MAX2659 GPS LNA优于十款同类产品 (MAX26

电子工程师 2008-08-11 3454

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MAX2659 GPS LNA优于十款同类产品-MAX2659 GPS LNA Outperforms Ten Competitive Parts

Abstract: This application note objectively compares performance and solution size/cost of the MAX2659 GPS LNA against 10 current competing GPS LNA products. Figures of merit such as gain-current efficiency, output power efficiency, and GPS input signal dynamic range are used for comparative evaluation.

Purpose of This Report

The MAX2659 was developed for mobile/handheld equipment to meet the size and solution cost requirements of low-cost consumer electronics equipment. It was designed to outperform all other low-cost, high-performance handheld GPS LNAs.

Due to highly positive reports from Maxim's IC design engineers, Maxim's Application Engineering group decided to test whether this new LNA indeed lived up to such encouraging reports. Therefore, we set up a comparison, including GPS LNAs from both Maxim and the competition that had roughly similar performance and existed in our customer's designs. We looked for:

1575MHz low-powered operation
15dB to 20dB gain and less than 1.5dB NF
3V operation with less than 10mA supply current
Whether it had been designed into commercial consumer equipment with high volumes (to ensure that the LNA is deliverable, stable, and cost-competitive)

This set of requirements represents the class of GPS LNAs akin to the MAX2659. We then selected 10 other LNAs that compete more or less fairly with the MAX2659 and performed a comparison. If you would like to skip all the analysis, you will find our summary at the end of this article.

Performance Figures of Merit

There are many ways to compare RF amplifiers, and we selected metrics that are objective, measurable, and focus on low power, high gain, and dynamic range. We decided that the most meaningful figures of merit for a comparison of GPS LNAs were:

Gain-current efficiency (dB gain/mA)
Output-power efficiency (% ratio P1dB/P_DISS)
GPS input-signal dynamic range in dB

We also compared the passive-circuit parts count, which relates to board-area size and solution cost.

Gain-Current Efficiency (dB Gain/mA)

A traditional way to assess an amplifier's "enthusiasm", or responsiveness, can be made by measuring the ratio of its signal gain to the DC energy applied. For example, transconductance is the traditional responsiveness figure-of-merit used for comparing individual semiconductor transistor devices. For comparing a group of LNAs, we use gain-current efficiency.

For our comparison, we calculated the dB gain available for the standing bias current. The less current required to obtain higher gain, the better the LNA's gain-current efficiency. A 20dB amplifier that draws 5mA bias current has a figure of merit equal to 4.0dB/mA.

Output-Power Efficiency (% Ratio P1dB/P_DISS)

Output-power efficiency is important for comparing how the LNA manages large interfering signals (Figure 1) with minimal battery drain. It can be measured by how much signal power the amplifier can deliver compared to the DC power it consumes. An RF amplifier's drive capability is often measured as 1dB gain compression point, which is the point at which the output-signal gain compresses by 1dB (i.e., a 9dB output-level change is observed for an input-level increase of 10dB).

LNA

Figure 1. Typical amplifier signals.

We use a ratio of the output-signal compression point (P1dB), to the total power dissipated (P_DISS). Suppose an amplifier has an input P1dB compression point (IP1dB) of -20dBm and a gain of 17dB, we estimate its P1dB at -3dBm, or 0.5mW. If that amplifier drew 5mA from 3V supply, its P_DISS would be 15mW, and the % output-power efficiency is calculated at:

0.5/15 = 3.3%

This represents a mediocre performance figure for a GPS LNA, as Table 1 shows. Note that, if the amplifier's IP1dB was higher at -12dBm, then P1dB could be estimated at +5dBm or 7mW, and the output-power efficiency would increase to 5%.

GPS Input-Signal Dynamic Range in dB

Spurious-free dynamic range (as in Figure 2) indicates ability to handle very small signals in the presence of large interferers—for GPS input amplifiers, this capability is very important.

LNA

Figure 2. Spurious-free dynamic range.

