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HMC821 frequency synthesizer


We designed a PCB for @HMC821 frequency synthesizer as much as similar to the one depicted in the datasheet of HMC821. and we load registers as below to obtain 1900MHz RF at the output

reg02  : 00000001

reg06:  0003074a

reg09: 00003264

reg05: 0000e80d

reg05:   0000ab95

reg05: 0000d11d

reg05: 00000005

reg03: 000000be

reg04: 00000000

but we obtain in spectrum three different frequencies at approxiametly 900MHz, 1821 MHz ,and 2700MHz, although it says LOCKED.

We couldn't find what can cause this.

is it possible to find source code ?

We also have the eval board of the HMC821 but we are not able to set frequency via its interface when we use an external reference frequency of 10 MHz. I think it works only with 50 MHz reference, which is on the board.

We would appreciate if you suggest.

thank you.


  • Attached is a list of register values that will lock the HMC821 to a 10MHz reference and output a 1900MHz tone, Integer Mode.

    The eval board can operate with 10MHz reference input, however, if in Fractional Mode the CP Offset current (Reg09h[20:14]) should be reduced for the lower comparison frequency.  In Integer Mode, the CP Offset current should be disabled (Reg09h[22:21]=00b).

  • We don't give out the evaluation software source code.

    Can you provide more information about the problem you are observing?

    Are you able to write to and read from the device registers?  For example can you read from Reg00h and get the correct Chip ID (which is 95187h).  Customers frequently have issues with SPI communication because the HMC821 has two SPI modes which can cause problems.  This is discussed here -->

    and here -->

    If you are not able to communicate with the device then you will see tones at 900MHz (high amplitude), 1800 & 2700MHz (lower amplitude) because the device powers up in 'divider' mode (VCO output 'divide-by-2' enabled).

  • This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin