Return Path Issues and Answers

Return Path Issues and Answers
Rev. # Feb. 2002 The next slide will go through the components of the return system.

Return System Design and Operational Goals
Operate the Return TX at its “optimum” drive level. Optimum is based on the maximum TOTAL power at the TX Align the return amplifiers so they all provide the same signal levels at the node input. Set the amplifiers for “Unity Gain” Adjust the modems so they all provide the same signal levels at the amplifier inputs. Modem transmit levels are controlled by long loop AGC based on the receive level at the head-end The modem with the longest (dB) return path must be capable of reaching the head-end demodulator. The entire digital return system requirements can be summed up in these rules. 1. Operate the Return TX at its “optimum” drive level. Optimum is based on the maximum TOTAL power at the transmitter for a fully loaded system. This power is determined by characterizing the transmitter’s dynamic range, which is usually specified by curves of BER v. level, or by NPR. 2. Align the return amplifiers so they all provide the same signal levels at the node input. Set the amplifiers for “Unity Gain” in the return path. A similar concept to the forward system, except it is the return INPUT levels that are kept constant. With a unity gain return, any signal that gets to the RF amp input at the correct level will automatically be at the optimum level at the node’s return transmitter. 3. Adjust the modems so they all provide the same signal levels at the amplifier inputs. Modem transmit levels are controlled by long loop AGC based on the receive level at the head-end. There will be a large range of losses from the modems to the first RF active. The modem with the longest return path (in dB’s, not footage) must be capable of reaching the head-end demodulator at the proper level. Modems with shorter return paths will be turned down by the long loop AGC to achieve the proper level.

Return Path Alignment Steps
1. Determine the optimum drive level at the laser, 2. Inject an equivalent level reference signal at the transmitter. 3. Adjust receiver output level and head-end combining to achieve proper levels at the CMTS demodulator. 4. Establish reference levels at the CMTS demodulator, or other head-end reference point. 5. Determine the optimum RF input level for the RF actives. 6. Adjust return amps for unity gain. Work from node outward, inject known levels at the RF amp input, adjust gain and equalizer to get the same reference levels at the head-end. When the modem demodulator has the proper level, the optical transmitter will be operating at optimum drive level. In order to meet the setup rules, the return system components have to be aligned in a very specific order. 1. Determine the optimum drive level for the return laser. This is the most critical step, because the laser establishes the dynamic range limits for the system. Also, all subsequent steps are based on the reference levels established here. These levels must be based on the TOTAL power expected at the transmitter, even though at system turn on fewer signals may be present. 2. The total RF drive power is then divided among the signals to be carried to establish the power in each RF signal. For mixed signal types, this is done on a power per Hz basis. For setup, one or two CW carriers will be substituted for the digital signals. The level of the CW setup signal is set equal to the power of the digital signal it has replaced. The CW carriers are injected at the node in order to establish an easily measurable reference. 3. At the head-end, the return signals may be split and combined several times. The RF level at the optical receiver must be adjusted to overcome these losses so the modem demodulator sees its required level. 4. After the Rx adjustment is made, any point in the signal path at the head-end may be used as a reference point. The CMTS demodulator input is a good choice. 5. Determine the optimum drive level for the RF amplifiers. For digital signals, the BER dynamic range of the RF amplifiers is very wide, so this level is less critical than for the laser. Ideally, the input levels at the RF amplifiers would be the same as for the node. 6. Start with the first amp out from the node. Based on the available testpoint configuration, inject the CW test signal that gives the equivalent desired level at the amplifier input. Adjust the gain and equalizer of the return amp to give the previously establisher reference level back at the head-end. Move to the next amp and do the same. All return amps should have the same input levels, and be adjusted to give the same reference level back at the head-end. When all the amps are adjusted for proper levels at the head-end, they will also drive the optimum levels at the return optical transmitter. The long-loop AGC in the CMTS will adjust the modem transmit level to match these levels.

