The internal 0° and 90° LO phases are digitally generated by a
divide-by-4 logic circuit. The divider is dc-coupled and inherently
broadband; the maximum LO frequency is limited only by its
switching speed. The duty cycle of the quadrature LO signals
is intrinsically 50% and is unaffected by the asymmetry of the
externally connected 4LO input. Furthermore, the divider is
implemented such that the 4LO signal reclocks the final flipflops
that generate the internal LO signals and thereby minimizes
noise introduced by the divide circuitry.
For optimum performance, the 4LO input is driven differentially,
but it can also be driven single-ended. A good choice for a drive
is an LVDS device as is done on the AD8339 evaluation board.
The common-mode range on each pin is approximately 0.2 V to
3.8 V with the nominal ±5 V supplies.
The minimum 4LO level is frequency dependent when driven
by a sine wave. For optimum noise performance, it is important
to ensure that the LO source has very low phase noise (jitter)
and adequate input level to ensure stable mixer core switching.
The gain through the divider determines the LO signal level vs.
RF frequency. The AD8339 can be operated at very low frequencies
at the LO inputs if a square wave is used to drive the LO, as
is done with the LVDS driver on the evaluation board.
Beamforming applications require a precise channel-to-channel
phase relationship for coherence among multiple channels. A
reset pin is provided to synchronize the LO divider circuits in
different AD8339s when they are used in arrays. The RSET pin
resets the dividers to a known state after power is applied to
multiple AD8339s. A logic input must be provided to the RSET
pin when using more than one AD8339. Note that at least one
channel must be enabled for the LO interface to also be enabled
and the LO reset to work. See the Reset Input section for more
information.