1, project background
A grid group needs to change the past infrared meter reading into a remote 408 MHz band wireless meter reading. Zhiyuan R & D team provides a solution for HPC-3000EWBM handlers + 408MHz modules based on customer needs. Many difficulties have encountered many difficulties in the implementation of this solution, but the end of the R & D team that is good at the ground is overcome.
2, send a link
Mockout machine, the communication distance between the wireless module installed on the handset and the wireless module mounted in the closed meter is only 3 meters! What we first suspect is that the wireless module has no power output, so that the transmit power of the transmission module has been tested. As shown in FIG. 2, the output power of the module has reached 10 dBm from the vector analyzer N9030A, and the decisor is excluded. The module PA works abnormal possibilities. Second, it is tested on the antenna connected to the module. As shown in FIG. 1, it can be seen on the N9912A to see the VSWR of 408 MHz at 408 MHz. The visible antenna is good to match the output impedance of the module. Since the launch circuit is working properly, will it be the right to send a packet format? In order to solve the problem, the N9030A combined with the VsA89600B software to capture the radio module to the spatial electromagnetic wave, and after multiple captain analysis: The wireless module has been in the correct package format Data is transmitted to the space in the form of electromagnetic waves. At this point, it is very certain that the entire transmission link is working properly, and the problem must be on the receiving link.
3, receiving link
When debugging the receiving link, it is done: connect a wireless module string one 20dB of attenuator to another wireless module through the feeder, then send and receive data, and increase the amount of attenuation of the serial failure, when the decrease is increased Data can also be stably sent and received during 115dB. However, remove the feeders and attenuators, screw the antenna, still only 3 meters or so. Can a 408 MHz electromagnetic wave at 115dB of the distance 3 meter distance? Obviously it is impossible, what is otherwise? At this point, an empiric engineer put forward the idea of testing the noise of handheld.
4, noise discovery
This measures taken when performing noise testing: the handset in the normal working state is adjacent to the antenna fixed in the shield box, and the output of the antenna is coupled to N9030A through the feeder, as shown in FIG.
Sure enough, there is a frequency of about -78 dBm in the frequency of 407.993 MHz, and the source is constantly radiant electromagnetic wave, and the test results are shown in Figure 4.
5, noise source positioning
The noise spectrum analysis found that there is a relatively strong power in these frequencies in these frequencies ... 404 MHz, 408MHz, 412 MHz ..., so that the noise source is a 4MHz signal, and the noise of these frequencies is its highly harmonic. Subsequently, start the crystal and clock signal on the handheld, and struggle for two days or no sources of noise. Later, during the measurement process, when everyone is right, the handset has entered the screen protection status, the noise power drops to -105dBm! Everyone immediately thought that this noise must be from the screen. After the screen circuit hardware and software analysis, this noise source is the 4MHz square wave clock output after the screen clock is divided. Since the hardware needs to take a long time, the cross-division coefficient of the screen is adjusted, so the screen clock is slightly deviated from 4 MHz so that its 12 harmonic is not falling within the communication band (408MHz ± 100 kHz). The noise power level after modifying the screen division factor is shown in Figure 5, and the noise in the communication band will decrease by about 25 dB, and the communication distance has increased to 50 meters. It is clear that the requirements cannot be met, and it is also necessary to improve the communication distance.
6, solution
The radio module is at the level of -107dBm, and wants to ensure that the bit error rate is 100,000. The signal to noise ratio (S / N) is more necessary, that is, when the noise power level is below -119dBm, There is little impact on communications. After rectification of the screen clock signal, the noise peak is worth a large weakening, but there are many spikes in -105dBM, these noise spectrums are similar to white noise spectrum, which is superimposed from multiple devices to generate noise, want to further reduce handheld The noise of the machine increases the distance, it is obviously unrealistic. Although it can't reduce the noise of the handset, we can still reduce the noise of the module working environment, and the method is actually very simple, as long as the wireless module is masked. When the wireless module is coated with a wireless module (see Figure 7) with a copper foil paper, the meter reading is performed, and the communication distance reaches more than 100 meters. This little attempt verifies that the idea of increasing the communication distance is correct, and this is the final plan, just use the shield cover instead of copper foil.
7, conclude
Among the development of RF products, single engineers have not been able to meet the current new technology development requirements, but also to borrow advanced instruments (such as N9030A + N9912A + N5182A) to assist in the development of products.
Source: Wiku Electronic Market Network
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