FREQUENCY CHANNEL COMBINER

TECHNICAL FIELD

[0001] The present invention relates to electronics and, more specifically, it relates to methods and systems for processing multiple information bearing signals.

BACKGROUND OF THE INVENTION

[0002] In certain applications, it is advantageous to receive multiple signals located at different frequencies simultaneously. For example, in GPS it is useful to be able to receive the LI, L2, and L5 signals located at 1575.42 MHz, 1227.60 MHz and 1176.45 MHz simultaneously. To do so in a conventional manner, three parallel receiver chains are required, including three parallel analog to digital converters.

[0003] It would advantageous to provide a means for receiving multiple signals with a single receiver chain, and to arrange for the multiple signals to occupy a contiguous base-band spectrum just before digitizing with a single analog-to-digital converter. Such a receiver

architecture can be realized with the use of the Sampling IF Filter.

[0004] The Sampling IF Filter provides filtering and

downconversion functions. A time continuous signal is first sampled (at a rate above the Nyquist rate) . The sampled analog signal is then delayed by different time increments, weighted, and summed to form a Finite Impulse Response (FIR) filter response. The filtered Attorney Docket No. 1002P010WO02 signal is then sampled at a lower rate, often less than the Nyquist rate. As a result of this second sampling, multiple frequencies from the input spectrum appear as aliases in the output. This aliasing allows the Sampling IF filter to also act as a frequency converter, and for the output to be at a different frequency than the input. In particular, the output can be extracted at a frequency band equal to the difference between the incoming signal and the Sampling IF filter clock frequency, in a manner identical to a down-conversion achieved with a mixer and a local oscillator.

[0005] The Sampling IF Filter has other properties that are

advantageous. Since it is a Finite Impulse Response filter, the signal undergoes a constant group delay across all frequencies. In addition, since the weighting functions can include amplification, the filter response can provide gain.

SUMMARY OF INVENTION

[0006] The present invention provides methods and systems for filtering and downconverting input signals. Cascaded sampling IF filters are used to filter and downconvert signals at specific frequencies to simplify their processing. A first sampling IF filter with multiple passbands receives multiple signals. The multiple passbands are preferably centered around the frequencies of interest and the sampling frequency of the filter is carefully selected. Passing the signals through the first filter will downconvert the signals. Second and subsequent sampling IF filters with multiple selected Attorney Docket No. 1002P010WO02 passbands and sampling frequencies will perform the same function until the desired or workable frequencies result .

[0007] In a first aspect, the present invention provides a

method for processing signals comprising: a) receiving a plurality of signals at a first device which also receives a first sampling clock signal b) filtering and downconverting said plurality of signals using said first device and based on a

plurality of passbands, each passband being centered around specific predetermined frequencies, a number of passbands being equal to a number of said plurality of signals, said plurality of signals being downconverted to frequencies based on said first sampling clock signal c) receiving outputs of said first device at a second device which also receives a second sampling clock signal d) filtering and downconverting said plurality of signals using said second device and based on a plurality of passbands, each passband being centered around a frequency of said outputs of said first device, said outputs of said first device being downconverted to frequencies based on said second sampling clock signal. Attorney Docket No. 1002P010WO02

[0008] In a second aspect, the present invention provides a method for processing at least two input signals, the method comprising:

- receiving said at least two input signals, each input signal being received at a separate

transconductance circuit element;

- converting each input signal into a current signal resulting in at least two current signals;

- summing said at least two current signals into a single internal signal;

- filtering said internal signal to result in an output signal.

[0009] In a further aspect, the present invention provides a circuit for processing input signals, the circuit comprising :

- at least two transconductance circuit elements, each transconductance circuit element receiving an input signal and each transconductance circuit element converting said input signal into a current signal;

- a summing node receiving current signals from said transconductance circuit elements, said summing node adding all of said current signals into an internal signal ;

- a signal filter receiving and filtering said

internal signal, said signal filter outputting an output signal . Attorney Docket No. 1002P010WO02

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The drawings show features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which :

FIGURE 1 is an illustration of the frequency spectrum of the input signal.

FIGURE 2 is an illustration of the frequency spectrum of the output of the first device;

FIGURE 3 is an illustration of the frequency spectrum of the final output of the second device;

FIGURE 4 is an illustration of a circuit for combining input signals while filtering the resulting combined signal; and

FIGURE 5 is an illustration of a generalized version of the circuit in Figure 4.

DETAILED DESCRIPTION OF THE INVENTION

[0011] In one aspect of the invention, two or more noncontiguous desired signals can be applied to a Sampling IF Filter which has its finite impulse response coefficients set to create two or more pass-bands, one around each of the desired signals. The Sampling IF Filter clock frequency can be selected such that its frequency or one of the harmonics of its frequency is between the frequencies of the desired signals, such that at the output the non-contiguous signals are now Attorney Docket No. 1002P010WO02 closer to one another in frequency so that less bandwidth is required to hold all of the desired signals .

[0012] The use of a Sampling IF Filter is also advantageous because of its ability to place zeros at arbitrary frequencies. One aspect of the invention is to place these zeros at frequencies that would also alias to the desired output frequencies.

[0013] Another feature of the Sampling IF Filter that has not been previously exploited is that effectively an LO (local oscillator) is available at all multiples of the output frequency. This allows the use of different harmonic multiples of the second sampling clock to serve as an LO for different desired frequencies.

[0014] In one implementation, as an example, the three GPS

bands, LI, L2, and L5 can be applied to a first Sampling IF Filter device which has a pass-band filter response around each GPS band. (See Figure 1) If the Sampling IF Filter clock frequency is chosen to be 1299 MHz, then the three GPS bands will exit the Sampling IF Filter at the Frequencies shown in Figure 2. It should be noted that the spectrum for L2 and L5 will be inverted at this point .

