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Microwave System Analyzer uses the satellite end-group delay measurements

In Electronic Infomation Category: M | on April 21,2011

Like almost all telecommunications systems, the bandwidth on the satellite systems require higher and 74F543 datasheet and higher, because these links are hosted on the Internet business, digital TV and 74F543 price and other digital traffic continues to climb. Results of satellite operators to use until the band was forced to complete the edge of all the available bandwidth, while the band edge of the signal quality of the transmission path of the satellite transponder and 74F543 suppliers and ground systems and other components used in filters and declined.

Existing satellite transponder in a typical configuration of channel bandwidth 36MHz to 72MHz range - depending on the satellite system - although the update the system configuration of the bandwidth of several hundred MHz are not uncommon. Since the band edge of these channels will increase the range of delay variation, so it may cause signal distortion or signal deterioration. To determine the amount of signal distortion or deterioration, measurement of these transponder channel bandwidth relative group delay is very important. Compensatory measures in order to take accurate data in order to avoid possible confusion, this delay test is necessary.

Error-free transmission of information in order to require the bandwidth of interest within the flat amplitude response and linear phase response. Indicators of group delay flatness, phase linearity can be used to measure the indicators measuring a variety of methods. The two most commonly used method is to direct the phase and envelope delay (delay modulation). Most commonly used vector network analyzer directly to the phase method, although these instruments can provide high accuracy, but the characterization of frequency conversion equipment is very complex. Sophisticated microwave system analyzer can overcome these problems. This instrument will be the receiver spectrum analyzer has been tuned for the scalar input, and using envelope delay measurement techniques.

Microwave System Analyzer (MSA)

Microwave System Analyzer (MSA) usually incorporates a spectrum analyzer and a comprehensive source and scalar analyzer instruments, and increased the frequency modulation and envelope group delay option. So the Microwave System Analyzer can measure the frequency conversion equipment and network, but also on the components and subsystems for group delay measurements.

Microwave system analyzer source and the receiver can be independent of each other, so that incentives can work in a frequency source, and the receiver to receive different frequencies in another. Group delay measurements from the envelope modulation, the modulation can be preserved through the frequency conversion, it can be the direct representation, do not access the local oscillator in the test device does not need to use an external mixer and oscillator.

Figure 1: In order to get the best performance of satellite links, the need for precise measurement and compensation in the band edge caused by the group delay distortion.

Group delay using the MSA for measurement and calibration

Order to measure the group delay, first with a known low-frequency signal source frequency modulation, and the result is applied to the device under test (DUT). Through the DUT, the signal is demodulated to recover the low-frequency signal phase with the original phase modulated signal can be compared to the results of group delay. Modulation envelope delay is the average group delay bandwidth. Modulated signal bandwidth is known as the measurement aperture, smaller than the group delay changes needed to get accurate results. The maximum absolute group delay measurements is equal to half of the positive and negative modulation frequency cycle.

Calibration inherent delay within the MSA can be a simple direct connection very fast to complete, as shown in Figure 2. This frequency conversion equipment, precision may be some impact, but in most cases a small decline this accuracy is not a problem. Using the "gold standard" technology can achieve higher accuracy, this method first measurement had a perfect characterization of the components, and then compared with the DUT, which described in detail in the next section.

Figure 2

: Microwave System Analyzers direct line calibration

Figure 3 shows the measured 2.2GHz to 500MHz with the MSA under the drive amplitude and delay response of the screen shots, then the MSA direct connection method is calibrated. In some cases, the direct connection may not provide sufficient calibration accuracy. Replace the method is to use the delay performance with a known or set the "gold standard" equipment calibration instruments. Appropriate gold standard device can be matched with external local oscillator good broadband mixer. Another option is to use the improved DUT, a key component of its group delay (usually a filter) needs to be bypassed.

Figure 3: The next drive amplitude and delay response

Converter accuracy and drift of the oscillator

Aeroflex * 0 MSA using the FM envelope delay method has great advantages. VNA with some methods, it does not require access to the local oscillator frequency converter, but the accuracy of the oscillator has certain requirements.

* 0 maximum resolution spectrum analyzer (aperture) when the bandwidth is measured in the group delay is fixed at 3MHz. Taking into account such factors, the resolution bandwidth and modem, which means that the MSA in the actual measurement frequency offset must be located in the actual frequency of 500kHz range.

Error will cause the spectrum analyzer filter frequency response of a certain group delay offset, if only interested in flatness, it no problem. However, if the converter local oscillator frequency stability is poor, then this offset would drift up and down, resulting in greater difficulty to solve. Although this situation will certainly help automatically adjust, but the only practical solution is to stabilize the converter LO. The magnitude of this effect by measuring the frequency error is generally produced 0.1ns per 1kHz group delay variation.

