A. Isolation
Study for isolation between RF blocks can be divided into two
stages: In the 1950’s to the 1980’s before the RFIC development,
studies were focused on the shielding. As long as an electronic
equipment contained RF blocks, a metallic box made by material with
high conductivity usually covers each RF block for the good
shielding. It is very heavily involved the mechanical works and the
cost of the shielding boxes is sometimes more expensive than the
device and all of the parts in the circuits. However, nobody dare
to think about the implementing of the RF blocks with integrated
circuit technology. On the other hand, in the implementing of the
digital integrated circuit, the dramatic reduction of cost was very
attractive and encouraging. In the 1980’s, the scientists and
engineers started to integrate RF circuit using the integrated
technology. It was, of course, a very brave action. Up to 1995, the
experiment for isolation had been achieved with a great step
forward. The isolation between blocks approached 40-50dB. RFIC
technology was becoming realistic and mass production began.
There are three kinds of isolation. The first is the isolation
between RF blocks. The second is the isolation between digital
blocks. The third one is the isolation between RF and digital
blocks. The first two have similar difficult points and have the
same issue of the solution with the exception of the case when the
PA block, which must be specially treated, is involved. The third
one is somewhat more complicated. Normally the power of the RF
block is much higher than that of the digital block since the
current in the RF block is in the order of mA while the current in
the digital block is uA. From the power viewpoint it looks like
that a digital block is easier to be disturbed than the RF block.
From the frequency viewpoint, an RF block is easier to be disturbed
than a digital block because the signal in a digital block is
basically a pulse. A narrow pulse is a wide band signal. It
contains high frequencies, including RF components even its digital
frequency is lower than the RF. Therefore, the third isolation is
somehow difficult than others.
A question might be raised up to a sensitive reader: how many dB of
isolation is enough for the practical purpose? In the wireless
communication system, the ideal isolation level should be around
130dB, which is 10 dB higher than the total gain of the useful
signal from antenna to the data output terminal. When the
un-isolated signal goes through the entire communication system and
gets a maximum of gain, 120dB, it is still at a level under the
sensitivity of 10dB. Then, the communication system never find its
existence. As mentioned above, when the isolation between blocks is
achieved up to 50-60dB, it makes RFIC becomes realistic and
available to applied into a system, say, communication system.
However, it is still far from the ideal goal and the research of
isolation must be continued.
B. High Q Inductor for IC
The project of high Q IC inductor is the key growing point in the
future RFIC as well as SOC development.
At present, the Q value of an IC inductor at RF range is about 5 to
10 in al l of IC processes. It is much poorer than that of a chip
of discrete inductor, in which the Q value of inductor is about 80
to 150. The low Q value of spiral inductors brings about the
additional noise to the RF blocks, and it is hard to be applied in
the implementation of a filter or an oscillator.
The second problem is that a large die area must be provided to the
spiral inductor. In order to reduce the cost and to simplify the
hardware works, most IC inductors are constructed on the chip. The
die area for each spiral inductor is about 10 to 100 times larger
than the area of the device or other parts. Looking at any of
existing RFIC layouts, about 60% to 80% of the area is occupied by
the IC spiral inductors. The IC spiral inductor becomes the main
factor of the total IC cost.
;){kind=link}