[0015] This composite signal, the output of the first device, can then be passed through a second Sampling IF Filter device, again having a pass-band filter response around each of the down-converted GPS bands. If this second Sampling IF Filter clock frequency is selected to be 225 Attorney Docket No. 1002P010WO02

MHz, then the three GPS bands will exit the second

Sampling IF Filter with equal channel separation, as shown in Figure 3. It should be noted that the spectrum for L2 and L5 have again been inverted, and as a result emerge from the second Sampling IF Filter correctly.

[0016] The three GPS channels have been filtered and down

converted, and have emerged at the base-band as contiguous signals which can be digitized in a single analog-to-digital converter.

[0017] It should be noted that closer frequency spacing between the LI, L2, and L5 outputs can be achieved as long as at least one GPS band is allowed to be frequency inverted at the input to the analog-to-digital converter. For example, if the clock frequency of the first Sampling IF Filter is chosen to be 1373 MHz, then the three GPS channels will emerge from this first Sampling IF Filter at 146 MHz for a frequency inverted L2 channel, at 197 MHz for a frequency inverted L5 channel, and at 202 MHz for the LI channel. If these signals are then applied to the second Sampling IF Filter having a clock frequency of 169 MHz, the three GPS bands will exit this second Sampling IF Filter with equal channel separation, at 23 MHz for the L2 channel, at 28 MHz for the frequency inverted L5 channel, and at 33 MHz for the LI channel .

[0018] Alternatively, a filter with an output sampling

frequency that placed an effective LO close to each desired RF channel can be used. With a sampling frequency of 42 times the chip-rate (1.023 MHz), Attorney Docket No. 1002P010WO02 appropriate LO frequencies can be found at the 37th , 29 ^th , and 27th multiples of the output sampling frequency for the LI, L2 and L5 bands respectively as indicated in Table I. Thus the filter requires a sampling frequency of 42*1.023=42.966 MHz.

Table I

[0019] Other frequencies that those listed in Table 1 are, of course, possible. It should also be noted that the downconversion process (the process of downconverting a signal from an RF frequency to a lower IF frequency) is well-known to those skilled in the art.

[0020] After the first filter which downconverts incoming

signals, the incoming signals may be combined in another manner. Another embodiment of the invention involves the use of multiple transconductance circuit elements, each of which receives a downconverted signal and each of which converts its downconverted signal into a current. The various currents (derived from the various downconverted signals) are then summed at a summing node and the resulting internal signal (a summed current signal) is filtered. Figure 4 illustrates an example configuration of such a circuit.

[0021] Referring to Fig. 4, three input signals INI, IN2, IN3 are each received separately by three transconductance circuit elements Gmi(t), Gm ₂ (t), and Gm ₃ (t) . Each Attorney Docket No. 1002P010WO02 transconductance circuit element converts the input signal into a current (ii(t)), i ₂ (t), and i ₃ (t)) . The various currents are then summed into a single current signal by way of a summing node SN. The single current signal can then filtered. In the example, the signal is filtered by way of a capacitor C. Of course, any suitable filtering means or circuits or circuit elements can be used in lieu of the capacitor. A generalized version of the circuit is illustrated in Figure 5 where the filter (FILTER) is shown generally along with the various transconductance circuit elements.

[0022] The output signal (OUT), a summed, filtered version of the input signals, can be processed further if desired. The output signal can be upconverted, filtered, or can be processed in any manner desired.

[0023] It should be noted that the input signals are,

preferably, downconverted versions of original signals. As an example, the original signals may be RF signals and the downconverted signals may be IF signals. Some example center frequencies for IF signals are shown as IF frequencies in Table 1 above. Some sample RF signals are also given the same Table 1.

[0024] It should further be noted that while Figure 4

illustrates 3 transconductance circuit elements, multiple (i.e. more than 2 or 3) instances of these circuit elements may be used, each one receiving an input signal and each one sending its output to a summing node . Attorney Docket No. 1002P010WO02

[0025] One advantage to the above embodiment is that signals can be more easily combined in the IF sphere as opposed to the RF sphere. As is well-known, combining signals in RF can lead to issues such as handling the power of the signals. By combining the signals in the IF sphere, the RF issues are thereby avoided.

[0026] It should further be noted that the transconductance

circuit elements are active devices and are

programmable. Preferably, the transconductance circuit elements are programmable such that the conversion parameters from the input signal (INX) to the current signal (i _x (t)) are adjustable. Example methods for programmable transconductances can be found in the PCT patent application published under publication number WO2009062306, the entirety of which is hereby

incorporated by reference.

[0027] Of course, the principles of the method described above may be used for other frequencies as well. Furthermore, it is to be noted that while a sampling IF filter is preferable, other devices or combinations of devices which perform the same function as the sampling IF filter may also be used.

[0028] Further information regarding sampling IF filters may be found in the following references, both of which are hereby incorporated by reference in their entirety:

Muhammad K & Staszewski RB (2004) Direct RF sampling mixer with recursive filtering in charge domain. Proceedings of the IEEE International Symposium on Attorney Docket No. 1002P010WO02

Circuits and Systems (ISCAS'04), Vancouver, Canada, May 23-26 2004, 1: 577-580.

Karvonen S, Riley T & Kostamovaara J (2001) A low noise quadrature subsampling mixer. Proceedings of the International Symposium on Circuits and Systems (ISCAS ' 2001) , Sydney, Australia, 4: 790-793.

[0029] A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow .