`Moore et al.
`
`I 1111111111111111 11111 lllll lllll 111111111111111 1111111111 1111111111 11111111
`US006577855Bl
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,577,855 Bl
`Jun.10,2003
`
`(54) LOW IF RECEIVER
`
`OTHER PUBLICATIONS
`
`(75)
`
`Inventors: Paul A. Moore, Seaford (GB); Michael
`E. Barnard, Redhill (GB); Astrid
`Paula Maria Schweer, Niirnberg (DE)
`
`(73) Assignee: U.S. Philips Corporation, New York,
`NY (US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by O days.
`
`(21) Appl. No.: 09/377,357
`
`(22) Filed:
`
`Aug. 19, 1999
`
`(30)
`
`Foreign Application Priority Data
`
`Aug. 25, 1998
`
`(GB) ............................................. 9818397
`
`Int. Cl.7 .................................................. H04B 1/26
`(51)
`(52) U.S. Cl. ........................................ 455/324; 455/303
`(58) Field of Search ................................. 455/303, 324,
`455/255, 257-259, 265, 192.1, 192.2, 302;
`348/235
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`5,003,621 A * 3/1991 Gailus ........................ 455/209
`5,398,080 A * 3/1995 Sakashita et al. ........... 348/735
`5,548,619 A * 8/1996 Horiike et al. .............. 375/344
`5,584,068 A * 12/1996 Mohindra ................... 455/324
`5,715,529 A * 2/1998 Kianush et al. ............. 455/266
`6,148,181 A * 11/2000 Otaka .......................... 455/86
`
`By RF Design, Rod Green & Richard Hosking, "High
`Performance Direct Conversion" Electronic World, Jan.
`1996, pp. 18-22.
`* cited by examiner
`Primary Examiner-Nguyen T. Vo
`Assistant Examiner-Nghi H. Ly
`
`(57)
`
`ABSTRACT
`
`A low IF receiver suitable for use in cellular and cordless
`telephones and TV tuners, comprises input signal receiving
`means(l0), quadrature related frequency down conversion
`means including first and second mixing means(14, 15)
`having first inputs(12, 13) coupled to the input signal
`receiving means and a local oscillator means(16) for gen(cid:173)
`erating a local oscillator signal having a frequency offset
`from a nominal centre frequency of the input signal coupled
`to second inputs(18, 19) of the first and second mixing
`means, a channel selectivity polyphase filtering means(20)
`for selecting a wanted channel signal from outputs of the
`first and second mixing means, means(26, 28, 30) for
`detecting when the quality of the frequency down converted
`signal becomes unacceptable due a large adjacent channel
`interfering component and means responsive to said detec(cid:173)
`tion for changing the local oscillator frequency by at least
`twice the low IF or one channel bandwidth to displace the
`large interfering component outside the bandwidth of the
`channel selectivity polyphase filtering means(20) and simul(cid:173)
`taneously changing the centre frequency of the polyphase
`filter.
`
`11 Claims, 4 Drawing Sheets
`
`14
`
`I
`
`17
`
`16
`
`34
`
`-
`
`-
`
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`
`10
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`I
`
`32
`'
`---+---'-----7
`I
`I
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`I
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`I
`
`L..l.. J
`
`a
`
`34
`
`15
`
`28
`
`30
`
`Ex.1015
`APPLE INC. / Page 1 of 9
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`
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`U.S. Patent
`
`Jun.10,2003
`
`Sheet 1 of 4
`
`US 6,577,855 Bl
`
`14
`
`10
`
`17
`
`20
`
`I
`
`16
`
`Q
`
`FIG. 1
`
`22
`
`10
`
`34
`
`14
`
`I
`
`17
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`Ti/
`2
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`16
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`34
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`I
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`I
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`32
`---+---L----7
`I
`I
`I
`I
`I
`I
`I
`
`~ ,-..,,:::::,
`
`µC
`
`30
`
`28
`
`FIG. 2
`
`Ex.1015
`APPLE INC. / Page 2 of 9
`
`
`
`i,-
`~
`(It
`(It
`~
`_,.""-l
`-...,l
`(It
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`FIG. 3
`
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`I I
`i
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`te:lf!vi 1i~ i TC7 i i T
`i I I I IJC9 ~ 1 ~11 ~ 18 ~13 ~ 19
`J2I
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`h i JC7 il lcsl ~l
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`
`~40 I~ I
`
`Ex.1015
`APPLE INC. / Page 3 of 9
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`U.S. Patent
`
`Jun. 10, 2003
`
`Sheet 3 of 4
`
`US 6,577,855 Bl
`
`FIG. 4A
`
`FIG. 48
`
`FIG. 5A
`
`FIG. 58
`
`FIG. 6A
`
`FIG. 68
`
`Ex.1015
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`U.S. Patent
`
`Jun.10,2003
`
`Sheet 4 of 4
`
`US 6,577,855 Bl
`
`50
`
`54
`
`56
`
`N
`
`N
`
`y
`
`y
`
`FIG. 7
`
`60
`
`Ex.1015
`APPLE INC. / Page 5 of 9
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`US 6,577,855 Bl
`
`1
`LOW IF RECEIVER
`
`FIELD OF THE INVENTION
`The present invention relates to a low IF receiver, which 5
`expression is intended to cover a low IF receiver section of
`a transceiver. Such a receiver is suitable for fabrication as an
`integrated circuit for use in telecommunications products,
`such as cellular and cordless telephones, and TV tuners.
`BACKGROUND OF THE INVENTION
`Low IF receivers are frequently implemented as
`polyphase or sequence asymmetric receivers. FIG. 1 of the
`accompanying drawings illustrates a typical polyphase
`receiver. A signal received by an antenna 10 is applied to first
`inputs 12, 13 of quadrature related mixers 14, 15. A local 15
`oscillator 16. generating a local oscillator frequency which
`is offset typically by half a channel spacing from the nominal
`carrier frequency of the received signal is applied by way of
`a ninety degree phase shifter 17 to a second input of the
`mixer 14 and directly to a second input 19 of the mixer 15. 20
`The outputs of the mixers 14, 15 are the I and Q channels,
`respectively. These channels are applied to a polyphase filter
`20 functioning as a band pass filter. The outputs of the
`polyphase filter 20 are applied to respective inputs of a
`demodulator 22 which provides an output, audio frequency 25
`signal on an output terminal 24.
`From FIG. 1 it can be seen that the basic polyphase
`receiver structure is very similar to a zero IF receiver in that
`the IF consists of a quadrature pair of channels, the in-phase
`or I channel and the quadrature-phase or Q channel. 30
`However, unlike the zero-IF receiver where the local oscil(cid:173)
`lator is run at the centre frequency of the wanted signal, in
`the polyphase case the low-IF is achieved by running the
`receiver local oscillator at typically half a channel spacing
`from the centre frequency of the wanted signal. Thus for 35
`example in GSM (Global System for Mobile
`Communications) with a channel spacing of 200 kHz, if the
`desired signal is at 950 MHz then the local oscillator could
`be at 950.1 MHz giving an IF of 100 kHz. The principal
`advantage of the polyphase approach over the zero-IF 40
`approach is that large DC offsets which are unavoidably
`generated by the two mixers are no longer in the IF band(cid:173)
`width and therefore do not interfere with the demodulation
`of the wanted signal.
`From the example frequencies given it is obvious that any 45
`signal present at 950.2 MHz will form an image signal also
`at an IF frequency of 100 kHz. In principle this is not a
`problem since the polyphase filter is able to distinguish
`signals on its complex inputs having opposing phase
`relationships, as is the case between the desired and the 50
`image frequency. However, this image rejection capability is
`in practice limited by the level of matching and balance that
`can be achieved in amplitude and phase between the I and
`Q channels, which means that an extremely strong signal at
`the image frequency will cause interference to the desired 55
`signal in the IF if the desired signal is too weak. Such a
`problem does not appear too serious for the DECT and GSM
`systems because there is a relatively relaxed specification on
`the adjacent channel rejection required by a receiver. How(cid:173)
`ever if the polyphase approach is extended to other systems 60
`not having such a relaxed specification on the adjacent
`channel rejection then image rejection will become more of
`an issue.
`
`SUMMARY OF THE INVENTION
`An object of the present invention is to improve image
`rejection in low IF receivers.
`
`2
`According to one aspect of the present invention there is
`provided a low IF receiver comprising frequency down
`conversion means including a local oscillator, a channel
`selectivity polyphase filtering means for selecting a wanted
`channel signal from the output of the frequency down
`conversion means, means for detecting when the quality of
`the frequency down converted signal becomes unacceptable
`and means responsive to said detection for changing the
`local oscillator frequency and the centre frequency of the
`10 polyphase filter.