Typical signals levels that are expected at the LNA output include antenna (source) thermal noise gained up by the LNA (plus noise figure) and any narrowband interfering signals. For GPS processing, we will assume 2MHz RF noise bandwidth (BW), which is the typical GPS-under-noise signal BW presented to the baseband digital processor. However, it is certainly possible to gather more spectral width (military systems) or less BW (less than around 1.5MHz BW begins to throw away valuable signal power). We also assume a mild input filter with roughly 100MHz of RF BW that greatly suppresses cellular, PCS, and Wi-Fi band signals from co-located transmitters.

The following are gained up to form the LNA's output spectrum:

kT noise plus 1dB noise figure in 2MHz BW produces -110dBm input noise
Input signals from a co-located 802.11 transmitter are around -45dBm after input RF filter
Input signals from a PCS or cellular band transmitter are around -25dBm after input RF filter

Large spurious signals drive the LNA into gain compression, so we refer our dynamic range from noise floor to 1dB compression point at the LNA output. For example, an LNA with a gain of 20dB and -15dBm interfering input signal produces 5dBm at its output. If its output compression point is 0dBm, then it will be operating very heavily into compression. This will reduce the gain by perhaps 3dB or 4dB, thus greatly reducing GPS receiver sensitivity.

Therefore, we specified the dynamic range figure of merit as P1dB minus output NF in dB.

Passive Circuit Parts Count

The number of external matching and power-supply decoupling components figures into the decision for designing in an LNA, even though these parts costs represent a small fraction of the overall solution cost. The main objection to having many external parts is circuit performance variation and circuit size. For this reason, Avago™ integrated a GPS bandpass filter into its ALM-1412 LNA. However, as we found out during this exercise, lackluster performance in other areas detracted from it being a clear performance leader.

Results of the MAX2659 LNA Comparison

Here we show two tables of LNA performance, representing competitive 10 LNAs that we have seen in commercial consumer-product use, roughly in the same class as the MAX2659.

Note that we sought datasheets from the web, and the figures we present here are as accurate as we were able to obtain through normal public advertisement channels. In the case of some of these devices, there may be improved versions, but the models presented here are what we have run across in our regular work over roughly the past year.

Table 1. GPS LNA Competitive Vendors Performance Comparison (1575MHz Data)
Note: Specifications are data sheet "typical" performance.

Specification	Avago ALM-1106	Avago ALM-1412	Maxim MAX2641	Maxim MAX2654	Hitachi® HA22022	Maxim MAX2659
V_CC range (V)	1.0 to 3.6	2.7 to 3.6	2.7 to 5.5	2.7 to 5.5	2.7 to 5.0	1.6 to 3.3
V_SUPPLY test (V)	2.85	2.85	2.8	3.0	3	2.85
I_BIAS (mA)	8.0	9.0	3.5	5.8	3.0	4.1
Gain (dB)	14.3	13.1	15.7	15.1	16.0	20.5
NF (dB)	0.8	0.8	1.2	1.5	1.3	0.8
IIP3 (dBm)	+4.7	+7.0	+1.4	-7.2	-2.0	-5.0
IP1dB (dBm)	-12.5	-9.7	-21.0	-18.0	-18.0	-12.5
S11 (dB)	-11.8	-9.0	-12.0	-9.7	4.6	-16
S22 (dB)	-12.4	-10.0	-12.0	-8.4	4.6	-7.5
S12 (dB)	—	-23.0	-30.0	-30.0	—	-32
Gain efficiency (dB/mA)	1.79	1.46	4.49	2.60	5.33	5.00
Power efficiency (% mW/mW)	7	11.7	3	3	7	54
Dynamic range (dB)	97.7	100.5	88.8	91.5	91.7	97.7
No. of external parts	8	Integrated BPF	7	5	4	3
Specification	Infineon® BFP740F	Infineon BGA615L	National Japan Radio NJG1107HB3	NEC® uPC8321	NEC uPC8322	Maxim MAX2659
V_CC range (V)	2.7 to 3.5	2.4 to 3.2	2.5 to 5.5	2.7 to 3.3	2.5 to 3.3	1.6 to 3.3
V_SUPPLY test (V)	3.0	2.8	2.7	3	3	2.85
I_BIAS (mA)	8.2	5.6	2.5	3.8	3.0	4.1
Gain (dB)	19.8	18.0	17.0	20.0	17.0	20.5
NF (dB)	0.7	0.9	1.1	0.8	1.0	0.8
IIP3 (dBm)	-1.7	-1.0	-4.0	-10.0	-8.0	-5.0
IP1dB (dBm)	-18.0	-14.0	-16.0	-22.0	-21.0	-12.5
S11 (dB)	-9.0	-13.0	-6	-10.0	-10.0	-16
S22 (dB)	14.4	-15	-6	-18.0	-20.0	-7.5
S12 (dB)	28.7	-35	-24	-35.0	-40.0	-32
Gain efficiency (dB/mA)	2.41	3.21	6.80	5.26	5.67	5.00
Power efficiency (% mW/mW)	6	16	19	6	4	54
Dynamic range (dB)	92.3	96.1	93.9	88.2	89.1	97.7
No. of external parts	8	7	8	7	5	3