Return System Headend equipment Satellite, Transmitter Optics
26 23 20 17 14 Public switch Com21 HCX comController Analog Video 55-319MHz 2 Way RF splitter RF splitter 430MHz 600MHz Headend equipment Satellite, Transmitter Optics Node plus houses Return receivers Return combiners CMTS and router The next slide shows an incorrect combining network

Headend Combining Sweep Modem Phone Analyzer Receivers Headend Devices
RX Sweep Modem Phone Analyzer Receivers Headend Devices Combining network( the wrong way) Additional Receivers The next slide shows the correct way to combine RX

Headend Combining Sweep Modem Phone Analyzer Same as previous slide.
RX Sweep Modem Phone Analyzer Same as previous slide. Before doing any combining know the following: How many nodes will there be in the entire system. Increase this amount by at least 10% What levels are needed at the termination equipment? Set the receiver outputs to obtain these levels. The next slide is the funnel effect RX

Energy Accumulation Return Path Signal
Mouse clicks will add arrows showing all energy forced to one pipe called the Funnel Effect. The next slide is tree and branch with four mouse clicks adding nodes.

Funnel Effect Cont.. Tree and branch. Explain the accumulated energy of the return cascade. Click and nodes(4) will drop in. The next slide shows how the funnel effect was recreated in the headend.

Funnel Effect Cont.. Click to reshow the nodes.
CPU Combiner Monitoring Device Click to reshow the nodes. Click to add Transmitter and Receivers Click to add cable Click to add combiner Click to add CPU Click to add Monitoring device. The point is that the funnel effect has been re-created. The next slide starts system behavior with modem carriers.

System behavior Thermal noise funneling Laser Clip
Most important cause of thermal noise: 1) subscribers 2) amplifiers 3) optical link Laser Clip Ingress and Impulse Noise Next slide is system behavior Average Case Modem carriers.

System behavior Average case Return rx and TX Amplifier with taps
Return TX Return Rx Return rx and TX Amplifier with taps Modem carriers starting top to bottom from right to left ending at the receiver. Shows the accumulative or additive effect of a cascade. Also explain how the different frequencies are used so that the speed of the modems remains fairly constant. The more devices at the same frequency, the slower the speed or the reduced bits per second being delivered. The next slide shows a good noise floor using a stealth meter in the spectrum mode.

Return Noise Floor The next slide shows noise build up in a node.

System behavior Noise funneling (amplifiers + optics) Return Rx
Return TX Noise funneling is the combination of all of the amplifiers in conjunction with the transmitter and receiver. This is an inherent problem of all amplifiers. The noise figure of the amplifier is specified by the manufacturer. The noise level to peak carrier is input dependant. The higher the input level the better c/n ratio. Go through an exercise to determine the c/n. The next slide shows what noise and ingress can do to the return path spectrum.

Return Noise Floor The next slide shows what modems contribute.

System behavior noise from modem Return TX Return Rx
Amplifiers are not the only source of noise. Any one modem can also create enough noise or the noise environment of the house can disrupt the return path. Same as the previous slide. Click the mouse(5) times and see the noise increase from one modem to the headend. The next slide explain the effect of hot levels or incorrect balancing of the return path. noise from modem

System behavior Laser Clip Return Rx Return TX
With correct inputs at the Transmitter the output of the transmitter as well as the received signal at the headend are fine. At the first active the inputs are increased forming distortions at the TX and rx outputs. At the next device the inputs are also at the wrong level which now puts the laser into clip which creates what appears to be other carriers. The next slide is a picture of ingress vs. noise.

Ingress Example 70 % from the home 25% from the drop cable
The next slide is noise vs. time. Ingress/ noise content Noise / ingress content

Random Noise VS. Time The next slide is impulse noise.

Impulse Noise The next section starts return path alignment.

Return path alignment Technical Support
The next slide again goes through return path components. Click the mouse and explain what the numbers refer to.

Return System Components
2 TX Head-End RX Combiner CMTS Demod 3 1 5 What is the CMTS device and what is the input needed? What output is needed to overcome the combining losses to achieve the inputs needed at the CMTS? What is the manufacturers set up procedure for the node to achieve the best c/n and distortions. What is the required input to the return amplifier? How much passive loss can I accept? How much loss can I expect in the home? What is the output level of the device in the home? What is the rated operating level? The next covers a typical return cascade. 4 7 6

Optical return path link
Rx Forward TX Return TX Return Rx Headend Node Optimum level input level too low ==> low thermal and RIN CNR input level too high ==> high Intermodulation noise ==> low CNR RIN = relative intensity noise This intensity is attenuated in fiber at the same rate as the signal, so the c/n due to the laser is independent of the fiber length. For short links RIN is the principle noise source. High inputs cause Intermodulation noise, clipping and other distortions which result in a lower carrier to noise floor differential. The next slide starts the optical link set up. CNR BER input level input level optimum level optimum level

Node-Hub Return Link

Alignment in the Field (1)
Person 1 The two man operation was the first method of balancing the return. Person #2 would inject carriers at the last active. Person #1 would then inform #2 if the levels received were at the right amplitude and if they were flat. If not #2 would change the pad and equalizer until the system return levels were correct. Once this was completed #2 would then go to the amplifier preceding #1. #1 would then inject while #2 would now read the levels. #1 now has to change the pad and equalizer. This was called the leap frog system. It was and probably still is the most accurate method of return balancing. The draw back is that the response between these carriers is unknown. The next slide is alignment #2

Alignment in the Field (2)
Or Spectrum Analyzer with Video Out Function The camera method was the second method of return balancing. The plus for this is that manpower was cut in half. As the tech. Injected the carriers at the node or amplifier another tech. In the headend directed the carriers into a spectrum analyzer. He then set the receiver levels to a system specified number. On the analyzer he would adjust these carriers to a horizontal gradical at 1 or 2 dB per division. He would then enhance the horizontal gradical with a grease pencil. This would make it easier for the tech. In the field looking at the handheld TV to see. The tech in the field would then proceed to the next active and inject the carriers. While watching the TV he would then change the pad and equalizer until the carriers rested on the horizontal line outline by the headend tech. This method was only as good at the resolution of the TV being used in the field. The next slide shows today's method. Or Baseband Output of Analyzer into Modulator

Alignment In the Field #3
Combining Network Out FREQ CHAN ENTER FCN CLEAR help status alpha light abc def ghi jkl mno pqr stu vwx yz space +/- 1 2 3 4 5 6 7 8 9 x . FILE AUTO SETUP TILT SCAN LEVEL C/N HUM MOD SWEEP SPECT PRINT System Sweep Transmitter 3SR Stealth Sweep Node TP The node is still set up in the standard way. Use two CW carriers to align the transmitter and receiver. The the stealth or Calan meters take over. As long as the embedding losses of the active devices remains constant, a reference is taken at the node. This results in a flat line with markers reading zero. As in the downstream the end result should be a response that is within a specified peak to valley while maintaining a set amplitude. With these meters this can be done by a single technician. The next slide shows expected in home losses.

Node-Hub Return Link Set up link to carry max (example) 23 (QPSK) ch
OT drive spec for 2 Video channels  dBmV optimum for 4 ch = 10Log(2/4) = -3 dB reduction in drive level Apply 2 carriers at “X”dBmV to node Adjust gain of node return transmitter to obtain correct drive level Measure received Hub optical power Measure RF out from Hub receiver Read the slide The next slide shows the difference in levels of CW carriers(2) vs. 23 QPSK channels.

Optimum drive levels for the NRT
+8 dBmV/ch Ch. width =1.6 MHz (42-5)/1.6=23 channels 10*log(23ch)=13.6 dB The left side curves show the BER limits in the presence of thermal noise and sporadic noise. The right side curves show the limits due to laser clipping. Notice that these curves are different for different modulation types. There are three scales on the bottom axis. The first is TOTAL power. By spreading this power evenly over the 5 to 42 return bandwidth, we get the second scale in dB/Hz for North American systems. The third scale is for 5-65 MHz systems. 10 log (37 MHz/1 Hz) = -76dB relative to total power for a 5-42 MHz system If this transmitter were to be fully loaded, we want the total power to be less than the clip limit, as determined by the BER curves. The red dashed line on the plot shows an operating point 6-8 dB below the BER limit for QPSK signals. The total power at this level is about +22dBmV. If we were to fill the entire return band with 1.6 MHz wide QPSK channels (23 would fit), each channel would be allocated 8dBmV 22dBmV - 10*log(23) = 8dBmV per 1.6 MHz channel. Alternately, we could use the dB/Hz scale and start with the -54dBmV/Hz and convert up to the 1.6MHz channel bandwidth. -54dBmV/Hz + 10*log(1.6x10^6) = 8dBmV/ 1.6 MHz This method gives the optimum level for individual channels, based on keeping the total fully loaded power within the BER limits. Notice that this level happens to be “10dB below analog levels”, and can serve as a rule of thumb for field alignment. +2dBmv total = -12 dBmV/ch +24dBmv total = +10dBmV/ch

