A-to-D Converter Does Frequency Translation – Design Note 259
Derek Redmayne
The need to characterize frequency sources, both in the
laboratory and in the п¬Ѓeld, is increasingly important. The
circuit in Figure 1 offers some interesting attributes in
a compact and relatively inexpensive scheme. It uses
an LTC В®1420 ADC to undersample a higher frequency,
driving an LTC1668 DAC, followed by a п¬Ѓlter to perform a
down conversion. The output of the п¬Ѓlter is subsequently
resampled to produce a manageable sample rate for a
single-chip microcontroller. In addition to characterizing
the carrier in an IF strip or the output of a local oscillator,
this technique is also useful for characterizing ADCs,
DACs, clock sources, signal sources or the effects of
logic devices or phase-locked loops on phase noise.
Frequency conversion or translation is usually performed by a diode mixer or a Gilbert cell mixer. Down
conversion is most often encountered in radio receivers;
up conversion is more commonly used in transmitters. The common superheterodyne receiver usually
involves one conversion to produce a п¬Ѓxed intermediate
frequency (IF). Spectrum analyzers, cellular base stations, cable modems, microwave and satellite receivers,
radar and optical communications systems all include
frequency conversion blocks. Down Conversion with an ADC
It may not be commonly known that down conversion …