A reference signal level for the system must be determined. This is the amplitude of the RF signal that appears at each outlet and is measured at the network's highest frequency. It is the value used to design the distribution network and to determine amplifier gain. Maintaining this standard reference amplitude in the entire network produces a transparent distribution system, as described in chapter two. This section describes using the video carrier level as a reference, and how signals with different bandwidths can be related to that reference level.
The Video Reference Level
The most common reference level for a broadband system is the video carrier signal measured in dBm V within a 6-MHz passband.
The video reference level specifies both input and output signal levels:
~ The amplitude of video signals received at any outlet
~ The amplitude of video Signals injected into any outlet from a transmitting device By designing the network to convey television signals properly, any other signal level can be related to the video reference level and the proper receive and transmit levels can be calculated for them. Any interface device can be attached to the network, aligned for proper transmit and receive levels, and operated successfully. Each
broadband equipment manufacturer uses different signal amplitudes and bandwidths for their interface devices. Establishing a reference level eases the design of a network that uses equipment from several different manufacturers.
The following statement can be used to specify a transpareht system.
The broadband system shall provide for the distribution of color or monochrome television Signals to any outlet in the system. In addition, the capability to originate television signals from any outlet in the system shall be considered in every design.
Other non-television signals shall be compatible with the system for distribution, using a variety of transmission techniques and access schemes. Signals such as data,
The following values refer to a 6-MHz video signal and satisfy the transparency criteria. These values are used in many operating networks.
~ The distribution system supplies a signal level of + 6 dBm V to each outlet. This is the device receive level.
~ The interface device supplies a signal level of + 56 dBm V to the distribution system. This is the device transmit level.
~ The forward path loss is about 50 dB. (This is the loss from the headend to a typical outlet.)
The receive level is near the middle of the input signal range of a typical television set, which is from 0 to + 15 dBm V. Normally, it is desirable to have the receive level between 6 and 10 dBm Vat each visual carrier frequency, with the aural carrier level 15 dBm V below the visual carrier level to minimize interference and to provide good reception. The cable network should be designed with all outlet signal amplitudes within 3 dB of each other. This means that the variation from the lowest signal level to the highest Signal level at the outlets must be 3 dB or less.
The transmit level of +56 dBmV also originated with television equipment specifications. This is a standard output level for television modulators. The forward loss of 50 dB is found by subtracting the receive level from the transmit level.
Figure 5-2 shows a small portion of a distribution system and how the desired receive level can be obtained at an outlet.
*20 dBmV
@ 216 MHz '42 dBmV '39.8 dBmV TRUNK CABLE
r----{>---22dB ~-+---+
200' of 0.5" cable 8 dB (actual loss = 9.2 dB) loss = 1.1 dB!100'
@216MHz
TAP
30.6 dBmV FEEDER CABLE (negligible loss)
+10 dBmV
Select Tap Value = 20.6 (20 dB tap)
Figure 5-2. Signal Levels from Trunk to Outlet
100' of drop cable
loss=4dB OUTLET
+6 dBmV
A different network, providing 35-channel service with a cascade of 20 amplifiers, uses the following levels. (Both network designs are based on a video carrier reference level.)
~ +8 to + 10 dBmV amplifier input level.
~ + 33 to + 35 dBm V amplifier output level.
This network has 66-dB rejection of second-order beat frequencies, which is within the range required for good network performance.
Bridging amplifiers for the same channel capacity usually have higher output signal levels (about +45 to +47 dBm V), but fewer such units can be connected in cascade.
Narrow Bandwidth Carrier Levels
Broadband amplifiers are specified in terms of visual carrier levels and 6-MHz video channels (see figure 5-3). When data communications devices with many carrier signals in that same 6-MHz bandwidth are used on the system, the transmission level of each data sub carrier must be lower than the video reference level; otherwise, the amplifiers in the system could be overdriven. This would distort their output Signals, and create interfering harmonic signals across the entire frequency spectrum of the cable. The necessary carrier level can be calculated with the following carrier derating formula.
DCL = VCL - IOlog(NC)
where DCL = carrier level for the desired data signal VCL = carrier level for a 6 MHz video signal log = the base ten logarithm
NC = the maximum number of data carrier signals that can occupy a 6-MHz assignment
The level difference between the video and data signal levels depends on the number of data subchannels within a 6-MHz channel. As more subchannels are squeezed into a 6-MHz bandwidth, less gain is available for each signal. The derating formula can be used to determine the maximum signal amplitude for each data carrier, and the manufacturer's specifications should be checked to ensure that this level is adequate for the application.
