Differential matching of AD5356 dual mixer on CN-0140

Hi all,

I am designing a dual IF receiver and studying the Circuit From The Lab 0140 which is indeed very interesting for me. Maybe someone could explain how the two matching networks between Mixer/SAW filter/VGA have been calculated as I cannot get such values with the Analog Devices RF impedance matching calculator nor with mine.

The network between Mixer/SAW filter is a high-pass (right?) but according to my calculations those values are beyond the freq of interest (which is 153.6MHz).

Could someone shed some light on that?

Cheers,

Michele

Parents
  • Hi Michele,

    The RF impedance calculator is not really applicable in this case.  The SAW filter requires a broadband match and the online tool tends to be too narrowband.  To further complicate the matter, the input match affects the output match and vice versa (making it rather cumbersome).  Nonetheless, I put in the s-parameters of the filter's center frequency just see what it would give.  While it gave a similar configuration to what we used in the Circuit from the Lab, the values were quite off from what was ultimately needed.

    So what is it that is needed?  A match and, more importantly, the correct passband shape.

    The starting point of the circuit design can be any of the following: the simple online matching tool, optimization tools within ADS or the like, or recommendations directly from the SAW filter manufacturer.  Regardless of the starting point, you will likely need to tune to your specific needs (for flat response in the passband).  Finding the right simulation tool is key to speeding up the inevitably empirical nature of the process.

    I am most familiar with ADS and that is why I suggested it, though it can be more than you need (or want).  I had never heard of RFSim99. I downloaded a version this afternoon and it works pretty well with some limitations which you may be able to work around, e.g. only S2P files can be loaded.  S4P files do not seem to be supported on the version that I had, but I may be wrong - it was my first time using it.  I ended up converting the S4P file that I had for the SAW filter to an S2P file.  I loaded the new S2P file in RFSim99 and below are the results of both.  They are virtually the same. You can get to the end point through both avenues.

    I hope this helps.

    Best regards,

    - Carlos

    (note: I attached both the S4P and S2P if you would like to repeat this on your own workstation).

     

    Schematic in ADS

    Schematic in RFSim99

     

     

    Raw SAW filter s-parameters in ADS

    Raw SAW filter s-parameters in RFSim99

    Matched SAW filter in ADS

      

    Matched SAW filter in RFSim99

    attachments.zip
Reply
  • Hi Michele,

    The RF impedance calculator is not really applicable in this case.  The SAW filter requires a broadband match and the online tool tends to be too narrowband.  To further complicate the matter, the input match affects the output match and vice versa (making it rather cumbersome).  Nonetheless, I put in the s-parameters of the filter's center frequency just see what it would give.  While it gave a similar configuration to what we used in the Circuit from the Lab, the values were quite off from what was ultimately needed.

    So what is it that is needed?  A match and, more importantly, the correct passband shape.

    The starting point of the circuit design can be any of the following: the simple online matching tool, optimization tools within ADS or the like, or recommendations directly from the SAW filter manufacturer.  Regardless of the starting point, you will likely need to tune to your specific needs (for flat response in the passband).  Finding the right simulation tool is key to speeding up the inevitably empirical nature of the process.

    I am most familiar with ADS and that is why I suggested it, though it can be more than you need (or want).  I had never heard of RFSim99. I downloaded a version this afternoon and it works pretty well with some limitations which you may be able to work around, e.g. only S2P files can be loaded.  S4P files do not seem to be supported on the version that I had, but I may be wrong - it was my first time using it.  I ended up converting the S4P file that I had for the SAW filter to an S2P file.  I loaded the new S2P file in RFSim99 and below are the results of both.  They are virtually the same. You can get to the end point through both avenues.

    I hope this helps.

    Best regards,

    - Carlos

    (note: I attached both the S4P and S2P if you would like to repeat this on your own workstation).

     

    Schematic in ADS

    Schematic in RFSim99

     

     

    Raw SAW filter s-parameters in ADS

    Raw SAW filter s-parameters in RFSim99

    Matched SAW filter in ADS

      

    Matched SAW filter in RFSim99

    attachments.zip
Children
No Data