`More particularly the present invention provides a low IF
`receiver comprising input signal receiving means, quadra(cid:173)
`ture related frequency down conversion means including
`first and second mixing means having first inputs coupled to
`the input signal receiving means and second inputs coupled
`to a local oscillator means for generating a local oscillator
`signal having a frequency offset from a nominal centre
`frequency of the input signal, a channel selectivity
`polyphase filtering means for selecting a wanted channel
`signal from outputs of the first and second mixing means,
`means for detecting when the quality of the frequency down
`converted signal becomes unacceptable and means respon(cid:173)
`sive to said detection for changing the local oscillator
`frequency and the centre frequency of the polyphase filter.
`In one embodiment of the present invention the means for
`changing the local oscillator frequency also inverts the sign
`of one of the local oscillator signals. The inversion of one of
`the local oscillator signals effectively tunes the polyphase
`filter by twice the IF.
`In another embodiment of the present invention the
`detecting means monitors the potential image frequencies
`and causes the local oscillator or frequency changing means
`to change the local oscillator frequency in response to
`deciding which image frequency to use.
`The detecting means may comprise a wideband polyphase
`filter, means for identifying the frequencies present in the
`output of the wideband polyphase filter, and image fre(cid:173)
`quency analysing means for providing a control signal for
`altering the frequency of the local oscillator and the centre
`frequency of the channel selectivity polyphase filtering
`means and the wideband polyphase filter.
`By being able to simultaneously alter the local oscillator
`frequency and the centre frequency of channel selectivity
`filter and other complex signal processing means, such as
`signal de-rotation rate, it is possible to allow the image
`frequency to be dynamically steered away from that of any
`unwanted strong signals. This feature can be used to enhance
`the performance of inexpensive receivers so that they can
`meet more stringent type approval requirements or alterna(cid:173)
`tively enable high performance products to be built which
`would have an on-the-air performance better than that which
`can be met with conventional superheterodyne and homo(cid:173)
`dyne designs.
`According to another aspect of the present invention there
`is provided an integrated circuit comprising quadrature
`related frequency down conversion means including first
`and second mixing means having first inputs for an input
`signal and second inputs for a local oscillator signal having
`a frequency offset from a nominal centre frequency of the
`input signal, a channel selectivity polyphase filtering means
`for selecting a wanted channel signal from outputs of the
`first and second mixing means, means for detecting when the
`quality of the frequency down converted signal becomes
`65 unacceptable and means responsive to said detection for
`providing a control signal for changing the local oscillator
`frequency and the centre frequency of the polyphase filter.
`
`Ex.1015
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`US 6,577,855 Bl
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`3
`According to a further aspect of the present invention
`there is provided a method of operating a low IF receiver,
`comprising the steps of monitoring the quality of a fre(cid:173)
`quency down-converted signal; if the signal quality is
`unacceptable, changing the local oscillator frequency by the
`equivalent of a channel bandwidth; determining if the signal
`quality is better; and if not, reverting back to the prior local
`oscillator frequency.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention will now be described, by way of
`example, with reference to the accompanying drawings,
`wherein:
`FIG. 1 is a block schematic diagram of the already 15
`described zero IF polyphase receiver.
`FIG. 2 is a block schematic diagram of one embodiment
`of a low IF receiver made in accordance with the present
`invention,
`FIG. 3 shows a typical polyphase filter which can be used
`in the embodiment shown in FIG. 2,
`FIGS. 4A and 4B illustrate frequency distribution at RF,
`FIGS. SA and SB illustrate infradyne operation,
`FIGS. 6A and 6B illustrate supradyne operation, and
`FIG. 7 is a flow chart of an algorithm which effects a
`change of frequency conversion mode.
`In the drawings the same reference numerals have been
`used to indicate corresponding features.
`
`4
`The filter comprises 2 sets of stages, corresponding stages in
`each set being identical. For convenience of description only
`one set of the stages will be described and the corresponding
`components in the non-described set will be shown in
`5 parenthesis. A first stage of the filter comprises a source
`resistor Rl (R3), a capacitor Cl (C6) and a transconductor
`JCl (JC9) coupled in parallel between signal rails 38, 40. A
`cross-coupled gyrator consisting transconductors JC26 and
`JCl 7 is coupled to the signal rails 38, 40. A current source
`10 Jl (12) representing the inputs is coupled to the signal rails
`38, 40. The current sources Jl, 12, correspond to signal
`inputs from the mixers 14, 15 (FIG. 2).
