Chapter 4: A Closer Look at Cable TV Network Transitions

Expanding or modifying an existing cable TV network may seem as easy as connecting a cable, but often that action can degrade the signal for every TV connected to the network, when it does, you will need to make adjustments.  In this chapter we will give you some ideas on how to do that.  We will do so from the perspective of adding a new RF source, such as an in-house property channel for a hotel, to a clear QAM network.  

Expanding an RF (coax) channel lineup is both a science and an art. Skilled RF technicians must ensure new channels are seamlessly integrated without degrading the overall system performance. The process involves meticulous signal power balancing, noise mitigation, and careful handling of amplification and attenuation. Here, we’ll explore the essential steps and common pitfalls in RF channel integration.  Before we start, let’s look at a simple wire diagram for adding a new TV channel source  to an existing in-house cable TV network.

Chapter Index:

Chapter 1: Adding Private TV Channels to Your In-House Cable TV System

Chapter 2: What Kind of In-House Cable TV Network Do You Have?

Chapter 3: The Basics of In-House Cable TV Distribution Networks

Chapter 4: A Closer Look at Cable TV Network Transmissions

Chapter 5: Integrating Antenna Channels Into an In-House Cable / Headend System

Chapter 6: The Use of Smart TVs in Prisons and Jails: Benefits, Risks, and Secure Alternatives

Step 1: Selecting a free channel

When adding a new channel at the RF level the first step is to identify unused carrier frequencies.  Carrier frequencies are, in the USA, 6 MHz blocks of bandwidth used by the modulator to transmit the digital channel, or channels, to the TV set.  The screen shot below was captured from a West Pond FlexStream MX-200CC monitor tuner.  This is a typical clear QAM linear TV network.  The first column is the carrier channel, the second is the frequency.  Notice that channel 2 and 3 are 6 MHz apart.  Since this originated with broadcast, there are some irregularities, but the 6 Mhz channel width is always honored.  Looking at the third column you can see that some carrier frequencies are unused.  These are prime targets for adding channels.  For example, channel 4 looks available on this scan.  Now that we have identified a potential carrier channel for our new programming, we can configure our QAM modulator and prepare to combine the two networks.

Step 2: Assessing Existing Signal Levels

Before adding a new channel, technicians must first measure the current RF signal levels across the existing channel lineup. This is typically done using an RF signal meter, measuring in decibels-millivolts (dBmV). Proper documentation of power levels at different points in the system ensures that new signals can be integrated effectively without causing unwanted interference.  If a meter is not available a monitor tuner output, such as the one shown above, is an acceptable alternative. 

 

Each RF system has a baseline noise floor—unwanted signals that can interfere with new channel additions. The noise floor should be measured to ensure that adding a new signal does not introduce excessive interference. Poorly managed noise levels can lead to signal distortion, ghosting, or digital tiling in video applications.  Every RF source has a noise floor and an amplified signal will amplify the noise floor.  In the image below you can see how two signals with similar SNR but significantly different signal strength and noise floors can result in a degraded channel after they are combined.  See below on using an RF meter for more details..

Step 3: Matching Signal Power Levels

New channels must be adjusted to match the power levels of the existing channels. This requires:

  • Amplification: If the new signal is too weak, an RF amplifier is used to boost the signal.
  • Attenuation: If the signal is too strong, an RF attenuator is introduced to bring the power level down to match the existing lineup.
  • Equalization: RF equalizers may be used to compensate for frequency-dependent loss, especially in longer cable runs.

Step 4: Combining the Signals

Once the new channel is properly adjusted, it must be merged with the existing RF feed using a signal combiner. This step must be carefully executed to prevent signal reflection and ensure minimal insertion loss. A poorly implemented combiner can lead to signal degradation across the entire network.  The most basic combiner is a passive splitter used in reverse, 1 to 2 becomes 2 to 1.  Be aware that combiners, like splitters, create signal loss.  Don’t use an 8 to 1 splitter when you only need 2 to 1. 

Common Pitfalls to Avoid

  • Overdriving Signals: Excessive amplification can lead to distortion and intermodulation issues, which degrade overall signal quality.  If you add amplification and the TV reception is worse, you are likely over driving the signal.  A common pitfall occurs on an unbalanced network.  Some legs are over driven and others are too weak.  This can make it difficult or impossible to find a sweat spot for all end points.  
  • Ignoring Splitter Losses: Each splitter introduces insertion loss, which must be accounted for when balancing power levels.  Typical loss is about 3 dB, but not all splitters are typical.
  • Underestimating Cable Length Effects: Longer coaxial cable runs introduce signal attenuation. Proper amplification and equalization are necessary to maintain consistent signal strength. Cable length signal degradation is frequency specific.  Lower frequencies (VHF) have less loss than higher frequencies (UHF).  For most TV signals the range is 2 – 6 dB per 100 feet of RG-6 cable. 
  • Failing to Check for Leakage: Unintended RF leakage can cause interference and regulatory non-compliance.  Old cables, bad terminations, uncapped combiner or splitter inputs, and loose cables can contribute to leakage.