Drive Levels for the NRT
Current factory alignment procedure Aligned with two CW carriers Reference drive level is listed on the sticker - as measured at the transmitter testpoint. Typically +18 dBmV Total voltage at clip point approximately 18dBmV = 24dBmV ( 20*log(2)=6 ) QPSK Channel width =1.6 MHz (42-5)/1.6=23 channels 10*log(23ch)=13.6 dB Based on Channel Bandwidth Read Next slide is drive levels continued. = 8.4dB per Channel (22dBmV value 2dBmV below QAM Clip)

Node adjustment Specified level into forward TP is 39dBmV
Test point sticker level is level for video carriers => for digital, target is TP level is 8dB Corresponding input level is 19dBmV (20dB) Note sticker level is not input level. From previous slide the input to the laser is 10 – 20dB. To get to the laser input the embedding lose of the node must be overcome. In this case the loss is 15 dB. Therefore the injected carrier level of 49 dBmV is needed to achieve the 10 –20 db input level required. This equates to 29dbmv at the amplifiers seizure assembly. The next slide shows the breakdown of a DNA node and the losses as related to the drive level.

(From the GNA Installation manual)
NRT Field Alignment (From the GNA Installation manual) Field alignment is done at “digital” levels, but using CW carriers. NTR gain is set “mid-range”, or -5dB. To get +8dBmV at the TP, +19dBmV is required at the node input ports. With a 20dB testpoint, a signal level of +39dBmv is injected at the node input TP. Step one of the field alignment procedure can now be established. 1. Optimum drive level at the testpoint has been established at +8dBmV/ch. 2. (Optional) To keep the tech happy, allow 5dB adjustment range in NRT gain control. 3. Level requirement at the node input is +19dBmV per channel +8dBmV at NRT +5dB for NRT gain set-back +6dB for Node losses = + 19dBmV 4. A signal level of +39dBmV is injected at the 20dB testpoint, which equals the +19dBmV at the node input port. 5. The NRT gain is adjusted to achieve the desired +8dBmV as measured at the NRT testpoint. 6. At the head-end, adjust the gain at the optical receiver to set the manufacturers recommended levels at the CMTS demodulator. With multiple optical links, set up the longest one first, since it will have the lowest RF output signal levels. 7. At some point in the head-end after the optical receiver, measure and establish a reference level. This level will correspond to the optimum drive level at the laser, and will be used for all subsequent field level adjustments.

Stealth Reverse Sweep Optical Transmitter Node Optical Receiver 3ST
Combining Network Node Optical Receiver TP Out FREQ CHAN ENTER FCN CLEAR help status alpha light abc def ghi jkl mno pqr stu vwx yz space +/- 1 2 3 4 5 6 7 8 9 x . FILE AUTO SETUP TILT SCAN LEVEL C/N HUM MOD SWEEP SPECT PRINT System Sweep Transmitter 3SR Stealth Sweep Optical Receiver The combining network is samples the downstream signals. This information is impressed onto the downstream telemetry signal where the receiver strips the information or points and represents these points as a sweep or plot. This plot includes the abnormalities on the response caused by the transmitter, receiver and RF module of the node. A reference is then taken removing all the abnormalities and tilt generated by the optics and leaving a flat line with minimal peak to valley deviation. This is then what the downstream balance is compared to. The same is done for the return. After the CW carriers are injected and the headend receive levels have been set a reference is taken for the return path. This is shown on the next slide. 3ST Reverse Sweep Displayed on 3SRV 3SRV 27 61

Reverse Sweep Reference
Point out the 5 and 42MHz markers. M1 and M2 are at the same level This also corresponds to the 0 Ref at the top of the sweep picture. Min/Max. will not be 0 but needs to be no greater that 0.5dB. The telemetry number(blue arrow) is a good reference but from here on it is disregarded. As shown on the next slide, if one balances to this number and there is an abnormality around this freq. , the balance will not be done correctly. 74

Reverse Sweep As shown, a dip has developed around 35MHz. If this is where the telemetry signal is located the unit cannot be balanced properly. The degradation does not have to be this bad but once the pads and or equalizers have been changed to accommodate this abnormality the effect will be additive. Also look at the Ref. Level at the top of the sweep picture. This also indicates that an improper balance procedure is being performed. The next slide starts unity inputs to return amplifiers.

