Digital Signal Processing Reference
In-Depth Information
a downconversion mixer and a very linear narrowband feedback-based output
buffer stage. Instead of a direct measurement of the entire frequency range, the
band-of-interest is first downconverted to the passband frequency of the buffer
stage. A sweep over the frequency band provides a reliable indirect measure-
ment of the linearity of the open-loop amplifier.
Also, the chip described in Section 7.6 does not provide a satisfactory means
to tune its gain during operation. Since the amplifier was designed to achieve
optimal distortion performance for a gain setting of 30 dB, changing the ratio
between the tail currents of the gain- and the loading pair would move the bias
point away from the optimal settings for distortion suppression. The correct
way to introduce a tunable gain in the system is to set the last few stages to
a fixed gain and implement a limited amount of gain tuning in the first few
stages. The validity of this suggestion is verified by recognizing that the final
stages of the amplifier are most susceptible to a deviation on their tail current
ratio, as they must handle the largest signal swing. On the other hand, the first
few stages have only a small signal swing over their output terminals. As a
result, a limited offset from the optimal bias point can be tolerated without
adverse consequences for the overall performance of the amplifier. One must
keep in mind that reducing the gain of the very first stage should be avoided,
since this may affect the noise figure of the amplifier. The contribution of the
successive stages to the noise floor, however, is reduced by the voltage gain of
the first stage.
9
The picture at the right shows a close-up of the am-
plifier which is wire-bonded on an alumina substrate.
One of the problems encountered during the measure-
ments was the parasitic impedance of the bonding
wires that connect the microstrip lines with the input
bonding pads.
10
In combination with the capacitance
of the on-chip esd structures, the inductance of the
bonding wires (1-2nH) causes reflections which is seen as ripple at the higher
end of the frequency spectrum (from 750 MHz and up). In the measurement
results of Section 7.9, this effect was eliminated by taking the S
11
reflection
coefficient into account. In a practical application, the effect of the bonding
wires should be moved out of the signal band. The wide fractional bandwidth
of the system, however, prevents that the parasitic bonding wire inductors can
be tuned to the frequency band of interest. A possible solution might be to elim-
inate bonding wires and turn to a flip-chip approach. Nonetheless, the flip-chip
technique is not for the faint-of-heart, so the author left it for future generations
to investigate.
9
Further information on the noise figure of a cascaded system can be found in Section 6.3.
10
The 50
termination resistors are located on-chip.
