Satellite Communications/SR measurement

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Question
Dear Sir,

I have few questions regarding measurement of spectral regrowth in HPAs.

1. How to measure spectral regrowth in HPAs. Will it be measured at the Tx side (I meant at the HPA) or can it be measured at the Rx side as well?

2. I have looked into the literature and it says that it should be -30dBc at 1 symbol-rate or -40dBc at 1.5 symbol rate. If for example I am receiving a a 35 MHz single carrier on a 36 MHz transponder. The CF is 11959 MHz and symbol-rate is 2900 Ksps. At which frequency points will I measure for this spectral regrowth?

Have a nice day.

Makil

Answer
First question…

Spectral regrowth is measured on the transmitter side, not the receiver side.

http://www.milsatmagazine.com/story.php?number=1336439260
Two general categories of HPAs are used on SATCOM uplinks: vacuum tube-based amplifiers and solid-state amplifiers. For vacuum-tube amplifiers, including both travelling wave tube amplifiers (TWTAs) and klystron-tube power amplifiers (KPAs), the industry practice is to use the tube’s saturated output power as the HPA’s rated power. The TWTA’s rated flange power is typically 0.5dB to 0.7dB lower due to component losses in the amplifier’s output section; i.e., the “750W” Ku-band TWTA provides 650W RF power at the flange. Solid-state power amplifiers (SSPAs) always use flange power, yet the industry nomenclature has greater variation because some suppliers use Psat and others use P1dB. SATCOM system designers must take careful note of these differences.

Question 2…

Same reference as above.

Typical specifications require SR be -30 dBc at an offset frequency from the carrier equal to the modulated signal’s symbol rate, or sometimes 1.5 times the symbol rate. These points are identified in Figure 3 on an SR plot for an OPSK modulated signal, where the amplifier meets a -30dBc SR at 1x symbol rate offset from center frequency, and -40 dBc at 1.5x symbol rate offset.

To answer your specific question…
11959 +/- 2.9 MHz at -30 dBc
11959 +/- 4.35 MHz at -40 dBc

I hope that this addresses your question.

Sincerely,

Thomas E. Burke, Ph.D., PMP

Satellite Communications

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Thomas E. Burke

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Experience

Forty years of experience with satellite communications. Have held roles as system engineer on JPL Mariner 9 program and a program manager for a number of classified communication satellite programs. Served as head of TRW / Defense Communications Division / Engineering Development Operation, a 1,400-person organization responsible for all aspects of classified communication satellite design and development.

Education/Credentials
Ph.D. Chemistry, California Institute of Technology (1969) B.A. magna cum laude Chemistry, University of Minnesota (1962)

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