Return Path Requirements
Signal Levels Passive Values Unity Inputs Next slide shows the unity gain principle

Signal Level Requirements at the RF actives
The next step is to adjust all the RF actives for unity gain, but first you need to determine the desired RF input levels. In general, you want the return signal to be high relative to system ingress. What signal level can be expected at the RF amplifier when the modem with the highest loss path transmits at its highest power? RF input levels are determined by the system. These levels are dependant on 1) what modulation scheme is being used? 2)how many actives are in the node? 3) How many nodes will be combined in the receive site? What is the optimum operating level of the modem? Can it overcome the passive loss to achieve the levels determined in the above statement? Signal level requirements continued------Click.

Signal Level Requirements at the RF actives
System should be designed for constant input level whether at the STATION ports or at the Input to the Return Amp.. Amplifiers are aligned for unity gain back to the Node, by inserting a reference signal and adjusting for the proper received level at the head-end. Internal combining losses should be taken into account when determining the correct CW carrier level to use as the reference signal. Some cable operators prefer to maintain a single injection level or maintain a constant signal input at the seizure assembly. They do not take into account the embedding losses in the amplifier itself. Unity gain to these systems is maintained at the transmitter. The down side to this is that the modems will be operating at random levels and the carrier to noise for the node will not be uniform. The next slide shows unity input in a cascade.

Determine Return Input Levels
Carrier to Noise at Transmitter Noise Figure Return Amp. Total Node Actives C/N Total = C/N single-10Log N C/N single = Input + 59 – N.F. -50 dbc 5 dB 75 Actives --50 = X – 10(LOG 75) -50 = X – 18.75 X = = dbc = X + 59 – 5 = X + 54 X = 54 – (-68.75) X = dB(15) Carrier to noise headend is based on the modulation scheme and the number of nodes being combined. Noise figure is a number set by the manufacturer Total node actives contains the node, amplifiers, and line extenders. It does not take into account the modems or the optical link. Thermal Noise(59) based on analog carriers or 4MHz bandwidth. Calculation done same as for a downstream cascade. The carrier to noise for the optics is based on the link. See manufacturers specs. This number then needs to be combined with the C/N at the TX. Next slide is a cascade overview. What return amplifier inputs are required?

Return path alignment example
Cascade example next slide.

Procedure Set-up RF Amps
Start with amplifier closest to node and work out Return amplifier has specified input level for a given channel plan Apply return input and adjust to obtain reference levels at headend Read The next slide goes through the node again.

Head End Reference 18dBmV 49dBmV Ref
The headend reference is based on the RF output based on the longest link and the combining losses suffered delivering the signal to the CMTS device. The combining losses include the splitters, couplers and pads needed to supply the CMTS with the proper input levels. The reference out needs to take into account these losses whether they are present or not. Based on the longest link, due to the lowest output levels, all other nodes need to match. This will maintain modem output continuity. The next slide shows how input levels for return amplifiers are determined. Note Reference levels at Headend and retain for rest of amp chain (Start with longest link)

Return Amplifier Set-Up
Headend L “X” Ref 15dBmV “X”dB “X”dB Level applied to return amp input (Take into account The test point loss and the Amplifier embedding loss) The tombstone on the system map should call out the pad and equalizer for each individual return amplifier. Pre-load these devices as the downstream is being rough balanced. When working from the node to end of line, without these devices installed, the balance is being done with an open or unterminated line. We have already determined the return amplifier input to be 15dBmV. What needs to be injected to obtain this level? To get the injection level one needs to know the losses of the amplifier in the return path. In the above example there is a diplex filter and a combining network to account for. The test point or injection point also needs to be addressed. TP = 30dB DP = -2dB Combiner = -5dB Input Return Amp. = +15dB Therefore +15 +(-2) + ( -5) + ( -30) = +42dB injection Level. Note: This may not be the same as the node so either a new reference needs to be taken or the test point and injection levels of the meter needs to be changed. The next slide goes through the equalizer selection. Output Equaliser (per map Design) Output Attenuator (per map design)

