III. Gm-C Filtering - Epublications - Université de Limoges

III.4.b Filter Limitations
In the following, Filter 2 has been simulated at higher frequencies in order to quantify
the degradation of the performances when increasing the tuning range of the filter.
Performances are summarized in Table 16.
Table 16. Filter 2 frequency limitations
Frequency Band (MHz) Performances
40 – 240
240 – 470
470 – 750
750 – 1000
- 116 -
Gain = 6dB
Q = 4
IIP3 = 7 to 12.5dBm
NF = 16dB
30mW
Gain = 7-8dB
Q = 4.5
IIP3 = 1dBm at 650MHz
NF = 17dB
30mW
Poor adjacent channels rejection
Gain = 8.5dB
Q = 5-6
IIP3 = -0.5dBm at 1GHz
NF = 17dB
30mW
Poor adjacent channels rejection
This table has been established using the same 10mS transconductances. Higher
frequencies are reached using a smaller fixed capacitance. It comes out that Q increases with
frequency, which explains the origin of the NF increase. This increase of the Q-factor is due
to the higher Q-factor of the fixed capacitance compared to the Q of the other capacitances
due to the Ron of the switch. At 650MHz and up to 1GHz, IIP3 falls down to 0dBm for a Qfactor
which increases up to 6 and a NF of 17dB.
Linearity strongly decreases with frequency. Indeed, using smaller capacitances to
reach higher frequencies of operation makes the Coff of the switches being relatively more
important and this degrades the filter linearity.
Hence, the dynamic range is considerably decreased. Furthermore, a Q-factor of 6 in
UHF is very poor in terms of adjacent channels rejection since N±5 channels are rejected by
only 1dB.