`The second, third and fourth stages are of identical layout
`and will be described collectively. A transconductor JC2
`(JClO), JC4 (JC12), JC6 (JC14) is connected in parallel with
`a capacitance C2 (C7), C3 (CS), C4 (C9) and another
`transconductor JC3 (JCll), JCS (JC13), JC7 (JC15) between
`signal rails 42, 44. Cross coupled gyrators JC19 (JC18),
`JC21 (JC20) and JC23 (JC22) are coupled to the signal rails
`20 42, 44 of the respective stages. The fifth stage of the filter
`comprises a transconductor JCS (JC16), capacitance CS
`(ClO) and resistor R2 (R4) connected in parallel between
`signal lines 46, 48. A cross coupled gyrator JC25 (JC24) is
`coupled to the signal rails 46, 48. Outputs are derived from
`25 the fifth stages. In practice the transconductors JCl 7 to JC26
`are all controlled by a common current control circuit that is
`adjusted to tune the centre frequency of the filter 36.
`Reverting to FIG. 2, the signal from the wideband
`polyphase filter 26 is applied to the FFT stage 28 which is
`30 sufficiently refined as to identify which channels potential
`interfering signals are located in. Finally the microcontroller
`30 analyses the signal strengths present in all of the potential
`image channels and selects the appropriate IF frequency to
`minimise potential image breakthrough by controlling the
`35 local oscillator 16 frequency and tuning the polyphase filters
`20 and 26 to the appropriate centre frequency using signals
`on a common control line 32.
`If there is some latitude in the choice of IF frequency then
`other considerations, such as the lowest power setting of the
`40 receiver, may then also be taken into account in the final IF
`frequency choice.
`The present invention is based on operating the frequency
`down conversion means, that is the mixers 14, 15 in two
`alternative modes, one in which the local oscillator fre(cid:173)
`quency fLo is greater than the nominal RF channel frequency
`fch, so called supradyne operation and another in which fLo
`is lower than fch, so called infradyne operation. In a simple
`mode of operation the microcontroller 30 switches the local
`50 oscillator frequency fLo by twice the IF, that is by 2fIF, to
`identify which channels the potential interfering signals are
`located in.
`In order to illustrate the operation of the receiver made in
`accordance with the present invention reference is made to
`55 FIGS. 4A, 4B, SA, SB, 6A and 6B.
`FIGS. 4A and 4B show the frequency distribution at RF
`with the stronger interfering components being below the
`wanted channel fch in FIG. 4A and above the wanted
`channel in FIG. 4B. The drawings also indicate the local
`oscillator frequencies fLoJ and fLo,s and their negative
`counterparts for infradyne and supradyne operation.
`FIGS. SA and SB show the frequency distributions at IF
`after quadrature conversion for infradyne operation. The
`bandwidth of the channel selectivity polyphase filter 20 is
`65 referenced B20 . The broken line indicates the practically
`limited rejection on the image side. By comparing FIGS. SA
`and SB it will be noted that in FIG. SA the larger interfering
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`45
`
`Compared to FIG. 1, the embodiment of the invention
`shown in FIG. 2 includes additional features, namely a
`wideband polyphase filter 26, Fast Fourier Transform (FFT)
`stage 28 and a microcontroller 30, which enable the receiver
`to monitor potential image frequencies and make an optimal
`choice at any instant of an IF and the consequent image
`frequency to use. One method of changing the sign of the IF
`in one channel, the Q channel, whilst leaving the sign of the
`other channel, the I channel, unchanged is to invert the phase
`of the local oscillator 16 output by providing a switchable
`inverter 34 in the local oscillator path to the input 19 of the
`mixer 15. An alternative method of changing the sign of the
`IF in one channel whilst leaving the sign of the other
`channel, the I channel, unchanged is to locate the inverter 34
`in the output path of the mixer 15, as shown in broken lines,
`thus inverting the Q channel. Optionally the inverter 34 may
`also be used in the I channel. Digital implementations are
`also possible. Irrespective of the actual method of changing
`the sign, this always has the effect of retuning the polyphase
`filters 20 and 26.
`More particularly the I and Q channels are coupled to the
`polyphase filter 26 which has a bandwidth wide enough to
`allow through the IF signals corresponding to potential
`image frequencies, which might extend for instance to three
`channels either side of the wanted signal. Since there is only
`a requirement to detect strong signals in this frequency range
`this filter 26 does not need a very high dynamic range, rather 60
`only enough sensitivity to guarantee detecting reliably an
`image signal that would cause an image rejection problem.
`This means that the polyphase filter 26 does not require so
`much resource in terms of silicon area or DSP instruction
`cycles as does the channel selectivity polyphase filter 20.
