The Sampling IF filter is based on Kaben’s patented Sampled RF Finite Impulse Response (FIR) technology. This technology is the RF equivalent of a digital baseband FIR filter. As such, center frequency, bandwidth, pass band ripple and adjacent channel attenuation can all be preselected to meet performance specification. Unlike traditional, un-tunable RF filters, the Sampled RF Finite Impulse Response technology provides a pass band response with no frequency dispersion similar to a digital baseband FIR filter.
The major blocks of the simplified Sampled RF Finite Impulse Response filter are shown in the accompanying figure. First, the RF input signal is applied to a bank of parallel RF transconductance amplifiers, each of which produces an RF current source that is weighted by a predetermined complex coefficient. The weighted RF current sources are then applied to an RF switching network, which sequentially “rotates” each weighted RF current source output to one (or more) parallel integrators. The outputs of the individual integrators are then sequentially clocked to a sampler, which provides the filtered, sampled analog, baseband representation of the input pass band RF signal.
A simplified example of the new, on-chip, reconfigurable RF filter is shown in the figure. This filter is comprised of four major elements: a current replicator, that generates multiple tap currents, each proportional to an input signal through constants TC0, TC1, and TC2; a “current rotator”, that sends the tap currents to multiple integrators; multiple integrators, where the number of integrators is one more than the number of tap currents; and an output sampling and resetting circuit.
The tap current coefficients chosen determine the filter response, that is achieved:
Here: and are the tap coefficients.
The current rotator that is connected to the tap currents, consists of a switch matrix, which is an array of switches coupled between any of the tap currents and any of the following integrators. Each of the integrators, CI, CI, CI, and CI consists of an operational amplifier and a capacitor. Finally, the output sampling and resetting circuit, which selects the correct integrator output at the correct sampling time, consists of output select switches, Ss1, Ss2, Ss3, etc.
Each of the integrators in the sampling RF filter periodically goes through two operating phases; an integrating phase, during which there is at least one current being received, and a rest phase, when no tap current is received. During the rest phase, the integrator’s charge is sampled by observing the voltage at its output. This is done by closing the corresponding output sampling switch, connecting the integrator to the subsequent circuits. The voltage on the integrating circuit output is then reset using its reset switch.
The above figure shows the timing diagram for the state changes of the 4 clock buses used in the current rotator of the filter shown in the block diagram Here, for example, when CK is high, the current from TC0 is integrated onto CI, then during the time when CK is high, TC1 is integrated onto CI, etc.
The performance of a 140 MHz center frequency, bandpass filter having 256 tap currents, is shown in the above figure (over a wide frequency range), and in the figure below (around the pass band). As expected for a sampling filter, undesired pass bands exist on both sides of the sampling frequency. Since the sampling frequency being use by the filter is high (1.61 GHz), these unwanted pass bands are far from the desired pass band. Furthermore, these unwanted pass bands are attenuated by the inherent sinc function operation of the integrating sampler.
As can be seen in the figure below, a 3 dB bandwidth of 10 MHz is achieved, with excellent pass band ripple (less than +/- 0.5 dB), and excellent adjacent band attenuation (greater than 64 dB).