This
year for EECE 690 the class goal was to design and fabricate a bluetooth
receiver on a chip. The block diagram is shown below with the area
that I designed.
My
job was to determine if it was feasible to develop a fully integrated loop
antenna for the receiver, and then design it. It was found from
the initial research that the radiation resistance of a loop antenna with
sides of 2mm was so small compared to the physical resistance of the antenna
that the idea of a fully integrated antenna was scrapped. So I then
decided to make a relatively small antenna using a trace on the circuit
board. The following discussion is centered around the design of a loop
antenna on the circuit board. The proposed antenna design is shown
below.
Diagram 1: Proposed Antenna Design (black box is the chip (2mm sides) &
square around it is the loop antenna (10mm sides))
The
following table here shows values that are
expected with a change in the distance of one side of the loop antenna
(D) and the number of loops in the antenna (N).
The
formulas used were:
Because
of the Q-enhanced filter that comes after this antenna an output resistance
of 1k Ohms is needed for the loop antenna. By placing a matching network
after the loop antenna we will be able to boost the radiation resistance
and get rid of the XL,, which as seen in the following table
is terrible. There is also a 35fF capacitive load that will be seen by
the antenna, so I will need to use this load when calculating the matching
network. To decide what loop antenna I would use (number of turns and diameter),
I had to first make sure that the Q of the antenna was not too big. The
on chip Q for the inductors and capacitors cannot be larger than 25. A
Q larger than this would make the inductor too large to put on the board.
The next restriction was the number of loops that I would need. I decided
that too many turns would cause capacitance so I restricted the number
of turns to 3. The difference in diameter for an antenna with 1, 2, and
3, loops were 14mm, 10mm, and 9mm respectively for an average Q of 25.
Because of the minimal difference in the 2 and 3 loop antenna I chose the
2 loop antenna with the values as shown below.
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Table 1:2 Turn Loop Antenna
This is the
antenna circuit shown with the matching network.
Figure
1: Antenna Circuit and proposed matching network circuit
To
get the value for C that will make the matching network have an output
resistance of 1k Ohms the following equations were used.
Because
we have a value of XL = j476.8 Ohms we will need to put a capacitor
in series with the inductor so that we can get the needed XL.
Xc needed is -j436.8 Ohms. So now we have:
Figure
2: Antenna Circuit and proposed matching network with Cap to adjust XL.
Matching
Network
Now
need to find the value of the matching network capacitor to get output
resistance of 1000 Ohms. Equations are below.
Because
of the 35 fF capacitive load we need to subtract the Cp above by 35 fF
to get the value needed. Therefore the Cp we need is 1.59 fF.
To
find the efficiency of the antenna I needed to first find the physical
resistance of the trace used to find an approximate Rphy. Other factors
that could change the Rphy could be the resistance in the board as well
as the resistance of the bond wires. These values were not calculated because
they were very small compared to the resistance of the trace. The trace
is approximately 76 mm long, 100 um wide and 10 um thick.
Summary
Problems will
occur with the capacitor that is placed to adjust the XL value. The equation
for the inductance of the antenna is at best still an approximate. If the
inductance of the antenna is different the matching network will not be
at the center frequency. This is a huge problem if the antenna is not passing
the desired frequency; the whole receiver will not operate. To fix this,
the capacitor in series must be tunable off the chip. Once the chip is
fabricated and the antenna is placed the capacitor will be tuned to be
able to pass the desired frequency. The biggest problem is the size of
the capacitor needed for the matching network. In the peregrine process
the smallest capacitor that can be used is 10 fF. So the matching network
capacitor cannot be placed on the chip. Going from 1.5 Ohms to 1000 Ohms
needs a small capacitor. To fix the Rant of 1.5 Ohms a larger loop diameter
is needed. Since there is a restriction to the size of the hearing aid,
this antenna has to be smaller than approximately 20 mm. I experimented
with other values to see if I could come up with a larger matching network
capacitor, but to no avail. So unless someone wants to have a loop antenna
coming out of his or her ear, the loop antenna is not a good choice
for this receiver.
For any questions
concerning the preceding document please email me at jkrause@ksu.edu.
For any other
questions concerning the receiver in whole please go to www.eece.ksu.edu/~tutor.