Satellite in orbit group delay

Components for the more portable test platform for testing the flatness, group delay is a good way, but there are many cases, test equipment can not be applied this approach. A special example is the DUT is a complete satellite link, which obviously can not be tested in the laboratory to measure a single instrument. In-orbit satellite test carried out for many reasons. In general after the satellite launch, the satellite prior to release to the user to measure traffic load and the purpose is to verify the integrity of the antenna platform. In daily operation need to perform periodic testing to verify performance or to solve unusual problems.

On the satellite link, the relative frequency of group delay, especially through the group delay after the frequency conversion, proved particularly difficult to measure the parameters. Figure 4 shows the linear and parabolic group delay, which is common in satellite networks delay type. Parabolic satellite transponder delay is usually used in communications equipment, and band-pass filter on.

Figure 4: The group delay and send the spectrum

Sinusoidal delay is usually caused by impedance mismatch in the system caused. Ideally, the group delay curve should be flat - no slope in a straight line - contains all frequencies within the frequency bandwidth of the link from time to time by the same delay. If you reached this situation, then the recovered there will be interference between digital, making it difficult to distinguish between them, thereby generating error.

Measurement system

Aeroflex satellite group delay test system equipped with two optional group delay of the microwave system analyzer 22, together with dedicated software running on the PC, and serial modem control and realize the whole relative links group delay measurements. First by the test system to produce test signals and applied to uplink and downlink reception analysis to the forward signal, resulting in changes in group delay, and the implementation of in-band gain flatness measurements. The two systems to keep pace in the frequency scan. The system can be used to measure

from multiple ground stations (in the same location or remote) until the in-orbit satellite transponder group delay links and other transmission characteristics. Setting the screen to select the input, output, and / or switching frequency and level.

Transfer time

Come and satellite transmission time is considerable, even in low Earth orbit satellites. Synchronous satellites in geostationary transfer time is about a typical 250ms. Because the source and the receiver frequency is synchronous, which means that in practical applications, the aperture is about 1MHz receiver received signal may be moved to the outside, thus the need to further shift the frequency of the source and receiver to compensate transmission time.

This offset formula is:

Foffset (MHz) = scan rate (MHz / ms) * transmission time (ms)

As an example, assume that the uplink (source) frequency is 14GHz to 14.5GHz, the downlink (receive) frequency is 11.2GHz to 11.7GHz, the satellite is in geostationary orbit, MSA scan time to 10 seconds, aperture (resolution bandwidth) is set to 1MHz or 3MHz. So if the transmission time

is 285ms, scanning speed is 0.05MHz/ms,

Foffset = 14.25MHz

Alarm transmitter to avoid, the best source of constant frequency (ie 14GHz to 14.5GHz). Receiver should be located between the 11.18575GHz and 11.68575GHz scan. The instrument will display the received frequency range, and the received frequency itself will fall well within the resolution bandwidth. Prediction may not be offset simply because of the geostationary satellite is actually not static, but also need to consider the Doppler component. Doppler frequency shift is a change in one day, the cycle is repeated every day, twice a day to zero. Doppler frequency is very easy to measure, it must apply the extra shift. Without considering the Doppler frequency shift may be generated on the group delay slope.

In-orbit measurement

Figure 5 shows a single ground station for satellites in geosynchronous orbit group delay measurements. Input (uplink) frequency is 14.47GHz to 14.5GHz, the output (downlink) frequency is 12.17GHz to 12.2GHz. Calibration completed at the input frequency - usually in the frequency instead of the source frequency receiver calibration, the purpose of switching through the band and frequency modulation hardware changes to eliminate the delay inside the instrument.

Instrument calibration and measurement of performance and group delay measurements above the same individual. In this case, the scan time is 10 seconds, the scanning speed is therefore 3kHz/ms. Transmission time offset of less than 1MHz, 3MHz therefore bore the case of data can be ignored.

Figure 5: measured synchronous satellite in geostationary orbit group delay

Remote ground station

Can also link ground station not in the same location group delay for such tests. MSA as the source of the control and operation of PC-specific software provider in the link master. MSA for the second receiver installed at the receiving end, the receiver can be located in the satellite signals can cover any corner of the earth. Local MSA use control interface, remote MSA using a serial connection via a modem, the instrument can be analyzed on the links section to the relative group delay measurements.

Either end because the system uses the local high-stability frequency reference (typically rubidium clock), two instruments in the frequency sweep can maintain accurate synchronization. MSA returned from the remote measurement data to the local PC, for the results of the inspection and storage. Any one MSA can be configured to source or receiver, because mobile devices do not need to be able to achieve two-way transmission path testing. If there are multiple receiving stations, each station can be installed in a remote MSA, to achieve a remote site in one place for all measurements.

This Summary

This article describes the end for satellite communication system as a test measurement system. This test system is equipped with two dedicated group delay measurements of microwave systems and proprietary software analyzer. Aeroflex * 0 to 46GHz series of MSA coverage of all current and future satellite band. The system can also be used as ground system * assessment and installation. Band group delay measurements of distortion make timely compensation, so as to ensure link integrity of the data transmitted.

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