Tuesday, February 18, 2014

Mixer LO Input Power Level

One item to be aware of on the B/C boards is the mixer LO power input.  The ADF4351 is differentially coupled to the LT5560 mixer.  The highest output power setting of the ADF4351 is +5dBm (pwr=3) while the LT5560 data sheet only quotes LO power inputs of +2dBm.  Power settings of 0, 1, 2 work fine, however, using a setting 3 results in F/2 outputs, where F is the LO input frequency. This only seems to occurs at LO frequencies above 1100MHz.  I speculate that there is some kind of analog frequency divider effect occurring either within the mixer or on the board.  Irrespective of the root cause, operating the mixer LO input above is data sheet level is not recommended.

Using a B-C board combination in the normal SDR configuration with no RF input allows a sinusoid output.  The following figure shows the spectrum of a test signal generated at 10.75MHz. The second and third harmonics are clearly visible at -17dBc and -26dBc respectively.  The fourth harmonic is not easily discernable but is present.

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