Essential Tools for RF Technicians

When making these changes, it is important to have a collection of essential tools and common devices to ensure a smooth and effective process. An RF technician’s toolkit should include an RF signal meter, spectrum analyzer, and RG-6 cabling tools for creating quality cables.  When resolving an issue you will likely find a need for amplifiers, splitters, combiners, attenuators, and terminators.  Having a well-stocked toolkit allows technicians to quickly diagnose issues, make adjustments, and maintain optimal signal performance across the network.  If you are going to do this often, here’s a list of tools and parts that will ensure smooth installations, troubleshooting, and repairs.

Tools:

  • RF signal meter
  • Spectrum analyzer
  • Coaxial cable stripper
  • Compression tool for connectors
  • Torque wrench for secure connections
  • Cable cutters
  • Multimeter for electrical measurements
  • Labeling tools for cable management

Spare Parts and Components:

  • Various types of coaxial cables (RG6, RG11, etc.)
  • F-connectors and compression fittings
  • RF amplifiers
  • Attenuators of different dB ratings
  • Splitters (2-way, 4-way, 8-way, etc.)
  • Signal combiners
  • Terminators to prevent signal reflections and leakage
  • Equalizers to balance signal strength over long transmission lines
  • Grounding wires and blocks for proper safety measures

Using an RF Meter

An RF meter is an essential tool for measuring and analyzing RF signals to ensure optimal performance in an RF network. Proper use of an RF meter involves understanding key metrics and how they affect signal quality.

Key Metrics and Definitions:

  • Frequency: The number of cycles per second of a radio wave, measured in Hertz (Hz). RF meters help technicians verify that signals are operating within the correct frequency ranges.
  • Carrier: The primary RF signal that carries data. The RF meter can identify the carrier frequency and power level to ensure proper signal transmission.
  • Signal Power: Measured in decibels-millivolts (dBmV), it determines the strength of an RF signal. Proper power levels prevent distortion and signal loss.
  • Noise Floor: The level of background RF noise in the system. A high noise floor can interfere with signal clarity and degrade performance.
  • Signal-to-Noise Ratio (SNR): The ratio of signal power to noise power, expressed in dB. Higher SNR values indicate clearer signals with less interference.
  • Bit Error Rate (BER): A measure of digital signal integrity that represents the number of erroneous bits received. Lower BER values indicate higher signal quality.
  • Modulation Error Ratio (MER): A metric used in digital modulation to assess signal distortion. A high MER value ensures stable and clear digital transmissions.
  • Quadrature Amplitude Modulation (QAM): A modulation technique used in digital RF systems, such as cable TV. RF meters measure QAM signal integrity to prevent artifacts like pixelation and signal loss.

How to Use an RF Meter:

1. Connect the RF meter to the signal source, ensuring a solid connection.

2. Select the frequency or channel you need to analyze.

3. Measure signal power and compare with expected levels.

4. Check the noise floor and ensure it is low enough to maintain a high SNR.

5. Analyze BER and MER to evaluate digital signal quality.

6. Adjust amplifiers, attenuators, and equalizers as necessary to optimize signal performance.

7. Re-test the signal after adjustments to confirm improvements.

Proper use of an RF meter allows technicians to diagnose signal issues, balance power levels, and ensure the integrity of RF transmissions.

 The Art of RF Network Balancing

Integrating new channels into an RF system requires patience, precision, and experience. Technicians must not only rely on measurement tools but also develop a keen sense for balancing amplification, attenuation, and distribution elements to maintain optimal performance.  Understanding these principles is crucial for any RF technician tasked with managing a complex coaxial network. Whether working on a cable TV headend, a private MATV system, or a specialized RF distribution network, these best practices help ensure a stable and high-quality signal across all channels.

Acceptable Signal Levels for Cable TV

To ensure reliable performance in a cable TV system, RF technicians should aim for the following values:

  • Signal Power: Typically between -10 dBmV and +10 dBmV at the subscriber’s outlet. Levels exceeding +15 dBmV can cause distortion, while levels below -15 dBmV can result in signal loss.
  • Signal-to-Noise Ratio (SNR): A minimum of 35 dB is ideal for digital QAM signals. Lower SNR values can lead to pixelation and signal degradation.
  • Noise floor (dBmV): The noise floor should typically be at least -40 dBmV or lower to ensure minimal interference. If the noise floor is too high, it may indicate external interference, faulty equipment, or improper grounding, all of which must be addressed before adding new channels.
  • Modulation Error Ratio (MER): Should be at least 32 dB for 256-QAM and 30 dB for 64-QAM. Higher MER values indicate better modulation quality.
  • Bit Error Rate (BER): Ideally <1.0E-9 (pre-FEC) and 0.0E+00 (post-FEC) to ensure error-free transmission. A high BER suggests data corruption and poor signal integrity.

These values help technicians assess signal quality and make necessary adjustments to optimize performance.