In most any wideband RF front end (RFFE), the system bandwidth is limited by some means, even if that limitation is a simple preselector. Often a wideband RFFE that is connected to an antenna must meet aggressive rejection parameters to avoid being desensitized by adjacent band transmissions. Attenuating out-of-band transmissions that are close by can be challenging, particularly since modern SWaP-C requirements force solutions to be very small. Not only must the corner (3 dB) frequencies be tightly controlled, but the solution must be compact, economical and sometimes crafty enough to achieve sufficient rejection of unwanted carriers while simultaneously suppressing spurious emissions. In this article we start with an LNA that is wide open to neighboring transmissions and show how cascading a couple LTCC filters and a single MMIC filter achieves both band-limiting and spurious mitigation. Physically utilizing components that come in tiny, cost-efficient packages enables the band-limited RFFE to meet aggressive SWaP-C requirements.
Original Lineup – Flat Gain in a Two-Stage Wideband LNA
The lineup at the center of our application of LTCC and MMIC filters is featured in [1] and is shown here in Figure 1. This LNA is designed for a frequency range of 400 to 6000 MHz and incorporates equalization to flatten the gain. The two performance attributes that we are most concerned with are the frequency range of 400 to 6000 MHz and its flat gain of 30 dB typical.
The input stage is the Mini-Circuits TAV2-14LN+. This choice was originally made based upon its very low noise figure (NF) of 0.69 to 0.75 dB over the 400-6000 MHz band, but most importantly is its relatively high gain of 16-22 dB over the band of interest, even though its gain slope necessitates equalization. The amplifier for the final stage is the PHA-83W+, selected previously since it performs quite well in the linearity category. Important for this exercise is the fact that the total gain flatness across the 400-6000 MHz band for the PHA-83W+ is only ±1 dB. The original LNA had required a very low NF (< 1 dB), which meant that no attenuation could be added to the input. The decision to split the required amount of equalization between two identical SMT equalizers, a pair of EQY-3-63+, was made to achieve a flat 30 dB of gain across the band.
Cascading LTCC and MMIC Filters – Tiny Additions, Big Benefits
The original LNA of Figure 1 was featured in a previous Mini-Circuits’ article on RF/microwave equalizers1 and spans 400 to 6000 MHz but is essentially wide open to neighboring transmissions. By cascading LTCC high pass and low pass filters with that original LNA, the RFFE design will remain wideband, with corner frequencies at 1000 MHz and 6000 MHz, but incorporates steep roll-off at the band edges to attenuate undesired signals. In addition to steepening the skirt above the 6000 MHz corner frequency, a reflectionless MMIC filter also prevents unwanted spurious emissions that are reflected from the mixer from being reintroduced into the second stage of the LNA where intermodulation can occur. This band-limiting and spurious mitigation is achieved with components that come in tiny, cost-efficient packages to meet aggressive SWaP-C requirements.
As shown in Figure 2 only three components are added. The first of these is an HFCG-1000+ LTCC filter that has a corner frequency of 1150 MHz, and its flat, low-loss passband extends to 8000 MHz. Although its corner frequency is typically 1150 MHz, the gain of the TAV2-14LN+ is running up faster than the HFCG-1000+ can accumulate roll-off, forcing the lower cutoff frequency for the RFFE downward in frequency. Following the original two-stage LNA cascade is the XLF-312H+ reflectionless low pass MMIC filter that has a cutoff frequency of 5650 MHz, which seems low at a glance until we consider that, as frequency increases, the equalizers continue to dial out some of their attenuation, which pushes the effective upper corner frequency of the RFFE beyond 6000 MHz. The Mini-Circuits’ LFHK-6000+ ultra-high rejection low pass filter does not determine the upper band edge as its 3 dB frequency is 6850 MHz, but it is used as a clean-up low pass filter, and it supplements the attenuation of the XLF-312H+.
Frequency Response for RFFE with Cascaded High Pass, Reflectionless, and Low Pass
Examine the frequency response curve for the entire RFFE of Figure 2 as it is illustrated in Figure 3. The first thing to note is that the mid-band gain for the cascade is 27 dB, which is only 3 dB less than the original design. On the lower skirt, the HFCG-1000+ does a remarkable job of attenuating out-of-band signals, with the frequency response curve of Figure 3 showing 27 dB of attenuation at 500 MHz and 56 dB of attenuation at 100 MHz. Additionally, the HFCG-1000+ is packaged in an industry-standard 0805 package, which is tiny by any stretch of the imagination, enabling stringent SWaP-C requirements to be met. The upper skirt has rolled off by 50 dB at 8000 MHz and 85 dB at 9000 MHz. This level of high-end attenuation is predominantly due to the XLF312H+ and LFHK-6000+ working in tandem. The XLF-312H+ is a 3 x 3 mm (.12 x .12”) MMIC and the LFHK-6000+ is an LTCC low pass in a 1008 package.
It’s Only Fitting
The footprint area occupied by the three cascaded filter components discussed in this article is approximately 20 mm2. In most RFFEs, that would be considered a very small amount of space to give up in order to achieve excellent out-of-band rejection and mixer spur mitigation.
From Wide-Open to Band-Limited
In this article, we demonstrated how we could take a previously wide-open LNA and fashion a band-limited RFFE that incorporates different LTCC filter types and a reflectionless MMIC filter, which continues to provide high gain and that adds excellent selectivity. In addition to enhancing RF performance, it was imperative to add physically small filter components to the LNA such that the RFFE design remained compact and cost-efficient, making it quite capable of meeting demanding SWaP-C requirements.
References:
1. RF/Microwave Equalizers: An Essential Ingredient for the Modern System Designer – Mini-Circuits Blog
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