Receiver and filter arrangement comprising polyphase filters

Details Receiver and filter arrangement comprising polyphase filters Receiver and filter arrangement comprising polyphase filters

1998-03-30

2001-05-22

Hunter, Daniel (Department: 2684)

Telecommunications

Receiver or analog modulated signal frequency converter

Frequency modifying or conversion

C455S307000, C455S190100

Reexamination Certificate

active

06236847

ABSTRACT:

FIELD OF THE INVENTION
The invention is related to a receiver comprising a frequency converter for deriving from the input signal of the receiver a plurality of intermediate polyphase signals, the receiver further comprises a polyphase filter for deriving filtered polyphase signals from the intermediate polyphase signals. The invention is also related to a filter arrangement using frequency conversion means and a polyphase filter.
BACKGROUND OF THE INVENTION
Such a receiver and filter arrangement are known from U.S. Pat. No. 4,723,318. In receivers an RF signal is down converted to an IF frequency substantially lower than said RF frequency in order to make it possible to obtain a good adjacent channel selectivity with easy realizable filters. A problem associated with this down conversion is the sensitivity of the receiver for so-called image signals. If the signal to be received has a frequency of f
RF
, the local oscillator to be used in the frequency conversion means has a frequency f
LO
and the IF frequency is equal to f
LO
−f
RF
, the receiver receives also RF signals with a frequency of f
LO
+f
IF
, causing interference to the reception of the desired RF signal with frequency f
RF
.
In order to suppress this undesired effect, often an RF filter is used in front of the frequency conversion means to suppress RF signals with a frequency of f
LO
+f
IF
. This RF filter can be quite expensive.
In the receiver according to the above mentioned US patent, a so called polyphase filter is used for filtering the output signal of the frequency conversion means. Polyphase filters can make use of multiple phase shifted input signals (to be provided by the frequency conversion means) to produce asymmetric transfer functions enabling suppression of signals at the image frequency without requiring an RF filter. The polyphase filter according to the above mentioned US patent is designed for suppressing a signal at an image frequency.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a receiver in which beside the image rejection also the adjacent channel selectivity is realized in a very cost effective way.
Therefor the invention is characterized in that the receiver comprises a further frequency converter for deriving a plurality of further intermediate polyphase signals from the filtered polyphase signals, the receiver comprises a further polyphase filter for deriving further filtered polyphase signals from the further intermediate polyphase signals, one of the polyphase filter and the further polyphase filter being arranged for attenuating signals above a first frequency, and one of the polyphase filter and the further polyphase filter being arranged for attenuating signals below a second frequency.
By using these measures it becomes possible to obtain a receiver with an asymmetric band pass transfer function. The first frequency converter with associated polyphase filter can suppress one part of the input spectrum (e.g. all frequencies below a frequency f
1
) by choosing suitable values for the IF frequency and the first frequency of the (first) polyphase filter, and the second frequency converter with associated polyphase filter can suppress another part of the input spectrum (e.g. all frequencies above a frequency f
2
<f
l
) so that a band pass transfer function is obtained suitable for realizing the adjacent channel selectivity.
It is observed that the article “High performance direct conversion” by R. Green and R. Hoskins in Electronics World, January 1996, pp. 18-22 discloses a receiver in which two polyphase filters are used. However this document does not use a combination of a first polyphase filter suppressing frequencies above (below) a certain frequency, and a second polyphase filter suppressing frequencies below (above) a certain frequency. The absence of said combination results in a spectrum inversion to take place in said receiver, which is in general undesirable in receivers used for receiving digital signals.
An embodiment of the invention is characterized in that the first frequency and the second frequency are substantially zero.
By using polyphase filters with a cut off frequency of substantially zero, it is obtained that this cut off frequency does not depend on the values of the components used in said polyphase filter. The cut off frequency of the combination of frequency converter and polyphase filter is now only determined by the frequency of the local oscillator used in the frequency converter. Because said frequency can be set very accurately, it becomes possible to realize very accurate filters in a cost effective way.
A further embodiment of the invention is characterized in that the first polyphase filter and the second polyphase filter comprise passive filters.
The use of passive filters does not require active elements such like opamps, resulting in lower costs, and often in a more stable operation of the receiver.
A still further embodiment of the invention is characterized in that the first polyphase filter and the second polyphase filter comprise passive filters using capacitors and resistors.
Using passive filters with resistors and capacitors results in a receiver which can easily be realized in standard IC technologies, resulting a very cost effective solution.
A still further embodiment of the invention is characterized in that the further frequency conversion means comprise image rejection mixing means.
By employing an image rejection mixer in the further frequency converter, it becomes possible to choose the intermediate polyphase signal and the further intermediate polyphase signal in the same frequency range without requiring expensive image rejection filters. The possibility to choose the intermediate polyphase signals and the further intermediate polyphase signals in the same frequency range enables the realization of receivers having a narrow IF bandwidth without requiring expensive band-pass filters.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be explained with reference to the drawings. Herein shows:
FIG. 1
, a block diagram of a receiver according to the invention;
FIG. 2
, a plot of the frequency spectra at different positions in the receiver according to
FIG. 1
;
FIG. 3
, a further plot of the frequency spectra at different positions in the receiver according to
FIG. 1
;
FIG. 4
, an implementation of the polyphase filter
16
in the receiver according to
FIG. 1
;
FIG. 5
, an implementation of the mixer
15
in the receiver according to
FIG. 1
;
FIG. 6
, an implementation of the polyphase filter
16
in the receiver according to FIG.
1
.


REFERENCES:
patent: 4434508 (1984-02-01), Sommer
patent: 4696055 (1987-09-01), Marshall
patent: 4723318 (1988-02-01), Marshall
patent: 5300838 (1994-04-01), Elizondo
patent: 5715529 (1998-02-01), Klamush et al.
patent: 5764708 (1998-06-01), Glass
patent: 5949878 (1999-09-01), Burdge et al.
patent: 5960364 (1999-09-01), Dent
“High performance direct conversion” By R. Green and R. Hosking. Electronic Word, pp. 18-22, Jan. 1996.*
“High Performance Direct Conversion” By R. Green and R. Hoskins in Electronics World, Jan. 1996, PP. 18-22.

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