Headend 15dBmV Ref “X”dB The first step( once the injection level has been set) is to install the correct return equalizer. It will, as in the downstream, attenuate the low end( 5MHz). Calculate the loss differential between 5 and 42MHz for the amount of cable preceding the amplifier and pick the equalizer to accommodate this difference. As seen by the sweep graph, 5 and 42MHz will be at the same amplitude with the correct eq. Selection. The actual level of the carriers is not important at this time. The next slide sets the attenuator value. Set Equaliser to get equal signal levels at both frequencies in Head End

Headend 15dBmV Ref In selecting the attenuator two parameters have to be taken into account. The M1 and M2 values need to be 0 and the Ref. Number at the top of the screen also needs to be 0. The first step is to manually adjust the meter ref. Level back to 0. This will leave the response above or below the center gradual. On 2dB/div. Pick the attenuator to move the response to the center gradual. Refresh the sweep. The trace should remain in the center and the M1 and M2 markers should read 0.0dB. If so the pad and equalizer selection are correct and the return amplifier has been set up properly. The next slide goes through an example. Set Attenuator to get correct signal level in Head End

In-Home Signal Losses Example of losses at 40MHz Drop -2.1 dB
Splitter dB RG dB ============= Total dB Example of losses at 40MHz Tap 150' -2.1 dB RG-6 50' -0.8 dB -3.5 dB House Losses Typical The next slide goes through levels and losses from the home to the active. We will use -10dB as the typical in-house and drop loss.

Relative to Return Amp Input
RF Plant Passive Losses Relative to Return Amp Input 7 dBmV embedding loss +15 dBmV at amp input B= Farthest from node low tap value +23 dBmV +25 dBmV +28 dBmV -5 -1.0 -7 -2.0 -11 -3.0 23 20 17 Cable Losses @870 MHz @40 MHz 26 +22 dBmV needed at Input to Housing +15 dBmV at amp input A= Closest to node High tap Value +45 dBmV into tap port +48 dBmV into tap port -10 dB internal and drop loss -10 dB internal and drop loss With a specified input to the return amplifier what will be the expected output levels of modems (A( closest to the active, B( farthest from the active.))? Each mouse click will add a new set of numbers. Starting with the yellow numbers or closest to the active. The next set will be the red or farthest from the active. The next slide goes through the embedding losses of actives. +55 dBmV Modem output +58 dBmV Modem output

Plant Actives - Type Amps Relative to Return Amp. Input
+42dBmV H H L L +22 dBmV H L +15 dBmV 5-LER-91 H L Typical Network amplifier. Allow 2dB of loss through the diplex filter along with copper runs. Allow 5dB of loss for the combiner which combines P2,P3 and P4, giving a single output or input to the return active device. The next slide goes through a typical line extender. -2 dB -7 dB Network Amplifier

Plant Actives - LE Relative to Return Amp. Input
+47dBmV H L -2 dB 5-LER-91 Line Extender +17 dBmV +15 dBmV +37dBmV A typical line extender only suffers 2dB of embedding loss. The thing to remember here is the test point or injection point loss. The older style has 30dB but the newer has 20dB. This will change the insertion level from that used with the network amplifier. The next slide goes through pad and equalizer selection for a cascade.

Return Set Up relative to Return Amplifier Input
-20dB +42 dBmV +35 dBmV -30dB +52 dBmV H L +15 dBmV +35 dBmV H L H L +22 dBmV +22 dBmV 9 Pad 5 EQ. +15 dBmV 2 Pad 5 EQ. L + 35 dBmV H L +17 dBmV +15 dBmV +47 dBmV 23 40 dBmV Input to Type Return Amp. = 15dBmV Amp. Embedding Losses = 7dB Cable Loss at 40MHz = 6dB Diplex Filter Loss = 2dB Station Gain = 24dBmV Input Level to Return Amp. = 15dBmV To TX Input at Node 15 dBmV Input Level With known values of embedding losses, cable losses, input levels and gain one can determine the pads and equalizers needed. The only variable again is what level is needed to be injected to obtain these levels. With mouse clicks go through the calculations to determine pads, equalizers and injection levels. The next slide starts Common Path Distortion. Input to TX =15dBmV Node Embedding Losses = 14dB Cable Loss at 40MHz = 6dB Diplex Filter Loss = 2dB Station Gain = 24dBmV Input Level to Return Amp. = 15dBmV 9 Pad 5 EQ.

