Digital Signal Processing Reference
In-Depth Information
The current level of implementation requires a cer-
tain degree of calibration to realize the best results.
The end user, however, does not want to be con-
fronted with this precarious task. For this, the bare-
bone open-loop amplifier setup can be extended with
an on-chip monitoring circuit to guarantee good per-
formance over a large range of circumstances (tem-
perature and process variations, varying power supply levels). Fortunately, if
the amplifier is going to be embedded in the signal path of a digital receiver,
most of the complexity required for this purpose is already on-board, in the
form of a digital signal processor. For a fully-featured automated calibration
system, the setup needs to be extended with an on-chip two-tone generator.
The only thing left to do for the signal processor in the back-end is to monitor
some interesting frequencies such as the third-order intermodulation frequency
of the two test tones. Based on measurements of the spurious signal levels at
this frequency, the ratio between the tail currents of the gain- and loading pair
can be fine-tuned in order to obtain the best possible biasing setting. The cali-
bration itself could be done while the receiver is online, by using some unused
frequency bands (for example below the 3 dB cut-off frequency at the lower
end of the spectrum) or off-line, during idle periods in between data reception.
In the meanwhile, though, the manual calibration method was just enough to
demonstrate the promising capabilities of the prototype open-loop amplifier.
