User Tools

Site Tools


subsystems:commsstudy
Untitled Document

OSSI Communication Systems Study

OSSI is not a good comms engineer and need lots of studies. So this page might be messy until we know what they are.

Dipole VS Monopole

ref: http://www.cubesat.org/images/cubesat/presentations/DevelopersWorkshop2007/Nunez_Paul.pdf

Mononpole needs ground plane for imager half part of the Dipole.
Satellite is floating in the space with small size finte ground plane.
—> Go with dipole or j-pole if you want to have better communications. ( but diplole system will require balun and increase other mechanical complexity.)

And as the size of the ground plane of the cubesat is very small, 
Monopole antenna's radiation pattern will be similar to Dipole antenna's pattern and also its gain will be similar or even less than Dipole antenna.

Balun

Need a balun to interface signals from unbalanced → balanced or balanced → unbalanced
Some transceivers have balanced out and some has unbalanced out
Other parts are:

  • Monopole Antenna: Unbalanced
  • Dipole Antenna: Balanced
  • Coax Cable: Unbalanced
  • Filter: Unbalanced parts are easy to get
  • Power Amp: Unbalanced parts are easy to get
  • Low Noise Amplifier: Unbalanced parts are easy to get


* Unbalanced = Single Ended / Balanced = Differential

RF stage Example)

Differential Transmitter -> Balun A -> Filter -> Power Amp -> Balun B -> Dipole Antenna


* Balun A is 50 Ohm to 50 Ohm type (or 1:1 type) from balanced input ports to unbalanced output port
* Balun B is 50 Ohm to 75 Ohm type (or 1:1.5 type) from unbalanced input port to balanced output ports(as the impedance of the dipole antenna is 75 Ohm)
* Balun B requires high power rating!!!

Balun B special case

When high power rating small sized balun is hard to get, we can solve the situation as below:

  1. Design a balun by LC matching: we use frequency 144~440MHz, so it's possible to design a balun using lumped elements (L, C) read article
  2. LC matching + Balun: if you can get high power balun but the ratio is 1:1 (not 1:1.5), then use LC match before the Balun B. ie. Power Amp - > 50 Ohm to75 Ohm LC Match → Balun B → Dipole

Balun for small satellite

  • SMD Balun: small size / good response / mostly low power for AMSAT radio band
  • LC matching Balun: relatively large / needs tuning / high power possible

LC matching Balun

Cautions when designing a LC matching Balun

Antenna Simulation

Cartesian coordinate system
Azimuth & Elevation
Place the antenna in the coord. system

Antenna Material

Dipole trimming with MFJ-269

The Adobe Flash Plugin is needed to display this content.

Microstrip Transmission Calculator

  • To calculate the impendance of PCB trace, you need to do some calulation
  • To put values in to the formula, you need to know your PCB characterstics
  • See datasheet of your PCB characteristic from the PCB manufacturer
  • We normally want to design our PCB trace so it can have 50Ohm impedance
  • This condition is only needed for that's RF sensitive trace-i.e. Transceiver to Antenna
  • Note: As the frequency range we are using here (144MHz / 430MHz) is not that high, we could just we L, C to match the impedance

Example Values

1. PCB

  • PCB type: FR-4 glass epoxy panel
  • Dielectric Constant of PCB: 4.2 ~ 4.7
  • Dielectric Height ( Height of PCB ): 0.8mm (recommended if you're designing dobuble sided PCB) / 1.6 mm ( possible but the width of the trace will be wider)
  • Height of Copper trace: 1/2 oz. (0.018mm) / 1 oz. (0.035mm)
    * The PCB material determines the dielectric constant
    * The height of the PCB is very important.

2. Targeting value

  • Targeting Impendance: 50 Ohm
  • Testing Frequency: 435MHz

3. Output

  • Width of Microstrip: 1.5 mm when using 0.8 mm PCB board / 3 mm when using 1.6 mm PCB board
  • Length of Microstrip: 1/4 wavelength = 370.983/4 = 93 mm (approximately) → make the microstrip shorter than 1/4 wavelength (this is considered as “short” and this means most of the RF power will be delivered to the other end http://www.tpub.com/neets/book10/41c.htm
  • Normally we make the length of the microstrip < wavelenth / 30: 370.983 / 30 = 12 mm in this example, Ref:Semtech AN 1200.04


AppCad example

PCB Design Calculator

Impedance Matching

Inter-stage matching VS 50 Ohm Matching

AX.25 UI Frame

BPSK implementation

  • good for Low power satellite communication
  • Delfi-C3 used BPSK transmitter and battery less comms with BPSK tx
  • NXP SA612A chip + OPAMP + MRF313 class A / AB final stage PA

VHF AGC Implementation

WPM to Hz

If you design a RF system that transmits morse code with CW signal, you need to know the data rates.
To set PLL Loop filter Bandwidth, and other filters BW to efficiently set your system, you need to know the data rates.
But when it comes to morse code we go by the WPM (words per minute) unit. and to convert WPM to bps (bits per second), we use

1 WPM = 3.984 Hz (4 Hz)

And to convert bps to Hz

1 Hz = 2 bps, 1 bit is a half of the period

An example:

ex) 12 WPM = 48 Hz = 25 bps

ref: http://www.vaibhavbirit.bravehost.com/qrp.html

Silicon Labs Si446x Series

A new low current trasceiver that covers from 119MHz to 1GHz and delivers 20dBm PA Power!!!
http://www.silabs.com/products/wireless/EZRadioPRO/Pages/Si446x.aspx
Plus side is SiLabs provide very kind app notes!!!!

AXSEM ax5042

RF6886 RFMD Power Amplifier

subsystems/commsstudy.txt · Last modified: 2012/12/16 13:36 by ossi