Common Mode Distortion
6 MHz Beats Cause Location The next slide is a picture of the beats.

Common Mode Distortion
REF 6.0 dBmV MKR 8.90 MHz dBmV ATTEN 10 dB PEAK LOG 5 dB/ On a mouse click arrows will show pointing to the CPD. They are beats every six megahertz. The amplitude may vary but the spacing remains constant. In the picture the beats look like IF channels vs. cable channels. The next slide is fully automated but shows the same thing. 0 Hz RES BW 300 KHz 40.00 MH SWP 20 msec VBW 100 KHz

Common Path Distortion
6MHz Interfering Beat FSK The carrier in the center of the display is an FSK of status monitoring. The important thing to point out here is that one of the CPD channels is encroaching on the carrier causing data errors. The next slide shows the same picture on a different analyzer.

Common Path Distortion
Modem Return Spectrum 25dB C/N @ Status Monitoring Frequency The next slide starts common problems.

Common Problems Seizure Coax Connector
First is the coax. Explain the problems that can occur using a knife to clean the center conductor. High end roll off---response problems. Next adds the connector and seizure screw. Cause is an antenna effect. Next is how not to over-tighten the seizure causing reflections and make corrosion easier. The next slide is a sweep of reflections or standing wave.

Common Problem Results
The next slide contains terminators.

Common Problems Terminator AC Blocking Terminator
The first terminator is a non AC block. If used with AC on the line it can be destroyed causing sweep response problems and noise generated onto the return path. The second terminator is an AC block. It is longer and heavier. The next set of slides starts return path testing with an analyzer.

Return Noise Floor The next slide shows what modems contribute.

Return Path Measurements
The next slide is the introduction to return path testing.

Introduction to Return Path Testing
Testing on the return path is significantly different than the forward path. Ingress from anywhere in the node can effect all subscribers on that node and interfere with data traffic. Subscriber’s modems must time share bandwidth on the return with all other users on that node. Spectrum displays of a spectrum analyzer are very useful tools for analyzing the return path and the signals carried on it. Carrier to noise and distortions cannot be tested easily as there are typically no continuous wave signals present. We know this from previous discussions. To do a BER on a modem it needs a separate frequency form the rest. Maximum hold and zero span are the best tools for doing return testing. The next slide again discusses noise and ingress.

Using a Spectrum Display to Track Ingress and Noise
Use a spectrum analyzer display to track the source of noise and ingress in the system. Return Modem Signal tap Node To Headend Return Modem Signal Go through slide. The next slide shows the limitations of the analyzer for catching fast transients. Check at various points in the system to locate source of ingress or noise Noise or Ingress

Limitations of Spectrum Displays for Catching Fast Transients.
Scanning Spectrum Analyzers measure only one band of frequencies at any given instant. Frequency Range Where Measurement is Being Made at That Instant A spectrum analyzer is like the Wavetek or Calan meters. It captures a moment in time. If a transient does not occur within this time it will not be seen. The same goes for the devices that are in the home. Modems are not continuous so unless one is looking at the exact right moment and the sweep is passing at that instant the carrier will not be seen. The next slide is limitations continued. Frequencies Stored From Last Pass of Filter

Limitations of Spectrum Displays for Catching Fast Transients.
If the spectrum analyzer is at another frequency when the transient appears it will not be displayed. The next slide goes into the max hold function. A transient happening at this time will be missed by the filter unless it is still there when the filter comes by again

Max Hold Function Max Hold allows the spectrum display to catch transient signals such as ingress and modems. Max hold displays the highest level measured and holds it until the trace is cleared by the user or a setting changed. Max hold will only catch a transient if it is present at the time the sweep passes the frequency of the transient. Allowing the trace to build up over time using max hold increases the chance of catching fast transients. Read The next slide is one of an analyzer with max hold on.

Max Hold Function Max Hold Trace Current Sweep
Note the difference between the current sweep and the max hold sweep. The max hold it a display of response built over time. The left most arrows could be interfering carriers and the right most could be modems captured over a time frame. The next slide starts zero span.

