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      • Slowing Down a CMOS Output

        Silicon_Labs | 12/344/2013 | 06:29 PM
        Question

         

        What are some practical ways to slow down a CMOS output edge?




        Answer

         

        Sometimes a CMOS output format buffer may have too high a drive strength and/or be too fast for a particular application.  In such a case, you may want to reduce the risetime and falltime for signal integrity or EMI reasons. Assuming you can't select a different part, or program a different edge rate, there are 2 circuit modifications you can try.  These can be done separately or together in an attempt to filter the output and slow it down.  Some experimentation may be necessary.

          1.  Add some series resistance at the buffer output, typically a few Ohms to low 10s of Ohms.  (You do not want to significantly impact any source matching if driving a transmission line.) 
          2.  Add some shunt capacitance at the load, typically a few pF.  
         



      • Si41xx-BM and Si41xx-BT Devices Unavailable

        Silicon_Labs | 12/344/2013 | 02:57 PM
        Question

         

        Why are Si41xx-BM and Si41xx-BT devices unavailable for order?




        Answer

         

        Per Process Change Notice #0501071 dated effective 07JAN05, the lead finish version RF Synthesizers were replaced by lead free versions.  So for example, P/Ns Si4133-BM and Si4133-BT were replaced by Si4133-D-GM and Si4133-D-GT.



      • Si4133W Unavailable

        Silicon_Labs | 12/344/2013 | 02:39 PM
        Question

         

        Why is the Si4133W unavailable for order?




        Answer

         

        The Si4133W RF Synthesizer, i.e. the wide temperature operation version of the Si4133, was sold as a product line by Silicon Labs to NXP as part of the Aerophone technology sale in 2007.  NXP then briefly sold it as the Aero4133, discontinuing it at the end of 2008.



      • Si53x/55x/57x/59x AC-Coupled LVPECL Bias Resistors

        Silicon_Labs | 12/343/2013 | 09:54 PM
        Question

         

        What value bias resistors are required for Si53x/55x/57x/59x AC-Coupled LVPECL?   




        Answer

         

        Application Note AN291 applies to the Si53x, Si55x, Si57x, and Si59x XOs and VCXOs.  Figure 2 shows 130 Ohm near end bias resistors being used for AC-Coupled LVPECL.  

        However, the LVPECL bias circuit is very robust and was designed to support bias resistors in the range 100 Ohms to 200 Ohms to allow for replacement compatibility with installed oscillators in old designs.  For new designs we recommend operating closer to the center or 'sweet spot' of this design range, e.g. 130 Ohms to 150 Ohms.    



      • Nominal Si55x VCXO Operation

        Silicon_Labs | 12/343/2013 | 09:23 PM
        Question

         

        What is the Vc recommendation for operating an Si55x VCXO at nominal output frequency?




        Answer

         

        Depending on your application (low Kv for example) you might get away with just letting Vc float to a high or low until your signal is connected.  However, if you want the unconnected control voltage to be “zero” or nominal VDD/2 you should set the control voltage using a voltage divider. You would have to account for the voltage source’s impedance to see how stiff to make the divider.  You could also follow all this with a non-inverting op amp to act as a buffer driving the VCXO.

        This KB entry applies to all current Si55x devices including the Si550, Si552, Si554, and also applies to the Si571, Si595, Si597, and Si599 devices. 



      • Default Si55x VCXO Control Voltage

        Silicon_Labs | 12/343/2013 | 08:58 PM
        Question

         

        Does the Si55x VCXO Vc input go to the nominal control voltage or VDD/2 when left unconnected?




        Answer

         

        No, you can't count on the Si55x VCXO Vc input going to the nominal control voltage or VDD/2 when left unconnected.

        The input sees a shunt nominal 50 pF to GND capacitor which is in turn connected to the high impedance gate input of a MOSFET.  There are also some calibration switches that are normally off that connect to this input line.  Therefore, the “floating” input control voltage depends on very small leakage currents on the switch devices attached to the signal path.  These will have a leakage current “fight” to see who wins and they will set an equivalent pull-up or pull-down. 

        This KB entry applies to all current Si55x devices including the Si550, Si552, Si554, and also applies to the Si571, Si595, Si597, and Si599 devices.