Meeting the Sirius XM™ Standard LNA Specification for Antenna Modules with and without a Co-located Cellular Transmitter
Shark Fin antenna modules containing SDARS LNAs are found in an ever-increasing number of new automobiles sold in North America. These LNAs serve to enhance the overall sensitivity of the SDARS receiver, as well as to compensate for the cable losses between the roof-mounted antenna and the dash-mounted radio.
Since the design of the LNA front end directly impacts the performance of the SDARS radio, Sirius XM™ has created strict guidelines which are outlined within the Standard LNA Performance Requirements section of their Vehicle Mobile Antenna Requirements document. This article will address these specific design constraints by providing a detailed comparison between the two basic antenna module configurations—namely with and without co-located cellular transmitters. Solutions are also provided for each use case.
Overview of Shark Fin Antenna Module Configurations
In addition to SDARS, many shark fin antenna modules include a variety of transmit and receive elements for cellular, GPS and WLAN radios as depicted in Figure 1. Note from the drawing that there are two basic configurations to consider: in the first case, the SDARS antenna and receiver front end are co-located with other radiating elements that could potentially desensitize the SDARS radio. Alternatively, the neighboring antennas could be devoid of any transmitter circuits, meaning that the entire antenna module is comprised of receivers only.
Between the two cases, the cascaded LNA requirements for parameters like noise figure (NF), gain and inter-modulation rejection ratio (IMRR) along with input-referenced, 1 dB compression point (IP1dB) and input-referenced, third-order intercept point (IIP3) are essentially identical with one significant exception: the out-of-band (OOB) blocker specifications are greatly relaxed for the second configuration where there are no co-located transmitters. The following sections will outline the design requirements associated with each antenna configuration. Proposed solutions are also provided using commercially available LNAs and SAW filters.
Design Requirements Common to Both Antenna Configurations
As a starting point, it is often best to consider the common filtering requirements that will apply to both antenna configurations. Including a Wireless Communications Service (WCS)-band SAW filter situated between the two LNAs in the cascade is a common prerequisite that is dictated by the WCS-band rejection requirements from within the specification. Figure 2 shows the WCS bands located just above and below the targeted 2320-2345 MHz SDARS band.
As a point of reference, the Sirius XM™ specification calls for better than 16 dBc of LNA selectivity from 2305 – 2310 MHz and 2355 – 2360 MHz, and better than 4 dBc of close-in selectivity from 2310-2315 MHz and 2350-2355 MHz. Only a purpose-built SAW filter can effectively attenuate these WCS channels. Since this SAW filter will introduce 3 – 5 dB of insertion loss into the receiver lineup, it must be preceded by a high-gain, ultra-LNA in order to pass the cascaded 1.0 dB NF specification.
Design Requirements Specific to Cases with Co-located Cellular Transmitters
When an SDARS LNA front end is co-located with a transmitting element, provisions must be made to prevent the neighboring transmit signal from overpowering the SDARS receiver. These relatively high-power transmit signals can saturate the SDARS LNAs, leading to a degradation in gain (and hence a reduction in receiver sensitivity). In essence, the neighboring transmit signal desensitizes the SDARS receiver, “blocking” its ability to “see” faint signals.
Figure 3 illustrates the effect that a cellular blocking signal can have on the gain response of an SDARS Single SAW LNA front end. This simple blocker test measures the gain compression of a center-band 2332.5 MHz tone in the presence of OOB blocker signals at the LNA input. As the power of the blocker increases, the in-band gain of the reference tone begins to decrease (compress), thus this measurement is somewhat like a standard IP1dB test.
Curves have been provided for select cellular and 802.11p frequencies ranging from 698 MHz to 5925 MHz. As shown, the front end’s gain can be compromised with blocker power levels as low as -8 dBm.
This level of performance is fine for meeting Sirius XM™’s Configuration 2 requirements. In these scenarios, the SDARS LNA front end only needs to demonstrate blocker resiliency up to a maximum of -10 dBm of input power. The Configuration 1 scenario, on the other hand, must pass the blocker requirement with equivalent input levels of up to +10 dBm. Clearly, an alternative design is needed to meet this stringent specification.
Since the neighboring blocker is “out of band” (OOB), it is relatively straightforward to attenuate this interfering signal from the SDARS LNA front end by adding a first stage SAW filter (refer back to the SDARS LNA lineup for Configuration 1 as shown in Figure 1). Figure 4 illustrates the filtering response typically associated with this first stage SAW device.
With this additional first stage SAW filter in place, the SDARS LNA front end becomes much more resilient to high level OOB blockers. Figure 5 shows the dramatic improvement in OOB blocking performance when employing the dual SAW implementation.
There is some latitude, of course, when choosing between SAW filters for this particular application. There is typically a direct tradeoff between OOB rejection and the filter’s insertion loss; these differences can be compared and analzyed within a simple cascade analysis to ascertain the best blend of sensitivity and resiliency to OOB blocker interference. The solutions provided at the end of the article will outline some of these tradeoffs when using commecially available SAW filters from various component vendors.
Additional Design Considerations
At this point, a brief discussion of the IMRR requirements of the spec is in order. As noted earlier, these requirements are common for both antenna configurations. An SDARS LNA designer will typically find that the LNA cascade easily passes the IMRR2 and IMRR3 requirements for Configuration 1 due to the ample filtering provided by the input SAW device.
