WO1988002200A1

WO1988002200A1 - Power control system for very-small-aperture-terminal

Info

Publication number: WO1988002200A1
Application number: PCT/US1987/002330
Authority: WO
Inventors: William E. Mcgann
Original assignee: Harris Corporation
Priority date: 1986-09-15
Filing date: 1987-09-15
Publication date: 1988-03-24

Abstract

Operation of a solid state power amplifier (SSPA) (30) in a very small aperture terminal is controlled by a digitally controlled preemphasis attenuation operator upstream of the input of the SSPA. The attenuation operator contains a temperature compensation section (42) that produces an output control voltage which is complementary to the gain-versus-temperature characteristic of the SSPA. This control voltage is scaled and applied as one input to a analog-to-digital converter (41), the output of which drives a digitally controlled (PIN) attenuator (34) at an input to the SSPA-containing up-converter circuit. A second input to the attenuator is derived from the earth station's monitors and control processor (61) which monitors the operating frequency and accesses an associated look-up table containing a characteristic representative of the variation in the power output of the SSPA versus change in frequency.

Description

_Power Control System For Very-Small-Aperture-Terminal

FIELD OF THE INVENTION

The present invention relates in general to satellite communication networks, and is particularly directed to a mechanism for automatically maintaining a constant up-link effective irradiated RF power at a remote earth station independent of the master station. BACKGROUND OF THE INVENTION

The increasing costs of leasing telephone lines and the need for interconnect capability of small computer terminals to mainframe systems has led to a rapid expansion of the satellite communications industry, particularly for business applications. Until recently, nearly all satellite communication networks were dedicated to C-band systems. With the increasing availability and reliability of Ku-band systems, however, which are less subject to ground interference than C-band systems, more users are turning to very-small-aperture-terminals (VSATs) which are notably cheaper, considerably more mobile and easier to install than the larger, conventional C-band stations. For controlling the power level of its up-link signal transmission to the satellite, the VSAT earth station typically employs a solid state power amplifier (SSPA) as part of its up-converter (IF-RF) circuitry. Because of the nature of the device, the output power capability of an SSPA changes significantly with temperature (e.g. an lldB gain change for a 90°C temperature (-40° to +50°C), as shown in curve 11 of Figure 1) and therefore requires compensation. Conventional compensation has been provided by controlling the bias network of the field effect transistors, which, at best, is capable of reducing the gain variation to a narrower range of 4dB over the 90°C ambient temperature range (as shown by curve 12 in Figure 1) .

In addition to its above-described temperature sensitivity, the gain of the SSPA varies considerably with change in frequency (e.g. a 6-7dB fluctuation over a 500MHz One proposal for handling this latter variation has been to operate the SSPA at saturation, where the gain versus frequency characteristics is relatively fla _f as shown at curve 22 in Figure 2. Operating at saturation, however, is unacceptable due to the spectrum regeneration caused by using the SSPA in its nonlinear range. This problem could be handled by employing a travelling wave tube in the VSAT earth station transmission equipment. To do so, however, would defeat the relatively low cost attractiveness of the VSAT.

SUMMARY OF THE INVENTION

In accordance with the present invention, the above- described limitations on the use of solid-state power amplifiers in a very small aperture terminal are obviated by a power control mechanism that incorporates a digitally controlled preemphasis attenuation operator upstream of the input of the SSPA. This complementary attenuation operator effectively combines separate control functions each dedicated to addressing a respective cause of reduced performance and the gain variation versus frequency change in the characteristic of the SSPA.

For this purpose the attenuation network contains a temperature compensation section that produces an output control voltage which is complementary to the gain-versus- temperature characteristic of the SSPA. The characteristic of the section is determined by initially setting the SSPA at a fixed level of attenuation and measuring the variation in up-link output power with respect to change in ambient temperature over a prescribed temperature range (e.g. -40°C to +50°C) . Using this transfer function, a corresponding complementary function is generated using a network of temperature responsive components (thermistors, sensistor and resistors) . By applying a precision reference voltage as an input to this network there is obtained a precision output control voltage that faithfully mirrors the gain vs. voltage is scaled and applied as one input to an analog-to- digital converter, the output of which drives a digitally controlled (PIN) attenuator at the input to the SSPA- containing up-σonverter circuit. A second input to the attenuator is derived from the earth terminal's monitor and control processor which monitors the operating frequency and accesses an associated look-up table containing a characteristic representative of the variation in the power output of the SSPA versus change in frequency (e.g. over the 500 MHz band of interest) . For each frequency assignment a differential power correction code is read-out of the look-up table, scaled, and combined with the control code employed for temperature compensation.

