Some components of ADC12j4000EVM

Hi MHC

Please refer to the ADC12J4000EVM schematics that are included in the design package available at the following link (link found in the EVM product folder):

http://www.ti.com/lit/zip/slac649

Those EVM features are there for the following reasons:

1. The NCO (Numerically Controlled Oscillator) jumper header near R136 is provided to allow logic control of the NCO_0, NCO_1 and NCO_2 inputs when the ADC12J4000 is operating in any of the DDC (Digital Down Converter) modes. There are 8 possible NCO Frequency/Phase settings that can be selected through either internal register bits, or these 3 logic inputs. Please see section 7.3.6 of the ADC12J4000 datasheet for more information regarding the DDC and NCO.
2. The EXT_VCO input allows the user to apply an input signal to the TRF3765 EXTVCO_IN input pin. This is provided for debug/test of the EVM, but doesn't have any practical use for customers.
3. The OSCin connector allows an external reference clock to be connected to the TRF3765, instead of the on-board 100 MHz crystal oscillator. This could be used to input a reference frequency that is more common in the communications industry like 122.88 MHz. This would then allow communications related sampling frequencies (2949.12MHz or 3072MHz for example) to be generated using integer rather than fractional PLL modes.

Best regards,

Jim B

Hi MHC

To use the OSCin connector one very minor solder change is needed. Resistor R38 must be removed from the board. Please refer to the schematics in the design files referenced previously.

The shorting jack SH-J2_1_2 must also be removed from header J2, this can be done by hand with no tools necessary. This step removes power from the on-board oscillator to prevent crosstalk from that source onto the signal applied at OSCin.

The on-board crystal oscillator has low jitter. If the external source being applied to OSCin also has low jitter then performance using that method may be as good as using the on-board oscillator. In addition, using an external signal applied to OSCin will allow the EVM to perform coherent sampling of the input signal, if the input signal is frequency locked to the external OSCin signal.

If a high quality low noise signal generator is not available I would recommend changing the on-board oscillator to the desired reference frequency instead.

What are you trying to do that cannot be achieved using the standard EVM configuration? If you can explain that I can try to recommend the best approach.

Best regards,

Jim B