Using Low Noise Modules in Satellite Applications

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The low noise block (LNB) is a key functional block in the receiver chain of a satellite communication system. The LNB basically consists of a low noise amplifier (LNA) chain and a downconverter. Although the components and circuits used in the LNB have evolved over time and vary greatly between products, the basic requirements of the LNA remain the same, namely to provide the lowest possible noise figure and appropriate matching conditions at the amplifier input.

This technical note discusses some design considerations for an ultra-low noise amplifier used as the first-stage LNA in a satellite receiver. The frequency chosen is 12 GHz, which is at the lower end of the Ku-band and is widely used in high data rate satellite communications applications due to its ideal balance between three key system performance parameters: (1) usable bandwidth (~500 MHz), (2) microwave propagation characteristics (rain fade becomes more severe as frequency increases further), and (3) required antenna size (roughly proportional to frequency). The discussion is provided at a high level of general information. Actual design requires more in-depth product development knowledge and expertise.

equipment

The device discussed in this note is CEL's low-noise transistor, the CE3512K2. It is manufactured using state-of-the-art GaAs pseudomorphic high electron mobility transistor (pHEMT) technology to provide excellent noise performance. In addition, the CE3512K2 uses a hollow cavity package structure, which significantly eliminates the RF power losses associated with the packaging material. In order to keep the device cost consistent with commercial applications, the cavity package of the CE3512K2 is non-hermetic. Therefore, a no-clean flux soldering process is recommended for PCB assembly (see Recommended Flux and Cleaning Conditions for K2 and K3 Package Devices for details ). As an alternative, the CE3514M4, which uses a molded plastic package, does not require no-clean flux. Its noise figure is slightly higher than that of the CE3512K2.

The CE3512K2 is designed for Ku-band LNA applications. It also exhibits excellent phase noise characteristics when used in higher frequency oscillator designs. A typical example is a 24GHz oscillator in Doppler radar applications.

Bias Circuit

Figure 1 shows a typical biasing scheme for a field effect transistor (FET) used in RF amplifiers in the GHz range. Since the gate is in a high impedance state at DC, only the bias circuit for the drain needs to be considered. The inductor value is usually chosen high enough to provide adequate RF isolation at the operating frequency, although in some cases the inductor in the bias circuit can also be part of the matching network. The shunt capacitor provides a low impedance point that acts as a low-pass filter to filter noise and other unwanted signals from the DC supply. As frequency increases, parasitic effects in the inductor and capacitor become dominant. Therefore, for amplifiers operating above 10 GHz, transmission line-based components are often used for biasing and matching circuits.

figure 1

Typical LC bias circuit for a FET (matching network not shown)

It is well known that a quarter wavelength (denoted as λ/4) transmission line converts a short circuit to an open circuit. This property is used in bias circuit design where a λ/4 transmission line is implemented to replace an inductor to achieve RF isolation. In principle, this technique has no low frequency limitation. However, in practice, it is often used in circuits operating at gigahertz and higher frequencies due to PCB size limitations. For shunt capacitors, there are two common transmission line implementations, namely rectangular and radial open stubs. The role of a rectangular stub can be understood using the same concept of impedance transformation through a λ/4 transmission line. In this case, the open circuit is converted to a short circuit. The analysis of a radial stub is much more complex. Almost, the length (radius) required for a radial stub is shorter than its rectangular counterpart for the open to short circuit conversion. Moreover, in the case of a radial stub, the low impedance point is well defined because of its narrow width at the interface with the through transmission line. We will use a radial stub to illustrate the circuit performance in this description.

DC Blocking Capacitors

As with any transistor circuit, DC blocking capacitors are typically required at both the input and output of the CE3512K2 amplifier circuit. At 12GHz, a typical chip capacitor has two undesirable effects: 1) the insertion loss of the input capacitor can be as high as several dB, directly degrading the noise figure by the same amount; and 2) the parasitic effects of the capacitor complicate the matching network design. While special high frequency capacitors designed to minimize these effects are available, they are typically expensive. Fortunately, for LNB applications, input DC blocking capacitors are not required because the LNA circuit is typically interfaced with a waveguide component that is naturally DC isolated. At the output, a transmission line-based bandpass filter is typically used to suppress out-of-band noise. This type of filter circuit is also typically DC isolated, eliminating the need for a DC blocking capacitor. In cases where such a filter is not practical or required, a capacitor in series with the output has little effect on circuit performance and is therefore acceptable.

During the prototyping phase, circuits without DC capacitors can cause problems for noise figure measurements (and possibly S-parameter measurements as well) since the measurement instrumentation is usually DC coupled. A common practical solution is to insert a DC blocking element or bias tee between the DUT and the instrument. The noise figure of the DUT can then be estimated by subtracting the insertion loss of the input DC block from the measured noise figure. Note that this procedure corrects the effect of the DC block only in magnitude. The measurement uncertainty associated with phase changes remains. If more accurate measurements are required, a more complex procedure involving phase calibration can be implemented.

Matching circuit

_{When designing an} LNA input matching network, it is well known that perfect input return loss and minimum noise figure ( NFmin provided in the S-parameter file ) are usually not achievable at the same time. The typical procedure for commercial products is to first determine the minimum specification for return loss (10dB is usually considered acceptable), and then design the match to achieve the best possible noise figure under the return loss specification. For the CE3512K2, the two matching points for return loss and noise figure are very close at 12GHz, as shown in Figure 3. This is not a coincidence, as the device is designed specifically for this frequency band.

For frequencies above 10 GHz, the matching network usually consists of transmission line components only, usually a series transmission line and an open circuit stub. Multiple stubs can be used to improve the frequency response across the bandwidth.

Simulation results

Design in practice usually starts with simulation using noise and S-parameters, followed by bench tuning to optimize performance.

Figure 2 shows the schematic of a 12GHz LNA using the CE3512K2. This design incorporates the considerations discussed above. Simulation results for gain, S21, input and output return loss, S11 and S22, and noise figure are shown in Figure 3. Note that this design achieves good input return loss, while the noise figure is only slightly above NF _min (0.02dB). At the output, moderate S22 is achieved without any matching.

figure 2

12GHz LNA simulation setup using CE3512K2

image 3

Simulation results

In actual product development, the design process is much more complicated due to various imperfections of components/devices and limitations of circuit implementation. Although simulations show a noise figure of about 0.5dB, a product specification of 1dB or slightly lower is generally considered excellent, all things considered. However, the considerations outlined in this note are relevant to actual applications.