LNA Measurement
The LNA has been measured on the PCB from the packaged IC with the configuration shown in Figure 4-6(b). This is possible because the LNA is connected to the external SAW, making the input and the output directly available through the IC pins, as shown in Figure 4-4. The packaged LNA has been measured using the test setups described in Figures 4-7, 4-8, and 4-9.
Figures 4-10 and 4-11 show the input and output return loss for the high-gain mode of the LNA measured with the network analyser (Agilent E5071A). Both are below -10dB at the working frequency and therefore meet specifications.
Figure 4-7 Test setup for the characterisation of the gain and return loss of the LNA
Figure 4-8 Test setup for the characterisation of the noise figure of the LNA
Figure 4-9 Test setup for the characterisation of the 1dB compression point of the LNA
Figure 4-10 LNA high-gain mode return loss
Figure 4-11 LNA high-gain mode output return loss
The external input matching network has been implemented with MURATA components (0603) and is shown in Figure 4-12.
The input impedance matching is worse for low-gain mode than for high-gain mode. However, optimisation in the power transmission is preferred for high-gain mode, as low-gain mode will have an active antenna that will amplify the incoming signal. The input return loss for low-gain mode is shown in Figure 4-13.
Figure 4-12 LNA input matching network
Figure 4-13 Return loss of the LNA in low-gain mode
Figure 4-14 Gain of the LNA in high-gain mode
The reading of the VNA (Agilent E5071A) illustrated in Figure 4-14 shows the gain of the LNA in high-gain mode. Moreover, the noise figure for high-gain mode measured with the noise figure spectrum analyser option is shown in Figure 4-15. Additionally, the insertion loss of the SAW filter connected just before the LNA of 0.5dB has to be taken into account.
Figure 4-15 Noise figure (upper trace) and gain (lower trace) of the LNA in high-gain mode
Figure 4-16 Noise figure (upper trace) and gain (lower trace) of the LNA in low-gain mode
Therefore, a gain of 18dB and a noise figure of 3.2dB have been obtained for the LNA. For low-gain mode, as shown in Figure 4-16, the gain is 9.5dB and the noise figure 5dB, after considering the insertion loss of the SAW filter. The noise figure has been measured with the Agilent E4402B spectrum analyser with noise figure measurement option 219 and noise source 346A.
Figure 4-17 shows the input 1dB compression point for the LNA, measured when the signal generator sweeps the input power. It reaches -24dBm for high-gain mode.
Figure 4-18 shows the input 1dB compression point for low-gain mode, which is -30dBm. This is just slightly lower than the specified value and will not seriously affect receiver performance.
Figure 4-17 LNA high-gain mode 1dB compression point
Figure 4-18 LNA low-gain mode 1dB compression point
Table 4-1 summarises the specifications and the results of the post-layout simulation and measurement. It can be seen that the input and output return loss measurements differ from the post-layout simulation results. This is due to the matching network and the paths of the PCB. However, it meets high-mode gain specifications. The gain is around 1dB less and the noise figure is 0.4dB higher than the typical mean post-layout simulation results for high-mode gain, as could be expected from the experimental results. However, as explained in the previous topic, this is not a drawback. For low-gain mode, the gain and the noise figure meet specifications. The current consumption is slightly higher than specified. These are satisfactory results when considering the problems of amplifier stability that have arisen in the design stage.
Moreover, it must be remembered that the designed LNA includes ESD protections and has been measured in a package. Therefore, the results encompass the parasitic elements of these factors. Moreover, additional features are entailed, such as double-gain mode and the antenna detection sensor.
TABLE 4-1 LNA results
|
Parameter |
Specifications |
Post-layout |
Measurement |
Unit |
|||
|
Gmin |
Gmax |
Gmin |
Gmax |
Gmin |
Gmax |
||
|
S11 |
 |
-14 |
-18.2 |
-7.5 |
-16.9 |
dB |
|
|
S22 |
 |
-7.6 |
-21.8 |
-8.3 |
-11.4 |
dB |
|
|
Gain |
8 |
20 |
10 |
19.7 |
9.5 |
18.5 |
dB |
|
NF |
10 |
2.5 |
3.5 |
2.8 |
5 |
3.2 |
dB |
|
Current |
<7 |
7.5 |
8 |
mA |
|||