As a further control factor for optimizing terminal performance, the present invention monitors the downlink carrier fade variation of the automatic gain control signal and compares this value to a prescribed reference voltage. If the monitored level drops below the reference level, it assumed that the link has suffered a rain fade, requiring an increase in up-link power. The voltage differential from the AGC level comparison is added to the scaled temperature/frequency adjustment control voltage which is scaled and applied to the A-D converter. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graphical illustration of the relationship between gain and temperature of a solid-state power amplifier;

Figure 2 is a graphical illustration of the relationship between gain and frequency variation of a solid-state power amplifier; and

^' Figure 3 is a schematic block diagram of a VSAT power control system in accordance with the present invention. DETAILED DESCRIPTION

Before describing, in detail, the particular improved VSAT power control system in accordance with the present primarily in a novel structural combination of conventional signal amplified processing circuits and not in the particular detailed configuration thereof. Accordingly, the structure, control and arrangement of such conventional circuits have been illustrated in the drawings by readily understandable block representations, which show only those specific details that are pertinent to the invention, so as not to obscure the disclosure with structural details which will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagram illustration does not necessarily represent the mechanical structural arrangement of the exemplary system, but is primarily intended to illustrate the major structural components of the system in a convenient functional grouping_r whereby the invention may be more readily understood.

Referring now to Figure 3 of the drawings, there is illustrated a schematic block diagram of a power control system for providing digitally controlled preemphasis attenuation of the input signal to a solid-state power amplifier incorporated in the up-converter circuitry between the IF input and RF output of an earth terminal station.

More specifically, ^'"an input signal (e.g. an intermediate frequency signal having a frequency of 140MHZ, for example) to an up-converter 35 of the earth station transmitter is coupled to the up-converter via input link 31. The output produced by up-converter 35, on link 32, has a frequency corresponding to the transmission frequency of the earth terminal, as beamed from the earth terminal antenna to the satellite. For purposes of the present description, it will be assumed that this output frequency lies in a range of 14.0-14.5 GHz. As described briefly above, within the up-converter circuitry of the VSAT earth station the principal power control element of a solid state power amplifier (SSPA) 30 the input of which is output of which is coupled through a waveguide filter stage 36 to an orthomode transducer 37. The output of orthomode transducer 37 is coupled over link 32 to an antenna feed horn (not shown) . As described briefly above, the behavior characteristics of the SSPA 30 are such that its gain varies considerably with a change in temperature and frequency. Conventional schemes for reducing the gain variation with temperature involve controlling the FET bias network for SSPA 30. Such bias control, however, enjoys only limited success and still leaves a variation of approximately 4dB over an ambient temperature swing of 90°C, as shown in Figure 1, referenced above.

A second, an equally significant problem is the considerable gain variation of SSPA gain with frequency change. In the present day VSAT earth stations, the modulation format may include frequency hopping techniques. As can be seen from behavior characteristic 21 in Figure 2, excursions in operating frequency are accompanied by a dramatic change in SSPA gain.

In order to compensate for this dramatic variation in SSPA performance, at the input or upstream (relative to SSPA 30) end of the up-converter 35 a digitally controlled attenuator 34 is inserted between IF input link 31 and up- converter stage 35. Attenuator 34 may comprise a precise PIN attenuator the attenuation setting of which is controlled by a digital control code supplied by an analog- to-digital (A-D) converter 41. As will be explained in detail below, converter 41 receives a combination of inputs from individual compensation networks including a temperature compensation network 42 for compensating for the SSPA gain vs. temperature characteristic illustrated in Figure 1, a frequency compensation network 43 for compensating for the gain vs. frequency changes shown in Figure 2, and a rain fade compensation network 44 for due to a rain fade) .