However, for the second configuration, passing these IMRR specs will require the inclusion of a simple, 2- or 4-element notch filter at the input of the first stage. These filters must be tuned to provide near-optimal IMRR performance while still meeting the 1.0 dB cascaded NF specification for the LNA. Basically, the closer the notch of the discrete filter comes to the SDARS passband, the worse the NF becomes.
The IMRR performance for both configurations (using Guerrilla RF’s reference designs) is shown in Figure 6.
Detailed Component Selection Considerations
Stage 1 Input SAW Filter: Required for Configuration 1 applications, this filter is critical for setting the cascaded NF and the OOB blocker linearity performance of the SDARS LNA solution. As noted above, the design issue here is that improved OOB rejection comes at the expense of higher in-band insertion loss. This OOB rejection is critical to the overall linearity performance since the stage 2 LNA is typically the limiting stage in terms of both blocker and IMRR performance. From our experience, the lowest loss input SAW solutions have an insertion loss of around 0.45 dB over the SDARS band with an associated rejection at 800 MHz of only around 10 dB. The highest loss SAW device the NF requirement will allow can be no more than 0.6 dB. Devices with this level of insertion loss can have higher 800 MHz rejection of around 20 dB, thus offering superior blocker performance. Please consult with the Guerrilla RF applications engineering team for manufacturer and part number suggestions. As described above, isolation between the cellular transmit and SDARS antennas in excess of 20 dB can allow a reduction in the minimum blocker power requirement. Achieving high antenna isolation can allow the use of the lowest possible loss SAW filter, thus buying margin to the difficult cascaded NF spec of 1.0 dB.
Although not an exclusive list, specific devices to consider are as follows: B3416 from Qualcomm®/RF360 (0.45 dB loss) and B3404 from Qualcomm®/RF360 (0.6 dB loss).
Stage 1 LC Input Notch Filters: For Configuration 2 applications, these notch filters require the use of high Q components to minimize NF impacts. At Guerrilla RF, we find that capacitors from Murata’s GJM series are cost-effective choices. For the inductors, wire-wound parts have superior Q values, thus minimal NF impact. Coilcraft HP series and Murata LQW series parts are good choices here.
Stage 2 LNA: The NF of this ultra-low noise device is critical to achieving the 1.0 dB cascaded NF requirement. For configuration 1, having the lowest possible NF at this stage allows the use of an input SAW device with better OOB rejection characteristics. The automotive-qualified GRF2073W and GRF2074W/GRF2093W LNAs were specifically designed for this application. Please note that GRF2074W and GRF2093W use the same pHEMT die but have 2.0 mm DFN-8 and 1.5 mm DFN-6 packages, respectively. Over the 2320 to 2345 MHz band, GRF2073W and GRF2074W/GRF2093W have de-embedded NF values of 0.37 and 0.30 dB, respectively. These ultra-low NFs combined with their high gain allow the cascaded LNA solution to meet the NF requirement while still using an input SAW device with adequate OOB rejection. Specifically, GRF2074W/GRF2093W with their 0.30 dB de-embedded NF allow for the use of a 0.6 dB insertion loss SAW1 device along with its superior OOB attenuation characteristics. This results in the industry’s best combination of cascaded NF and linearity performance.
For the Single SAW solution, we recommend using the GRF2071W as the stage 2 LNA since its lower gain is better suited for meeting the cascade’s IMRR requirements.
Stage 3 WCS SAW: This filter supplies the required rejection for the WCS bands which occupy 2305 to 2320 MHz and 2345 to 2360 MHz. Achieving this close-in rejection leads to a SAW filter with a relatively high insertion loss of around 3.0 to 4.5 dB over the SDARS band. Not trivial, this loss is a major factor in determining how much stage 2 LNA gain is required to meet the overall NF requirement. The high in-band attenuation of this filter usually makes it the source of any excessive gain ripple within the SDARS band, thus some minor external matching is sometimes required. The B3447 WCS-band SAW filter from Qualcomm®/RF360 is a good choice for this particular stage.
Stage 4 LNA: Well protected by the preceding SAW filters, this LNA primarily provides any necessary gain to meet the design targets with only modest linearity requirements. The amount of gain required for this LNA varies by the length of coaxial cable required for a specific application and by the gain of the preceding LNA. Typical gain requirements are in the 14 to 20 dB range with NF of < 1.5 dB being sufficient so as not to significantly impact the cascaded NF. Guerrilla RF has four automotive-qualified recommendations for this socket: GRF2071W, GRF2073W, GRF4002W and GRF4014W. Note that the use of GRF207X devices for both LNAs in the cascade offers a powerful combination of ultra-low cascaded NF with a wide range of cascaded gain.
The components referenced above have been implemented within two reference designs which exceed the Sirius XM™ LNA requirements. A summary of each solution’s cascaded performance is provided in Figure 7.
When including an SDARS LNA front end within a shark fin antenna module, one must consider the two possible configurations referenced within the Sirius XM™ Vehicle Mobile Antenna Requirements document. Although there are certain requirements that are common between both of these use cases, the additional OOB blocking specifications associated the co-located transmitter case will often drive the overall architectural requirements of the LNA front end. Antenna designers will need to leverage their understanding of these architectural differences when creating the most cost-effective (yet spec-compliant) solutions for the automotive industry.