`FIG. 3 illustrates an embodiment of a 5th order polyphase
`filter 36 which can be used as filter 20 and/or 26 in FIG. 2.
`
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`US 6,577,855 Bl
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`6
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`5
`component lies within the limited image rejection of the
`filter 20 whereas in FIG. SB it is the smaller interfering
`component.
`FIGS. 6A and 6B show the frequency distributions at IF
`after quadrature conversion for supradyne operation. In FIG. 5
`6A, the smaller interfering component lies within the limited
`bandwidth B20 of the filter 20 whereas in FIG. 6B it is the
`larger component within the limited bandwidth B20 of the
`filter.
`
`10
`
`By making the bandwidth of the wideband polyphase
`filter 26 equal to at least 3 channel bandwidths it is possible
`to determine where a larger interfering component lies and
`to eliminate it by simply selecting a local oscillator fre(cid:173)
`quency fLo to give infradyne or supradyne operation, and 15
`changing the centre frequency of the filter 26 accordingly.
`FIG. 7 is a flow chart of an algorithm which effects a
`change of frequency conversion mode in the case of poor
`quality reception.
`
`Block 50 denotes monitoring signal quality by comparing
`the signal with a threshold value, for example by considering
`the bit error rate in a repeatable signal such as a training
`sequence and/or synchronisation code word, averaged over
`a predetermined number of bursts or repeats. In block 52 a 25
`check is made to see if the measured quality is unacceptable.
`If the answer is No (N) then the flow chart reverts to the
`block 50. However if the answer is Yes (Y), then in block 54
`the local oscillator frequency is changed by the equivalent of
`one channel bandwidth, for example 200 kHz in the case of 30
`GSM. In block 56 a figure of merit is determined and in
`block 58 a check is made to see if the reception is better. If
`the answer is Yes (Y) the flow chart reverts to the block 50.
`If the answer is No (N), in block 60 the local oscillator
`frequency is changed back to its original value and the flow 35
`chart reverts to the block 50.
`
`In operation the figure of merit of reception should be
`verified periodically because accumulating changes in
`operation may lead to the threshold being too small. Using
`the quality of the previously received signal, the threshold 40
`could be increased for a predetermined period.
`
`In situations of failing reception leading to the loss of a
`call-in-progress and a failed connection, the mode of fre(cid:173)
`quency conversion should be changed.
`
`quality of said frequency down(cid:173)
`is better for demodulation by a
`
`What is claimed is:
`1. A method of operating a low IF receiver, comprising the
`steps of:
`monitoring the quality of a frequency down-converted
`signal formed by a local oscillator;
`if the signal quality is unacceptable, changing a first local
`oscillator frequency of the local oscillator to a second
`local oscillator frequency by the equivalent of a chan(cid:173)
`nel bandwidth;
`determining if the
`converted signal
`demodulator; and
`if not, reverting back to the first local oscillator frequency.
`2. The method of claim 1, comprising changing a center
`frequency of a filter used in filtering the down-converted
`signal so that said quality becomes acceptable for demodu(cid:173)
`lation by said demodulator.
`3. A receiver for receiving a received signal having a
`20 received frequency comprising:
`a local oscillator which provides a local oscillating signal
`having a local frequency;
`first and second mixers which down-convert said received
`signal from said received frequency to an intermediate
`frequency using said local oscillating signal to form
`first and second signals;
`a first filter having a center frequency; said first filter
`receiving said first and second signals and forming first
`filtered signals;
`a second filter having said center frequency; said second
`filter receiving said first and second signals and form(cid:173)
`ing second filtered signals;
`a demodulator which demodulates said first filtered sig(cid:173)
`nals; and
`a controller which monitors said second filtered signals,
`and if said second filtered signals have an unacceptable
`quality, then said controller changes the local oscillat(cid:173)
`ing frequency of the local oscillator and the center
`frequency of at least one of the first filter and the second
`filter so that said quality of said second filtered signals
`becomes acceptable for demodulation of said first fil(cid:173)
`tered signals by said demodulator.
