Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1999-08-25
2003-06-03
Trost, William (Department: 2683)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S303000, C455S313000, C455S316000, C331S002000
Reexamination Certificate
active
06574462
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to receivers and transmitters for radio frequency (RF) signals, and particularly relates to local oscillators for radio frequency signal receivers.
Local oscillator signals are employed in radio frequency signal receivers, such as direct conversion receivers. A direct conversion receiver mixes down to an intermediate frequency of zero Hertz, and is therefore sometimes referred to as a zero IF receiver. The modulation information only is represented in the down conversion, and there is no carrier information that is typically associated with an intermediate frequency. In a direct conversion receiver the local oscillator signal is operating at the same frequency as the input RF signal. U.S. Pat. Nos. 5,438,692 and 5,548,068 disclose conventional direct conversion receivers.
In direct conversion, the modulation information is preserved through quadrature down conversion, which involves mixing the incoming line or carrier with a local oscillator signal along two different paths. The local oscillator signal along one path may be at zero phase (0°) with respect to the input RF signal, and may be phase shifted to 90° along the other path. Alternatively, one path may be at −45° while the other is at +45° with respect to the input signal. See for example, U.S. Pat. No. 5,303,417. In any event, the circuit paths are typically mutually 90° different in phase, and one path is referred to as the in-phase (I) channel while the other is referred to as the quadrature (Q) channel. The quadrature down conversion method preserves the necessary phase information within the input signal.
As shown in
FIG. 1
, a conventional radio frequency receiver includes an input port
10
for receiving an input RF signal from an RF antenna includes a local oscillator
22
. The input signal is divided between two input paths. The input signal in one input path is mixed at mixer
12
with the local oscillator signal
14
at zero degrees phase shift. The input signal in the other input path is mixed at mixer
16
with the local oscillator signal at 90° phase shift. The 90° phase shift in the local oscillator signal is achieved by phase shift device
20
. The local oscillator signal is produced by voltage controlled oscillator (VCO)
22
.
Interference may occur if the local oscillator signal couples to the input RF signal. Because the frequencies of these signals are the same, the local oscillator signal cannot be frequency filtered from the incoming signal. The incoming signal would, in effect, be blocked. U.S. Pat. Nos. 4,811,425 and 5,428,837 are directed to reducing the effects of leakage of local oscillator signals to RF input signals in zero IF receivers.
Moreover, interference may occur if the RF input signal radiates to the VCO. Since VCOs are typically very sensitive, any signal that is close in frequency to the frequency of the VCO may interact with it, even if the signal comprises only a small amount of energy. This is because the VCO will selectively amplify signals at or near its frequency.
One way of overcoming this problem is to employ a VCO that operates at a frequency different than the input RF signal. The frequency of the VCO signal is then modified to produce a local oscillator signal at the same frequency as the input RF signal. For example, two VCOs could be employed together with a mixer. The signal from one VCO (at frequency F
1
) may be combined with the signal from another VCO (at frequency F
2
) by the mixer. The product, however, of the F
1
and F
2
signals, will include spurious signals that must be filtered out to produce the local oscillator signal. For example, the product of two sine functions sin(&agr;)×sin(&bgr;) equals ½ cos(&agr;-&bgr;)−½ cos(&agr;+&bgr;). Two frequencies would be produced at the mixer (F
1
+F
2
and F
1
−F
2
), and one would have to be filtered out. It is typically necessary to do this type of filtering off IC, which further invites interference or leakage of the local oscillator signal to the input RF signal.
In other conventional local oscillator circuits, one VCO only might be employed and the output of the VCO would be input to a frequency doubler, then to a bandpass filter, and finally to the phase shift device
20
. The frequency of the VCO (F
1
) could be one half the frequency of the RF input signal, and the frequency of the local oscillator would then be 2F
1
. In further conventional local oscillator circuits, the frequency of the VCO (F
1
) could be twice the frequency of the RF input signal, and the frequency of the local oscillator signal may be equal to ½F
1
. This could also be achieved with one VCO (F
1
), whose output could be input to a one-half frequency divider to produce the local oscillator signal for input to the phase shift device
20
. In each such circuit however, the local oscillator signal may still radiate to the RF input signal, and the VCO may be sensitive to harmonic frequencies of the RF input signal.
Such conventional techniques do not fully alleviate the interference problems. It is an object of the present invention to provide a local oscillator signal for radio frequency transmitters and receivers that has reduced leakage or interference between a radio frequency input signal and the local oscillator. It is also desirable to provide a local oscillator circuit that may be employed in an integrated circuit environment. It is further desirable to provide a local oscillator signal for dual band operation where one frequency band (e.g., 1800 MHz.) is twice the frequency of the other frequency band (e.g., 900 MHz.).
SUMMARY OF THE INVENTION
The invention provides a local oscillator apparatus for use in radio frequency communication systems. The local oscillator apparatus comprises at least one mixer coupled to an oscillator input signal and to a feedback signal such that a local oscillator signal may be produced by fractional multiplication of the oscillator input signal. In an embodiment of the invention, the local oscillator apparatus includes a regenerative modulator comprising a pair of frequency dividers.
REFERENCES:
patent: 4451930 (1984-05-01), Chapman et al.
patent: 4912432 (1990-03-01), Galani et al.
patent: 5761615 (1998-06-01), Jaffee
patent: 6181212 (2001-01-01), Khoini-Poorfard et al.
patent: 6192225 (2001-02-01), Arpaia et al.
patent: 6243569 (2001-06-01), Atkinson
IBM Advances; Silicon-Germanium 3-V PCS-Band: GSM Tri-Band VCO; IBM Corporation; Aug., 1998.
Philips Semiconductors Product Specification; Low Voltage IF I/Q Transceiver SA1638; Figure 1. SA1638 Pin Configuration and Figure 2. SA1638 Block Diagram.
Proceeding of IRE; Fractional-Frequency Generators Utilizing Regenerative Modulation by R.L Miller, pp. 446-455.
Analog Devices Inc.
Ferguson Keith
Samuels , Gauthier & Stevens, LLP
Trost William
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