USER’S MANUAL__________________________________________________________________
188 _________________________________________________________________ M211322EN-D
sec pulse (1 MHz bandwidth), the dynamic range of the LNA may be
about 105 dB, and the dynamic range of a single channel of the IFDR
is about 84 dB (from -78 dBm to +6 dBm). In this case, the minimum
separation would be 21 dB. At minimum separation, the overlap of the
low gain channel and the high gain channel will be maximized, and
that overlap is equal to the dynamic range of a signal channel of the
IFDR minus the separation. In this case, the overlap is
(84 dB to 21 dB ) = 63 dB.
- The absolute maximum separation of the channels is the dynamic
range of a single channel of the IFDR. In the above example this
would be 84 dB. At maximum separation, the overlap of the low gain
channel and the high gain channel is zero; we begin using one as soon
as the other has begun to saturate.
We see that there can be a large difference between the absolute minimum
and maximum signal level separations; thus additional criteria must be
considered to choose an optimum value that is between these diverse
limits.
Choosing a proper separation value is a trade-off of several factors. If the
separation value is too low, the IFDRs may end up operating very close to
their noise floors. And if the separation is too high, then the overlap
between the two channels is reduced which makes it difficult for the IFDR
to make a smooth transition as it combines the data from both channels.
Too high a separation may also result in receiver components that are not
practical to build.
As a rule of thumb, channel separations in the 22 dB to 30 dB range
provide a good balance of the above criteria. In the case of a 1 µsec pulse
this results in an overlap interval of approximately 55 dB to 63 dB, which
is sufficient for good IFDR transitions and also leads to receiver
components that are practical to build.
Once a separation value has been chosen, one must consider how to build
the receiver to achieve this. The basic receiver will take the form of an
LNA and a mixer followed by a splitter resulting in a low gain channel and
a high gain channel. We know the gain difference in the two channels (the
separation value), but we must find the actual gain to use in each channel.
If we consider the total system dynamic range as generally set by the LNA
(105 dB in the above example), we can estimate the minimum detectable
signal input to the LNA as well as the maximum usable linear level at the
IFDR. If the LNA has a noise figure of 1dB and we are using a 1 µsec
pulse, the minimum detectable signal at the LNA input is -113 dBm, and
thus the maximum signal is 105 dB above this, or -8 dBm. If we add to
these number the gain of the LNA and the conversion loss of the mixer
(and any other losses experienced through the power splitter for the low
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