Up & Down Converter Design and Block Diagram in Satellite Communication

In satellite communication systems, up and down converters play a crucial role in ensuring efficient transmission and reception of signals. Up converters are responsible for converting baseband or low-frequency signals to high-frequency signals suitable for uplinking to a satellite. Conversely, down converters translate received high-frequency signals from the satellite down to a lower, usable frequency range for ground-based processing.

This article delves into the design, components, and working principles of C-band up and down frequency converters, with a detailed block diagram to illustrate their operations within satellite communication systems.

The design is carried out on a microstrip board using discrete RF components such as RF Mixers, Local oscillators, MMICs, synthesizers, OCXO reference oscillators, and attenuator pads.

RF Up Converter Design

RF frequency converter refers to the conversion of frequency from one value to another. The device which converts frequency from a low value to a high value is known as an up converter. As it works at radio frequencies, it is known as an RF up converter.

This RF Up converter module translates IF frequency in the range of about 52 to 88 MHz to an RF frequency of about 5925 to 6425 GHz. Hence, it is known as a C-band up converter. It is used as one part of the RF transceiver deployed in the VSAT used for satellite communication applications.

Figure-1: RF up converter block diagram

Let’s see the design of the RF Up converter part with a step-by-step guide.

Step 1: Find out Mixers, Local oscillators, MMICs, synthesizers, OCXO reference oscillators, and attenuator pads that are generally available.

Step 2: Do the power level calculation at various stages of the lineup, especially at the input of MMICs, such that it will not exceed the 1dB compression point of the device.

Step 3: Design proper Microstrip-based filters at various stages to filter out unwanted frequencies after mixers in the design, based on which part of the frequency range you want to pass.

Step 4: Do the simulation using microwave office or Agilent HP EEsof with proper conductor widths as required at various places on the PCB for the chosen dielectric as required for the RF carrier frequency. Do not forget to use shielding material as an enclosure during simulation. Check for S parameters.

Step 5: Get the PCB fabricated and solder the purchased components.

As depicted in the block diagram of Figure 1, appropriate attenuator pads of either 3 dB or 6dB need to be used in between to take care of the 1dB compression point of the devices (MMICs and Mixers). Local oscillators and Synthesizers of appropriate frequencies need to be used based on the conversion. For 70MHz to C band conversion, LO of 1112.5 MHz and Synthesizer of 4680-5375MHz frequency range is recommended.

The rule of thumb for choosing a mixer is that the LO power should be 10 dB greater than the highest input signal level at P1dB.

GCN stands for Gain Control Network, designed using PIN diode attenuators which vary attenuation based on analog voltage. Remember to use Band Pass and Low pass filters as and when required to filter out unwanted frequencies and pass the wanted frequencies.

RF Down Converter Design

The device which converts frequency from a high value to a low value is known as a down converter. As it works at radio frequencies, it is known as an RF down converter.

Let’s see the design of the RF down converter part with a step-by-step guide. This RF down converter module translates RF frequency in the range from 3700 to 4200 MHz to IF frequency in the range from 52 to 88 MHz. Hence, it is known as a C-band down converter.

Figure-2: RF down converter block diagram

Figure 2 depicts a block diagram of a C-band down converter using RF components. Let’s see the design of the RF down converter part with a step-by-step guide.

Step 1: Two RF mixers have been selected as per heterodyne design, which converts RF frequency from the 4 GHz range to the 1GHz range, and from the 1 GHz range to 70 MHz range. The RF mixer used in the design is MC24M, and the IF mixer is TUF-5H.

Step 2: Appropriate filters have been designed to be used at different stages of the RF down converter. This includes 3700 to 4200 MHz BPF, 1042.5 +/- 18 MHz BPF, and 52 to 88 MHz LPF.

Step 3: MMIC amplifier ICs and attenuation pads are used at appropriate places as shown in the block diagram to meet the power levels at the output and input of the devices. These are chosen as per the gain and 1 dB compression point requirement of the RF down converter.

Step 4: RF synthesizer and LO used in the up converter design are also used in the down converter design as shown.

Step 5: RF isolators are used at appropriate places to allow the RF signal to pass in one direction (i.e., forward) and to stop its RF reflection in the backward direction. Hence, it is known as a uni-directional device.

GCN stands for Gain control network. The GCN functions as a variable attenuation device which allows setting of the RF output as desired by the RF link budget.

Conclusion

Up and down converters are essential components in satellite communication, enabling seamless conversion between frequency bands to facilitate signal transmission and reception. Understanding their design and operational block diagrams helps optimize system performance, ensuring reliable and efficient satellite communications. Whether for broadcasting, data transfer, or navigation, these converters form the backbone of robust satellite systems.

RF Frequency Converter Applications

Following are a few of the applications in which rf frequency converters are being used:

• RF Signal Generation and Analysis
• C band RF Transceiver design and development
• RF up converter design and RF down converter design in mmwave bands,
• RF Frequency converter used in GSM mobile phone,
• L Band Satellite modem,