bandwidth is greater than 200kHz.

    In general, if the spurs are due to fpd, then reducing the charge pump current will also reduce these spurs. You could also reduce the loop bandwidth in order to reduce fpd spurs.

  • 0 in reply to Noel Fung
    Genius 5250 points

    Hi Neol:

    On customer's board, there are some PD spurs around Lmx2592 output 6GHz signal, especially at 5.2GHz/5.4GHz/6.8GHz, the worst value is about -67dBc.

    To lower the PD spurs, we have changed the Icp to 6.25mA or lower , but that spurs changed a little.

    Then we used a narrow loop filter as below, nothing seemed to help.

    Please help us to check lmx2592 register value, if changing some value can get help. We really need to lower these spurs 10dB at least.

    Hope your reply,

    Thanks. 

    0x40, 0x0077
    
0x3E, 0x0000
    
0x3D, 0x0001
    
0x3B, 0x0000
    
0x30, 0x03FD
    
0x2F, 0x00C5
    
0x2E, 0x0520
    
0x2D, 0x0000
    
0x2C, 0x0000
    
0x2B, 0x0000
    
0x2A, 0x0000
    
0x29, 0x0001
    
0x28, 0x0000
    
0x27, 0x8104
    
0x26, 0x001E
    
0x25, 0x4000
    
0x24, 0x0C11
    
0x23, 0x021B
    
0x22, 0xC3EA
    
0x21, 0x2A0A
    
0x20, 0x210A
    
0x1F, 0x0601
    
0x1E, 0x0034
    
0x1D, 0x0084
    
0x1C, 0x2924
    
0x19, 0x0000
    
0x18, 0x0509
    
0x17, 0x8842
    
0x16, 0x2300
    
0x14, 0x012C
    
0x13, 0x0965
    
0xE, 0x0631
    
0xD, 0x4000

     

     

     

  • 0 in reply to Lian Wu
    TI__Guru* 81580 points

    Hi Luck,

    Register setting is fine. 

    In the layout, where is C1? It is placed closed to CPout pin or not?

  • 0 in reply to Noel Fung
    Genius 5250 points

    Hi Noel:

    Customer has known the reason. 

    When they change frequency from 3GHz to 6GHz, they only set the 0x30 register and there are higher spurs.

    If set the 0x1F register to 0201 same time , the spurs will be lower. 

    Do we need refresh all registers when only changing frequency?

  • 0 in reply to Lian Wu
    TI__Guru* 81580 points

    Hi Luck,

    Please see datasheet section 7.3.9 for the details. Basically, we should turn OFF unused buffers in order to save current as well as reduce crosstalk.

  • 0 in reply to Noel Fung
    Genius 5250 points

    Hi Noel:

    Thanks for your support.

    The issue is closed.