Comparing Gain-Current Efficiency (dB Gain/mA)

Originally, we had thought that the higher-gain LNAs would probably fare best in this comparison, but that is not the case. Consider for example the MAX2641, an older but popular SiGe LNA introduced in 1998—it has an excellent 4.49dB/mA gain efficiency, but a gain of only 15.7dB.

The best of the group was New Japan Radio's NJG1107 at 6.8dB/mA gain efficiency, due to its low bias current of only 2.5mA. The worst were the Avago LNAs with less than 2dB/mA gain-current efficiency.

Overall the MAX2659 fared acceptably at 5.0dB/mA, where it ranks fifth from the top, placing it among the upper group of LNAs. Four of its peers have quite a similar gain efficiency.

Comparing Output Power Efficiency (% Ratio P1dB/P_DISS)

Here the MAX2659 really shines; it is clearly the best in its class. Most of the LNAs we investigated had 3% to 7% power-efficiency range. However, the MAX2659 provides an excellent 54% due to its high P1dB and modest power dissipation. Several peers offer 15% to 20% power efficiency.

Comparing Signal Dynamic Range in dB

The Avago ALM-1412 with integrated GPS bandpass filter ranked first in dynamic range at 100.5dB. The MAX2659 and the other Avago LNA we compared tied for second place at 97.7dB, about 3dB less than the ALM-1412. The lowest dynamic range we observed was 88.2dB for NEC's uP8321. Most LNAs we looked at appeared to offer roughly 91dB to 95dB of signal dynamic range.

Comparing Passive Circuit Parts Count

We obtained our external passive parts count by examining the datasheets and/or application circuit provided on the web and, in some cases, there may be lower parts count circuits than we were able to find. However, it is important to note that we represent typical RF application engineers looking through the web for data when making our decisions.

As most LNAs require six or seven external parts, the MAX2659's requirement of only three external parts makes it the clear leader for use in small design solutions.

Summary of Our GPS LNA Competitive Survey

All of the LNAs presented here offer some value, whether in price, size, or electrical performance. However, the MAX2659 not only has competitive features, but it is also priced "to compete aggressively."

Therefore, we ended up agreeing with the IC design engineers' claims that the MAX2659 is the best overall in its class, as it:

has the highest power efficiency and second-highest dynamic range,
provides the lowest parts count,
is among the top rankings for good gain efficiency,
and is priced to compete aggressively.

We think it is a genuine success. Apparently, so do many of our customers who have adopted it and designed it in.

Avago is a trademark of Avago Technologies Ltd.
Hitachi is a registered trademark of Hitachi, Ltd.
Infineon is a registered trademark of Infineon Technologies AG.
NEC is a registered trademark of NEC Corp.

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