Temperature compensation network 42 comprises a precision adjustable voltage reference source 51 the output of which is coupled to a temperature-responsive component- containing network 52. Network 52 includes one or more of temperature sensitive elements such as thermistors, sensistors and resistors, selectively interconnected to produce a prescribed temperature compensation voltage characteristic when drive by reference source 51. For this purpose, an input signal is coupled over link 31 to PIN attenuator network 34 for application to the up- converter 35 which is coupled to SSPA 30. Via a temperature monitor element in the hardware environment of SSPA 30, and monitoring the output of link 32, the variation of up- converter output with change in temperature is tracked. Using a controlled temperature source, the temperature of the environment in which the SSPA 30 is located is varied over a prescribed operating range (e.g. -40^βC to +50^βC) and the output power level on link 32 is measured. Given this output power versus temperature, behavior, a complementary function is implemented in temperature-sensitive component hardware corresponding to network 52. Thus, for changes in ambient temperature of the SSPA 30, network 52 will produce a prescribed output voltage which is coupled through summing junction 65 and scaled via scaling amplifier 53. This scaled output voltage is coupled through adder 69 to analog- to-digital converter 41 for setting the attenuation of attenuator 34 to compensate for variations of the gain of SSPA 33 with change in temperature. For providing compensation for variations in SSPA gain with changes in frequency, a lookup table, which may be accessed by the terminal's monitor and control processor is employed. More particularly, for a given level of attenuation of attenuator 34, the frequency of the input signal applied over input link 31 is varied over the converter 35 coupled to SSPA 30 is measured at the output of orthomode transducer 37 on link 32. From this measurement, a gain versus frequency characteristic, corresponding to curve 21 shown in Figure 2 is derived. A complement of this curve with reference to a prescribed gain level is then generated and programmed into a suitable read only memory 62. Memory 62 is accessible by the monitor and control processor of the VSAT earth station, shown in Figure 3 as monitor and control processor 61. Processor 61 receives as an input a frequency control signal indicating the frequency of the signals to be applied to link 31. In response to this input signal, it accesses from memory 62 a gain control code which is converted to analog voltage via digital-analog converter 63, scaled via scaling amplifier 64 and then supplied as an input to adder 65. Adder 65 combines the precision temperature compensation voltage at the output of network 52 with the frequency compensation voltage at the output of scaling amplifier 64 and supplies the resulting voltage via scaling amplifier 53 to adder 69. Adder 69 also receives an input from a rain fade compensation network 44, which essentially comprises a comparator 67 coupled to an input link 66 and a precision voltage reference 68. Input link 66 is coupled to the AGC bus of the VSAT's modem so as to monitor the AGC level of the received RF carrier. A drop in this level will indicate substantial attenuation over the link (presumably, corresponding to a rain fade) . The voltage differential produced by comparator 67 is coupled as second input to adder 69 and combined with the compensating voltage derived from networks 42 and 43 via scaling amplifier 53. The resulting analog voltage is coupled to analog-digital converter 41 which produces an output control code for setting the level of attenuator 34. As a result, attenuator 34 provides a digitally controlled pree phasis attenuation of the input signal supplied over link 31 based upon ambient link characteristics (rain attenuation/fade) , and operating frequency.

In the above described exemplary embodiment of the invention, temperature compensation of SSPA 30 is provided by a temperature-responsive component network 52, which effectively corresponds to a pieσewise implementation in circuit components of a gain versus frequency characteristic that is fairly simple to match to a behavior pattern such as illustrated in Figure 1. For the more dramatic variation of gain versus frequency, as shown in Figure 2, a lookup table is employed. Pursuant to the present invention, a lookup table may also be used to provide a compensating characteristic for gain versus temperature change in place of component 52 within temperature compensation network 42. In this circumstance, the monitor and control processor 61 of the VSAT accesses respective lookup tables for providing compensation for both temperature and frequency changes.

As will be appreciated from the foregoing description, the present invention provides a mechanism for compensating for temperature-dependent and frequency-dependent nonlinearities of the solid-state power amplifier portion of the up-converter circuitry of a VSAT up-link terminal. Not only does 'the invention provide a substantial improvement in performance over conventional approaches, but it does so without adding substantial increase in costs, such as by the addition of an expensive travelling wave tube, which would make the terminal effectively cost prohibitive and defeat the purpose of the simplicity and relatively inexpensive components of which VSAT stations are configured. While I have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art, and I therefore do not wish to be limited to the details shown and described herein but obvious to one of ordinary skill in the art^,

