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• The absolute minimum channel separation is equal to the total dynamic range of the
receiver minus the dynamic range of a single channel of the IFDR.
Generally, the total dynamic range of the receiver is set by the LNA.
For example, for a 1μ 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
(-78 ... +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 is 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 channel separation 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 0. We begin using one as soon as the other has begun to saturate.
There can be a large difference between the absolute minimum and maximum signal level
separations. You must consider additional criteria 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 operate very close to their noise floors. If the separation is too high,
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 ... 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 ... 63 dB, which is sufficient for good IFDR transitions and also leads to
receiver components that are practical to build.
Receiver
Once you have chosen a separation value, you must consider how to build the receiver to
achieve this value.
The basic receiver is 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 gain and high gain channels), we can use this information to
determine the signal values of the components in these two channels.
Chapter 7 – Processing Algorithms
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