1. Field of the Invention
This invention relates generally to an image reject sub-harmonic mixer and, more particularly, to an image reject sub-harmonic mixer that includes an RF 90xc2x0 hybrid, two high pass filters, two diode pair mixers, two low pass filters, and an intermediate frequency 90xc2x0 hybrid to provide reduced components over existing sub-harmonic mixers.
2. Discussion of the Related Art
Image reject sub-harmonic mixers are known in the art for mixing RF and local oscillator (LO) signals. These mixers are typically used in many communications systems, such as radio and radar communications systems, for frequency down-conversion purposes. Frequency down-conversion includes mixing the received RF signal with a local oscillator (LO) signal to generate an intermediate frequency (IF) signal suitable for signal processing. Mixing the received RF signal with the LO signal generates sum and difference signals from these two signals, where the low frequency difference signal is used as the IF signal and the sum signal is filtered out. Sub-harmonic mixers have an advantage over conventional mixers because it is easier to generate the fundamental LO signal from which the effective LO signal is used for the mixing, especially at higher frequencies (GHz).
Typically, the fundamental LO signal is selected to be about one-half of the frequency of the received RF signal so that the second harmonic of the fundamental LO signal is near the frequency of the RF signal. The second harmonic or effective LO signal is the signal that is actually mixed with the RF signal to generate the IF signal. The mixing process generates responses at the effective LO frequency, plus and minus the IF frequency. The image reject mixer rejects one of the responses as an image signal by selective phasing, so that it does not interfere with the signal processing as noise or the like.
The conventional image reject sub-harmonic mixer is implemented using two identical mixers with separate power dividers driving the LO and RF ports. FIG. 1 is a schematic diagram of a conventional image reject sub-harmonic mixer 10. An RF input signal is applied to an input port of a 90xc2x0 RF hybrid 12 (power divider), and in-phase and quadrature phase RF signals are provided at the output ports of the hybrid 12. The in-phase RF signal is applied to an RF band-pass filter 14 and the quadrature phase RF signal is applied to an RF band-pass filter 16. An LO signal is applied to an in-phase LO hybrid 18 (power divider) that splits the LO signal into copies of itself that are in-phase with each other. One of the LO signals is applied to an LO band-pass filter 20 and the other LO signal is applied to an LO band-pass filter 22.
The RF and effective LO signals from the filters 14 and 20 are mixed by an anti-parallel diode pair 24 to generate an in-phase IF signal that is the difference between the RF signal and the effective LO signal. Likewise, the RF and the effective LO signals from the filters 16 and 22 are mixed by an anti-parallel diode pair 26 to generate a quadrature phase IF signal that is the difference between the RF signal and the effective LO signal. The in-phase and the quadrature phase IF signals are 90xc2x0 apart in phase. Low pass filters 28 and 30 are provided to pass the IF signals from the diode pairs 24 and 26, respectively, and reject the high frequency RF and LO signals.
The in-phase and quadrature phase IF signals are combined in a power combiner or 90xc2x0 IF hybrid 32. Proper phasing is provided in the RF and LO hybrids 12 and 18 and the IF hybrid 32 to reject the image signal relative to the desired IF frequency. The LO hybrid 18 provides in-phase LO signals to the LO ports of the two diode pairs 24 and 26, and the RF hybrid 12 provides a 90-degree phase shift between the RF ports of the two diode pairs 24 and 26. The IF hybrid 32 provides 90-degree phasing to reduce the undesirable sideband. In this design, the upper sideband is provided on the lower output port of the hybrid 32, and the lower sideband is provided at the upper output port of the hybrid 32.
To maintain RF and LO signal separation to drive the mixer diode pairs 24 and 26 in the conventional sub-harmonic mixer 10, each diode pair 24 and 26 requires separate bandpass, low pass or high pass filters to eliminate undesired coupling among the RF, LO and IF signals. This implementation thus requires six separate filters, one at each of the LO, RF and IF frequencies for each mixer. Additionally, two power dividers for the LO and RF signals and one power combiner for the IF signal are required to implement the conventional image reject subharmonic mixer.
Typically, sub-harmonic mixers are implemented in a monolithic circuit in state-of-the-art receivers. Thus, there is a need in the art to simplify the circuitry required for an image reject sub-harmonic mixer to reduce hardware requirements and reduce the physical area required for the mixer. Reducing the number of components also reduces the cost of the mixer. It is therefore an object of the present invention to provide such a sub-harmonic mixer.
In accordance with the teachings of the present invention, an image reject sub-harmonic mixer is disclosed that employs less components than the image reject mixers known in the art. In one embodiment, the mixer includes a four-port 90xc2x0 RF coupler, two high pass filters, two anti-parallel diode pairs, two low pass filters and a 90xc2x0 IF coupler. RF and LO signals are applied to isolated input ports of the 90xc2x0 RF coupler. The RF coupler generates in-phase and quadrature phase RF and LO signals at the two output ports of the RF coupler. The RF and LO signals at one of the output ports of the RF coupler is applied to a first bandpass filter and the RF and LO signals at the other output port of the RF coupler are applied to a second high pass filter.
The anti-parallel diode pairs provide the mixing of the RF and effective LO signals, and generate two IF signals that are 90xc2x0 apart in phase. The high pass filters reject the IF signal to prevent IF power from coupling to the RF and LO ports of the RF coupler. The low pass filters pass the IF signals and reject the LO and RF signals. Image rejection is obtained by combining the IF outputs of the two diode pairs in the 90xc2x0 IF hybrid to separate the upper and lower sidebands of the effective LO signal.