Claims

WHAT IS CLAIMED 1. For use with a signal amplification network through which input signals to be transmitted over a communication link are coupled, an arrangement for controlling the operation of said signal amplification network in the presence of operating conditions which produce a variation in the manner in which said signal amplification network couples signals therethrough, comprising: first means for storing information representative of the variation in the manner in which said signal amplification network couples signals therethrough over a range of variation of at least one prescribed operating parameter; and - second means, coupled to a signal flow path through which said input signals are coupled to said signal^' amplification network, for modifying said input signals in accordance with the information stored by said first means. 2. An arrangement according to claim 1, wherein the information stored by said first means is representative of the variation in gain of said signal amplification network for variations in at least one prescribed operating parameter, and said second means includes means for controllably adjusting said input signals as a complementary function of said variation of gain. 3. An arrangement according to claim 1, wherein the information stored by said first means is representative of the variations in gain of said signal amplification network for variations in ambient temperature thereof, and said second means includes means for controllably adjusting said input signals as a complementary function of said variation of gain with temperature. 4. An arrangement according to claim 1, wherein the information stored by said first means is representative of the variation in gain of said signal amplification network ^ said second means includes means for controllably adjusting

5 said input signals as a complementary function of said

6 variation of gain with frequency.

1 5. An arrangement according to claim 3, wherein the

2 information stored by said first means is also

3 representative of the variation in gain of said signal ^ amplification network for variations in the frequency of

5 said input signal, and said second means further includes

6 means for controllably adjusting said input signals as a

7 complementary function of said variation of gain with

8 frequency .

1 6. An arrangement according to claim 3 , wherein said

2 first means comprises a temperature-responsive impedance

3 network hav ing an imp edance -versu s -temp erature ^ characteristic corresponding to said complementary function,

5 and means, coupled with said network, for coupling an

6 attenuation control voltage to said second means in

7 accordance with said impedance-versus-temperature

8 characteristic.

1 7. An arrangement according to claim 2, urther

2 comprising third means, coupled to said second means, for

3 controlling the operation thereof in accordance with a 4- prescribed transmission characteristic of said communication 5 link.

1 8. An arrangement according to claim 6, further

2 comprising third means, coupled to said second means, for

3 controlling the operation thereof in accordance with a prescribed transmission characteristic of said communication 5 link.

1 9. For use with an uplink earth terminal of a

2 satellite communications network having a controlled signal

3 amplification device through which signals to be transmitted ^ from said uplink earth terminal are coupled for

5 amplification and transmission over a satellite

6 communication link from said earth terminal, uplink to a 7 arrangement for controlling the operation of said controlled

8 signal amplification device in the presence of operating

9 conditions which produce a variation in the manner in which

10 said signal amplification device couples signals

11 therethrough comprising:

12 first means for storing information representative of

13 the variation in the manner in which said signal 14- amplification device couples signals therethrough over a

15 range of variation of at least one prescribed operating

16 parameter; and

17 second means, coupled with a signal flow path through

18 which said input signals are coupled to said signal

19 amplification device, for modifying said input signals in

20 accordance with the information stored by said first means.

1 10. An arrangement according to claim 9, wherein the

2 information stored by said first means is representative of

3 the variation in gain of said signal amplification device k- for variations in at least one prescribed operating

5 parameter, and said second means includes means for

6 controllably attenuating said input signals as a

7 complementary function of said variation of gain.

1 11. An arrangement according to claim 9, wherein the

2 information "stored by said first means is representative of

3 the variation in gain of said signal amplification device - for variations in ambient temperature thereof, and said

5 second means includes means for controllably attenuating

6 said input signals as a complementary function of said

7 variation of gain with temperature.

1 12. An arrangement according to claim 11, wherein the

2 information stored by said first means is representative of

3 the variation in gain of said signal amplification device ^ for variations in the frequency of said input signal, and

5 said second means includes means for controllably

6 attenuating said input signals as a complementary function

7 of said variation of gain with frequency. 8 controlled amplification device comprises a solid-state

9 power amplifier coupled with an up-converter section of a

10 signal transmitter of said uplink earth terminal.

1 14. An arrangement according to claim 13, further

2 comprising third means, coupled to said second means, for

3 controlling the operation thereof in accordance with a ^ prescribed transmission characteristic of said communication 5 link.

1 15. An arrangement according to claim 14, wherein said

2 prescribed transmission characteristic of said communication

3 link corresponds to attenuation of signals thereover during i the transmission of signals over said link.

1 16. An arrangement according to claim 15, wherein said

2 third means includes means for monitoring a signal

3 representative of the carrier level of signals received by said downlink receiving earth terminal.

1 17. An arrangement according to claim 8, further

2 comprising third means, coupled to said second means, for controlling the operation thereof in accordance with a prescribed transmission characteristic of said communication link.