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A&D Test & Measurement
Efficient Design and Analysis of Airborne Radomes
The loop test translator (LTT) is an extremely valuable tool for evaluating the performance of satellite earth stations. It allows the user to carry out analysis, alignment and testing without incurring satellite airtime costs and the risk of interfering with other satellite users. Thus, it has applications during equipment development, qualification, trouble-shooting and in-service routine monitoring.
A basic loop test translator comprises a mixer and local oscillator to translate the ground-to-space transmit frequency to the ground-from-space receive frequency in just the same way that the satellite does. In practice other features are provided to enhance usefulness and flexibility.
The ALT range of loop test translators from AtlanTecRF covers the satellite communication bands S, C, X, Ku, DBS and Ka. They are block converters, which means they convert the whole transmit band with a single frequency LO to the required receive band or directly to the first IF band (which is typically in L-band). Table 1 shows the breadth that the standard ALT product range covers; non-standard bands and non-standard LO frequencies can also be provided.
The local oscillators leverage the company’s expertise in low noise phase-locked oscillators. Standard products have internal 25, 50 or 100 MHz reference oscillators, according to model, but a range of options, including 10 MHz internal and/or external reference frequencies can be supplied for compatibility with station reference signals. Enhanced stability of internal references is available using OCXO and TCXO solutions. A reference frequency output is supplied together with a loss of phase lock alarm.
Double balanced mixers are used and the input path features a 30 dB variable attenuator, adjustable via a 10-turn control with dial. Optionally, a 69 dB range step attenuator can be provided. The purpose of the input attenuator is to set the input to the mixer so that it is operating at the correct level. The overall conversion loss is 20 dB nominal, or 35 dB for Ka-band.
These LTTs are normally ‘wide open’, i.e. they contain no filtering and no additional gain. This provides the optimum conditions for the investigation of spurious signals. However, both filtering and additional gain to compensate for conversion loss can be provided if required.
The situation arises in Ku-band where a common transmit frequency range is associated with three differing receive bands. This can be addressed by a three-band model in which three local oscillators can be switched via a front panel control to allow a single unit to be used for all three receive bands. This is useful for testing mobile or fly-away systems, which may have to be field-configurable to suit the region in which they are deployed.
Figure 1 Three-band switched unit with custom-designed test translator.
Figure 1 illustrates a three-band switched unit together with a custom-designed test translator. The same principle of combining several LOs in one product can be applied to provide a cost-effective combined C- and Ku-band unit. Custom translation frequencies can also be supplied.
The standard mechanical configuration is 19-inch rack mounting with a height of only 1U to minimize space in a crowded earth station rack, although other mechanical arrangements can be provided, including weatherproof enclosures designed for external mounting. Similarly, connector configurations can be changed to suit the application and waveguide interfaces can also be provided if necessary.
The ALT-1750-KuE LTT is an Extended Ku-band (13,750 to 14,500 MHz) unit translating to 12,000 to 12,750 MHz via a 1,750 MHz LO with internal reference. It has a nominal conversion loss of 20 dB. Figure 2 shows a typical plot of the output with a transmitter input of 0 dBm at 14.25 GHz. This unit has an LO of 1,750 MHz and its seventh harmonic can be observed at 12,250 MHz.
Figure 2 ALT1750-KuE with 0 dBm input at 14.25GHz.
Figure 3 Expanded centre portion of Fig. 2.
Figure 3 is an expansion of the centre portion of the display. The local oscillator in this unit has a typical phase noise of -105 dBc/Hz at 1 kHz, extending to -140 dBc/Hz at 1 MHz offset. Frequency stability with a standard internal reference is ±5 ppm maximum over 0° to 50°C, with a typical stability of ±2 ppm over +10° to +40°C and ±2 ppm maximum per day.
The ALT-9800-Ka and ALT10300-Ka models cater for the increasingly popular Ka-bands. They cover transmit frequency bands of 27.5 to 31.0 GHz and 28.0 to 31.5 GHz, respectively, both models downconverting to 17.7 to 21.2 GHz.
Most operational earth stations do not have the luxury of a continuously-available spectrum analyzer capable of observing waveforms at signal frequency, so the LTT provides a convenient and cost-effective way of downconverting the uplink signal to enable it to be seen in the receiver or with an IF spectrum analyzer. Typically this enables checks to be made on the modulation, power levels, spurious and noise. Used in conjunction with a baseband BER test set, BER can be measured. If group delay measurements are required, the LO in the LTT can be locked to the station reference to improve accuracy.
In the event of a hardware failure within a VSAT network it is common for the transceiver to be replaced in the field and returned to the service-provider’s central workshop. The LTT is the ideal tool for verifying the perceived fault under controlled conditions before embarking on a costly repair process or returning the transceiver to the supplier. The availability of multiple translation frequencies in a single unit adds to flexibility at minimum cost.
This ALT range of loop test translators is suitable for off-air testing and monitoring of satellite earth station equipment, including the analysis of spurious, modulation and alignment of transmitter chains. Models are available for the satellite bands S to Ka and a wide range of options can be specified.
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RS No. 301
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