Antenna Filter LNA Codesign Using reflections, rather than fighting them'

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Antenna - Filter - LNA CodesignUsing
reflections, rather than fighting them.

• Sayf Alalusi
• Robert Brodersen

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Why Not Match?

• On-chip inductors are low-Q avoid them.
• We are going to run the received signal straight
  into the gate of a MOSFET
• Gate of a MOSFET is an open-circuit

Open-Circuit
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Problem 1

• Assuming that the input to the LNA is a MOSFET,
  and that the MOSFET device is primarily a V to I
  converter, not a power gain device
• Then the Antenna and RF Filter should present the
  maximum possible voltage over the frequency band
  of interest only. They should attenuate voltages
  outside the band.

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Problem 2

• The antenna, filter and LNA should ultimately
  maximize the SNR at the output of the LNA.
• For a given gain, SNR is maximized by minimizing
  noise figure (NF).
• NF is a strong function of the impedance seen at
  the input to the LNA, in this case, the filter.

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Antenna to Filter Interface

• The system cannot exceed the received SNR at the
  terminals of the antenna.
• Any loss in the path from the antenna to the
  circuits shows up directly as noise.
• Therefore, we want an impedance match at the
  antenna to filter interface

G 0
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The Filter

• Microstrip coupled line filters are like a series
  of coupled resonators.
• Each resonator can only support frequencies with
  ?/2 L

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Filter to LNA Interface

• The last segment is terminated by the input to
  the LNA, so it will support a standing wave as
  well, due to the reflection.
• Also, since there is a full reflection, we will
  get double the input voltage to the LNA for free,
  versus a power (impedance) match.
• This situation also results in a very near
  optimum noise figure for the LNA.

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Backscatter/Re-radiation

• Due to the full reflection at the input to the
  LNA, all of the received energy is re-radiated
  into the air.
• However, the power levels we are dealing with are
  very low, and re-radiated energy being received
  at another receiver is spread over 2 spheres
  (1/r4)

1/r2 path loss
1/r2 path loss
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MEA Systems

• If we design MEA systems with todays
  methodology, the off-chip interface components
  will be too crowded (see Bob Frye)
• Since the reflection, and hence matching, are not
  an issue, we can essentially choose the line
  impedance, and therefore line width to suit our
  needs.