Two examples using the specifications from two different broadband data transmission units are provided to show how to calculate data carrier levels for a transparent system.
AMPLITUDE (dBmV)
56
38
VIDEO BANDPASS
VIDEO CARRIER
COLOR SUBCARRIER
- - + - - - -
+3.58o~ ________ ~~ ____________________________ ~ ______ ~
+4.5
·1
I I
o 2 . 3 4
FREQUENCY (MHz)
SOUND CARRIER 15-18dB DOWN
Example 1
Distribution system specifications:
Typical output VCL +56 dBmV
Typical input VCL +6dBmV
Manufacturer's specifications:
Data subchannel bandwidth = 300 kHz Number of carriers inside 6 MHz = 20 carriers Find the typical input and output DCL.
DCL = VCL - 1OIog(20)
= VCL - 13 dB
Typical output DCL = +56 - 13 = +43 dBmV Typical input DCL = +6 - 13 = -7 dBmV
Given the distribution network's reference levels and this particular interface device, the interface device's transmitter should supply +43 dBm V to the network, and its receiver should expect a -7 dBm V signal from the network. If the interface device cannot be adjusted for these levels, pads could be used or the reference levels could be changed.
Figure 5-4 shows the spectrum of the device used in this example referred to the same 6 MHz scale as the television channel.
AMPLITUDE (dBmV)
56
43
13dB DOWN 300kHzCARRIER
/ '/c '"'""'"'''"'
~ nm /.
I 1 I 1
'" 1«>1 a> ~ ~
..
.... '" «> a>f2 1 .... 1 0 f2 ;::: ;::: ;::: ;::' N .... ;::' '" .... ~ ~ '" ....
1 1
0 111 2 3 4 5 6 7 8 9 10 11 12 1 1
300 kHz CHANNEL - I ( 4 - -BANDWIDTH
! I
I
I IFREQUENCY (MHz)
Figure 5-4. A Typical Data Channel
'\
1
N "' '" ~
..
....;! ;! ;! "' .... ~ ~ it: FREQUENCY (MHz) 13 14 15 16 17 18 19 CHANNEL NUMBER
Example 2.
Distribution system specifications:
Typical output VCL +56 dBmV
Typical input VCL +6dBmV
Manufacturer's specifications:
Data subchannel bandwidth = 96 kHz Number of carriers inside 6 MHz = 56 DCL = VCL - 1000g (56)
= VCL -18 dB
Typical output DCL = + 56 - 18 = + 38 dBm V Typical input DCL = +6 - 18 = -12 dBmV
Comparing these two examples confirms that when more subchannels are used in the same 6-MHz bandwidth, the signal levels must be lower for equivalent quality transmission. Using the interface device's specifications, proper signal levels for use on a transparent system can be computed.
Figure 5-5 shows the relationship between signal levels and bandwidths for a television channel and for the two data channels discussed in/the two examples.
TELEVISION DATA TELEVISION DATA
/,.---~I\,---....,,/,....----,I\,---..,/,.---~I\'---..·v~----1I\~_---..,
VIDEO VIDEO
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 56 dBmV
AUDIO DATA CARRIERS AUDIO DATA CARRIERS
- - - - - - - - - - - - - - - - - - - - - 43 d8mV
- - - - - - - - - 38 dBmv
COLOR COLOR
--:---,----,-1 ~~...I....I...I..L.J..I-U.-.----'---~I~-~
OdBmVI...-
6MHz~
6MHz.1.
6MHz~
6MHz---l
20 CARRIERS 56 CARRIERS
300 kHz 96 kHz
Figure 5-5. Television and Data Carriers
Narrow Bandwidth Advantages
The previous two examples show that many narrow bandwidth data subchannels can be transmitted in a 6-MHz channel without interference by reducing their carrier amplitudes below the video carrier reference level. Two consequences of using narrow bandwidth signals arise.
A narrow bandwidth signal can operate successfully with a lower signal-to-noise ratio. This is because noise increases with bandwidth. The noise floor of the 300-kHz-wide data channel is -70 dBm V, while the noise floor of a 4-MHz-wide television channel is -59 dBm V.
~ Lower amplitude data signals create less intermodulation distortion than higher-level television signals.
Basing the system's design on video signal levels allows the construction of a transparent network that supports a wide range of interface equipment. Using the carrier derating formula provides the proper signal levels for operating each device.
In addition to channel bandwidth, noise is an important factor in determining operating levels. The following section discusses thermal noise and how it affects signal levels.