Friday, April 30, 2010

Creating a Terrain Profile for DTV RF Propagation Analysis

ATTN: This tutorial only works for FireFox, do not use IE, the profile website doesn't work with IE for some reason.

Step 1: Determine the coordinates of the transmitter (in decimal format by clicking on the coordinates in Wikipedia, uses geohack)

Step 2: Determine the coordinates of the receiver location (your house), also in decimal format. This step can be skipped as you can click directly on the map in step #3 to locate the receiver location.

Step 3: Create a profile using the coordinates from above. Enter the coordinates as 45.51611,-122.732778

Step 4: Adjust the height above terrain for the transmitter and receiver by clicking on the height hyperlink next to the coordinates.

Step 5: Note if you have line of sight (LOS) and the distance of the path. The path distance will be used later when we calculate path loss and link budgets.

Friday, April 16, 2010

Determining receiver sensitivity using kTB, NF, and S/N requirement

Receiver sensitivity is the minimum input signal required to produce a required signal to noise ratio. In other words, it answers the common question in virtually any communication system "Is my signal good enough?"

Many system specifics are based on receiver sensitivity. For example, in a cellular system it can help you determine "good" coverage from a cell tower antenna. In digital TV receiver sensitivity can be used to determine if you need an antenna on your house to have a 'good' signal.

In test and measurement, receiver sensitivity is used to determine what is required to make a 'good' (accurate & repeatable) measurement. If you know the receiver sensitivity for a spectrum analyzer for example, you can make judgments about the quality of measurements and how to best setup the measurement.

Calculating rx_sens (receiver sensitivity) is pretty straight forward.

Step 1: Calculate kTB (noise floor) for the bandwidth of interest
k = Boltzmann constant (1.38*10E-23)
T = temperature in Kelvin
B = bandwidth of interest in Hertz

Step 2: Convert to dBm

For example: For a bandwidth of 30KHz
example: For a signal with a bandwidth of be 30Khz
Noise Floor = kTB = 1.38E-23 * 300 * 30000 = -129dBm

Step 3: Estimate receiver senstivtiy
Rx_sens = Noise Floor + NF of the receiver + S/N required

Example: For 10dB noise figure receiver, with a 20dB S/N requirement:
RX_Sens = -129dBm + 10dB + 20dB = -99dBm

Now we know what level we need at the receiver. Stay tuned for the next post regarding link budgets. We'll see how to use this number in our link calculations to determine if the transmitted power level is 'good enough' to meet our RX_Sens requirement.

%% Receiver Sensitivity
% James Eastham
% Member, IEEE
% Revision: R1
clear all;
close all;
b = [1 10 100 1e3 10e3 100e3 500e3 1e6]; %bandwidth every decadeHz
k = 1.38E-23; %Boltzmann's constant
T = 300; %Temp
n_floor = k*T*b;
n_floor_dBm = 30 + (10*log10(n_floor));
figure('Color',[1 1 1]);
xlabel('Noise Floor (dBm)');
title('Receiver Sensitivity vs. Bandwidth');

Saturday, April 10, 2010

Railways of the World the Card Game

My first serious attempt at game design is going to print! RotWTCG is scheduled to hit stores June 2010.

Here is the boardgamegeek entry:

Interested in ordering a copy?

Friday, April 2, 2010

RF Connector Types & Proper Connector Care

Chess-set photo found on!

There are many different types of coaxial RF connectors. The standard types can be reviewed on wikipedia at:

The types which students will most encounter include the BNC, F connector, N connector, TNC connector, SMA, and UHF connector. All of these connectors can be reviewed online. Each connector has a male and female version. The N-Type is a very common threaded RF connector seen on most RF equipment.

There are a few important "Do's and Dont's" associated with connector care. I found this list on the microwaves101 site and found it usesful ( Here are the ones I find most important for students:

#1: Don't use pliers on a stuck connector - ever. A properly sized wrench is the tool you should be using.
#2: Don't make critical measurements with dirty connectors. Make sure connectors are clean (
#3: Don't use part of a calibration kit for adapters / connectors.
#4: Use a torque wrench (or gently tighten by hand for most non-critical classroom measurements).
#5: Don't overtighten, or tighten too much!!! These connectors in some cases cost more than your tuition. Be extra careful with them, pretend they are made of eggshells and filled with explosives.
#6: Don't 'spin' the connectors joining each other, only turn the threaded sleeve. If you 'spin' the junction you can wear our the mating surfaces.

Here is a photo of a damaged SMA from work. The cable this was connected to cost about $1,000. That's not the bad part, the damaged cable also caused about 4 hours of downtime for a well paid engineer! BE CAREFUL WITH CONNECTORS!!!!