`4. A receiver for receiving a received signal having a
`45 received frequency comprising:
`oscillator means for providing a local oscillating signal
`having a local frequency;
`first and second converter means for down-converting
`said received signal from said received frequency to an
`intermediate frequency using said local oscillating sig(cid:173)
`nal to form first and second signals;
`first filter means for filtering said first and second signals
`to form first filtered signals, said first filter means
`having a center frequency;
`second filter means for filtering said first and second
`signals to form second filtered signals, said second
`filter means having said center frequency;
`demodulator means for demodulating said first filtered
`signals; and
`control means for monitoring said second filtered signals,
`and if said second filtered signals have an unacceptable
`quality, then said control means changing the local
`oscillating frequency of the oscillator means and the
`center frequency of at least one of the first filter means
`and the second filter means so that said quality of said
`second filtered signals becomes acceptable for
`
`In the event of finding big interferers before and after
`shifting the IF by twice the IF frequency and the microcon(cid:173)
`troller 30 not being able to decide on the better IF, it can as
`a final option operate the receiver as a zero IF receiver by
`shifting the IF by one IF and altering the polyphase filter 50
`characteristic so that it appears effectively as 2 filters. This
`may be done by adjusting the cross coupling, that is,
`between transconductors JCl 7 and JC26; JC18 and JC19,
`JC20 and JC21, JC22 and JC23, and JC24 and JC25 so that
`the currents are zero.
`In the present specification and claims the word "a" or
`"an" preceding an element does not exclude the presence of
`a plurality of such elements. Further, the word "comprising"
`does not exclude the presence of other elements or steps than
`those listed.
`
`55
`
`60
`
`From reading the present disclosure, other modifications
`will be apparent to persons skilled in the art. Such modifi(cid:173)
`cations may involve other features which are already known
`in the design, manufacture and use of low IF receivers and 65
`component parts thereof and which may be used instead of
`or in addition to features already described herein.
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`Ex.1015
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`US 6,577,855 Bl
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`5
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`7
`demodulation of said first filtered signals by said
`demodulator means.
`5. An integrated circuit comprising:
`oscillator means for providing a local oscillating signal
`having a local frequency;
`first and second converter means for down-converting a
`received signal having a received frequency from said
`received frequency to an intermediate frequency using
`said local oscillating signal to form first and second
`signals;
`first filter means for filtering said first and second signals
`to form first filtered signals, said first filter means
`having a center frequency;
`second filter means for filtering said first and second 15
`signals to form second filtered signals, said second
`filter means having said center frequency;
`demodulator means for demodulating said first filtered
`signals; and
`control means for monitoring said second filtered signals,
`and if said second filtered signals have an unacceptable
`quality, then said control means changing the local
`oscillating frequency of the oscillator means and the
`center frequency of at least one of the first filter means
`and the second filter means so that said quality of said
`second filtered signals becomes acceptable for
`demodulation of said first filtered signals by said
`demodulator means.
`6. A receiver comprising:
`means for monitoring quality of a frequency down(cid:173)
`converted signal formed by a local oscillator;
`changing means for changing a first local oscillator fre(cid:173)
`quency of the local oscillator to a second local oscil(cid:173)
`lator frequency by the equivalent of a channel
`bandwidth, if the signal quality is unacceptable; and
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`10
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`8
`means for reverting back to the first local oscillator
`frequency if the quality of said frequency down(cid:173)
`converted signal is not better for demodulation by a
`demodulator.
`7. A receiver as claimed in claim 6, wherein said changing
`means changes the first local oscillator frequency by at least
`twice a frequency of said frequency down-converted signal.
`8. A receiver as claimed in claim 6, wherein the changing
`means also inverts a sign of a local oscillator signal from
`said local oscillator.
`9. A receiver as claimed in claim 6, wherein said changing
`means in response to finding that an unacceptable signal is
`present at both the respective local oscillator frequencies,
`adapts a polyphase filtering means to operate as two zero IF
`filters, said polyphase filtering means selecting a wanted
`signal from said frequency down-converted signal.
`10. A receiver as claimed in claim 6, further comprising
`detecting means for monitoring said quality of said fre(cid:173)
`quency down-converted signal, the detecting means moni-
`20 taring image frequencies and causing the changing means to
`change the first local oscillator frequency in response to
`deciding which image frequency to use.
`11. A receiver as claimed in claim 6, further comprising
`detecting means for monitoring said quality of said fre-
`25 quency down-converted signal; and channel selectivity
`polyphase filtering means for selecting a wanted signal from
`said frequency down-converted signal; the detecting means
`comprising a wideband polyphase filter, means for identi(cid:173)
`fying the frequencies present in the output of the wideband
`30 polyphase filter, and image frequency analysing means for
`providing a control signal for altering the frequency of the
`local oscillator and the centre frequency of the channel
`selectivity polyphase filtering means and the wideband
`polyphase filter.
`
`* * * * *
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`Ex.1015
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