This invention generally relates to an apparatus and method for a low loss low noise receiver for satellite transceivers and receivers.
A radio signal receiver may typically include an amplifier chain having a filter stage and amplifier stages through which the received radio frequency signal is passed in series. The filters filter out unwanted (out of band) signals and noise and the amplifiers amplify the remaining signal. The resulting signal may then be passed to a mixer where it is downconverted, and subsequently demodulated.
The amplifiers used in receivers contribute to the noise attendant to the recovered signal. The noise added to the signal by the amplifier results in a degradation of the signal-to-noise (S/N) ratio at the output of the amplifier. A figure of merit for the amount of noise added by the amplifier is the ratio of the signal to noise ratio at the input (S/N)IN to the signal-to-noise ratio at the amplifier output (S/N)out. This ratio is commonly referred to as the noise factor (F) of the amplifier, and is used to calculate the noise figure (NF) according to the formula NF=log10(F), where F=(S/N)IN/(S/N)out. To cope with the extreme sensitivity of the high frequency signal the receiver must have a very low noise figure. Otherwise, the noise will tend to be amplified so that is overshadows the desired signal.
For high frequency operation, low noise amplifiers (LNAs) are often desired. LNAs are special amplifiers which are fabricated to produce less noise during operation. However, although the LNA contributes less noise, it contributes noise nonetheless. To optimize the performance from a LNA, the input to the LNA is typically noise matched prior to providing the signal to the LNA input for amplification.
In most receivers the first stage of the receiver is noise matched. Noise matching is typically performed in the first stage because the first stage dominates the noise performance of the entire receiver. Thus, by noise matching the first stage, each successive stage will contribute less noise than the previous stage. Noise matching the first stage is especially important in the high frequency receiver and transceiver systems (e.g., satellite receivers and transceivers.) In these receivers and transceivers systems, noise performance is critical since the signal received typically travels long distances and through many environmental media.
To noise match in conventional microwave receivers, microstrip or coplanar waveguides are often used. However, using microstrip or coplanar transmission lines are disadvantageous because the dielectric media in their construction causes significant insertion loss. That is, there is a significant loss in the strength (e.g., power) of the signal due to the loss tangent/dissipation factor of the substrate used in the microstrip or coplanar transmission line design. In particular, exemplary substrates which may be used in the microstrip or coplanar transmission design include, for example, Arlon 45N with a dissipation factor of 0.025 and Rogers 4003 with a dissipation factor of 0.003.
A typical receiver may also include a circuit element designed to couple a carrier signal to aid in the retrieval of the desired signal. One such circuit element commonly used is a capacitor. Capacitors may typically be used between amplifiers in a chain to AC couple the received signal and provide a DC block. The capacitors used are typically single layer high Q producing capacitors which can be expensive relative to the overall system cost, especially for high frequency operation. In addition, the capacitors are often inconsistently manufactured due to different manufacturing process tolerances. In systems requiring increased sensitivity, such as in high frequency receivers, this inconsistency in operation leads to a heightened level of unacceptable performance unpredictability. Instead of using capacitors for coupling, some receivers may use xe2x80x9ccouplers,xe2x80x9d which are less expensive to manufacture and which provide a higher performance at RF and millimeter wave frequencies.
Typical couplers which may be used may be positioned between the successive amplifiers in the amplifier chain. One such coupler found in the prior art is the microstrip quarter wave coupler. The microstrip quarter wave coupler typically only has one or two ground plane with a conductor supported by a layer of dielectric.
Using microstrip couplers, however, is problematic because the width and gap between the coupling lines can be less than five mils. The width and gap are determined by the bandwidth, coupling, directivity, and impedance of the application. Additionally, there is a cost production issue associated with fabricating boards with controlled impedance lines of the fine spacing and width required in microstrip couplers.
Another coupler which is widely used is the stripline broadside coupler. The stripline broadside coupler uses the broadside of flat conductors to effect the signal coupling. Typically, such conductors are a quarter wave length long. The broadside couplers are typically preferred since the broadside coupler has two ground planes and homogeneous dielectric which promotes transverse electromagnetic mode (TEM) propagation. In addition, the even and odd phase velocities of the propagating waves are identical, which gives good bandwidth, directivity and voltage standing wave ratio (VSWR).
However, using stripline couplers has its disadvantageous. First, the stripline coupler typically has a higher insertion loss that the microstrip coupler. Second, the stripline coupler requires at least a four layer board (e.g., 4 metal layers, and 3 dielectric layers) and therefore is more costly to produce than a conventional two layer coupler (e.g., 2 metal layers and 1 dielectric layer). In addition, to control the impedances on the stripline coupler at RF and microwave frequencies is extremely difficult and cost prohibitive.
Consequently, a low loss receiver is needed which significantly improves the noise performance of a receiver system. Such a receiver may use a substrate with lower insertion loss than the prior art, and may additionally exclude the use of costly high Q capacitors, which may save time and money in the manufacturing process.
Generally, a low loss high frequency transmission system according to various aspects of the present invention includes an improved input noise match circuit and an improved interstage noise match circuit. An input noise match circuit and an interstage matching circuit in accordance with the present invention uses a suspended substrate, which has significantly lower insertion loss than any other planar transmission lines found in the prior art. By using a suspended substrate in accordance with the present invention, maximum performance from the first stage LNA is achieved. In particular, the present invention uses an input match stage and an interstage matching circuit with a free space substrate. Accordingly, the present invention use a smaller less expensive antenna, has a lower bit error rate or higher order constellations for digital communication, and provides continued service during inclement weather (e.g., rain, snow, fog, sleet, etc.).
As described more fully below, the present invention uses a suspended substrate circuit element input matching and a suspended substrate stripline broadside coupler which performs the function of a DC block in the interstage match between the first and second LNA. By using a suspended substrate in the input match, the present invention provides the least amount of signal loss since the system presents minimal resistance to signal flow. By using a suspended stripline substrate in the interstage match, the interstage match virtually eliminates any downstream noise contribution by the 1st LNA to the overall noise factor (NF).
In one exemplary embodiment, the system uses as an input noise match circuit a suspended substrate circuit. Since as noted, in conventional noise match circuits, there is significant loss in the power of the signal due to the dissipation factor of the substrate used, the present invention provides performance advantages over the prior art due to the effective dissipation factor associated with a suspended substrate circuit. In particular, it is known that the dissipation factor of a suspended substrate circuit rests between the loss tangent of free space and the laminate used in the circuit. Further, because the electric characteristics of the suspended substrate circuit is dominated by free space, then the suspended substrate dissipation factor is closer to that of free space. Free space has a loss tangent of zero. Thus, the dissipation factor of the suspended substrate circuit will be somewhere near zero.
In another exemplary embodiment, a quarter wave broadside coupler is disclosed again using a suspended substrate. The suspended substrate coupler may be used in an interstage noise match circuit. The use of the suspended stripline broadside coupler in the interstage noise match lowers the cost over conventional coupler systems by eliminating the single layer capacitor commonly found in receiver systems. In addition, since no single layer capacitor is required, the present invention eliminates the manufacturing step required to place the capacitor thereby lower the cost of the overall receiver system.
In yet another exemplary embodiment, an interstage noise match block is disclosed which includes the aforementioned suspended substrate coupler and additionally includes an interstage noise match circuit. The interstage noise match circuit may be of similar construction as the input stage circuit described above. Thus, when used in combination with the suspended substrate coupler, the overall performance of the interstage noise match block is improved above the prior art.
These features and other advantages of the system and method, as well as the structure and operation of various exemplary embodiments of the system and method, are described below.