Forward & Return Interaction
An increase in forward levels can create distortions that fall within the return path bandwidth This will appear on an analyzer to be poor diplex filter isolation or common path distortion (CPD)

Setting Up For Certification
A few words about test equipment If it’s not calibrated - it’s not valid Know your gear - what does it need to give you accurate results? An HP 8591C analyzer requires a minimum input signal of 27 dBmV to give an accurate CNR result. A maximum input signal level must be less than 37 dBmV to give an accurate distortion measurement.

Performing Certification Tests
The carrier-to-noise test for channel T-10.

The carrier-to-distortion test for channel T-10.

The hum test for channel T-10.

The in-band flatness response for channel T-10.

Peak-to-valley response of the entire return spectrum ( MHz).

The carrier-to-peak disturbance test. This test is done at 14 to 32 MHz for network. Additional tests from 10 to 40 are performed to document the entire return bandwidth.

DOCSIS Information To move from QPSK to 16 QAM requires an increase in CNR of ~7 dB to maintain a given BER To move from 16 QAM to 64 QAM an additional increase in CNR of ~6 dB is required to maintain the same BER The more complex the modulation format, the more error prone it becomes to transmission impairments

Return Shots From The Field
This problem still exists. It was “solved” by using a high pass filter on the drop going into the office.

Heavy impulse noise will exhibit itself as “modulation” on the hum test. This problem was resolved by locating and correcting a 150 microvolt leak caused by a bad input connector on a customer’s VCR.

CPD is caused by dissimilar metals creating a PN, or diode, junction.

This is an example of poor isolation in a diplex filter. The same problem will occur when an amplifier does not have diplex filter, and the standard jumper strap is used instead. Note that this impairment looks much like CPD!

This frequency response problem was caused by a defective span of .750 cable. The span was replaced and the problem disappeared. Of note -- the forward path response was unaffected. The problem affected only the return system.

The CB operator had installed several of his own outlets (mashed connectors, poor quality cable, loose connectors, etc.) CB (and ham) radio transmissions come up and remain at a relatively constant power if they are from a base station but tend to fluctuate in power if they are mobile.

A distribution end of line was found to be missing a 75 ohm line terminator.

Prior to doing a reverse sweep in the node area.

Note the roll-off above 30 MHz. The diplex filter being used is a 5-30 MHz variety rather than a 5-40 MHz type.

A squirrel had chewed a hole in the .500 cable near the home of a ham radio operator. A 200 microvolt leak was found inside the ham operator’s home. After fixing the rodent damage and replacing the the defective cable and connectors inside the ham’s home, the ingress went away. Connectors and cable work were done by CATV!

Return laser is being overdriven. Laser is DFB -- FP would be much worse.

A “one way” tech changed out a module in a two-way plant. The strap which had to be cut for diplex operation, wasn’t. What we’re seeing is an image carrier of a downstream signal.

An improperly seated SC connector created a “wave” like action rolling through the analyzer trace.

We still don’t know what caused this! We suspect it came from a nearby military base. It was there for several weeks and then disappeared. I’m sure it will return (pun intended) someday.

An example of a “new” (less than 1 1/2 years old) 860 MHz 2-way system that had not been maintained as 2-way plant. There is NO technology substitute for properly maintaining your plant and using good engineering practices.

This system was having a problem with converters that had a return frequency of 9 MHz. The only signal being generated by the cable system is the “haystack” at 37 MHz. The rest is ingress.

Return Path Testing - Conclusions
Testing on the return path is much different than on the forward path due to noise and ingress funneling and the bursty nature of return path signals. The spectrum analyzer display is a very useful tool for tracking down ingress and noise in the field. The zero span display of a spectrum analyzer stops the frequency sweep and turns the display into a time domain display of the signals in the frequency band of the resolution bandwidth filter. The zero span display of a spectrum analyzer allows you to see fast transients events such as modem bursts and ingress as well as the noise in between the events. The next slide is the last with return path conclusions.

Return Path Issues and Answers
Follow the Manufacturers Guidelines and Specifications. Complete Headend Combining Prior to Activation. Start with the Furthest Node and work toward the Headend. Align all Nodes Identically. Adjust Optic Receivers to accommodate the Termination Equipment. Check Return for Noise and Distortions. Set the return actives for Unity Gain. Know the in home devices capability and operational range. Maintain the System Integrity.

Return Path Issues and Answers

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Return Path Issues and Answers

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