Return Path Issues and Answers

1. Return Path Issues and Answers Rev. # 3 Feb. 2002

2. 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.

3. 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.

4. 26 Analog Video Com21 55-319MHz 23 splitter 2 Way RF 20 430MHz 2 Way HCX comController RF splitter 600MHz 17 Public switch 14 Return System

5. RX RX RX Sweep Modem RX RX RX Phone Analyzer RX RX RX Headend Combining

6. RX RX RX Sweep Modem RX RX RX Phone Analyzer RX RX RX Headend Combining

7. Energy Accumulation Return Path Signal

8. Funnel Effect Cont..

9. CPU Monitoring Device Combiner Funnel Effect Cont..

10. System behavior • Thermal noise funneling • Most important cause of thermal noise: • 1) subscribers • 2) amplifiers • 3) optical link • Laser Clip • Ingress and Impulse Noise

11. Return Rx Return TX System behavior Average case

12. Return Noise Floor

13. System behavior Noise funneling (amplifiers + optics) Return Rx Return TX

14. Return Noise Floor

15. Return Rx Return TX System behavior noise from modem

16. System behavior Laser Clip Return Rx Return TX

17. Ingress Example • 70 % from the home • 25% from the drop cable Ingress/ noise content Noise / ingress content

18. Random Noise VS. Time

19. Impulse Noise

20. Return path alignment Technical Support

21. Return System Components Head-End RX TX Head-End RX Head-End RX Head-End RX Combiner CMTS Demod 2 3 1 5 4 7 6

22. Node Rx ForwardTX Headend Return TX Return Rx Optical return path link • Optimum level • input level too low ==> low thermal and RIN CNR • input level too high ==> high Intermodulation noise ==> low CNR CNR BER input level input level optimum level optimum level

23. Node-Hub Return Link

24. Alignment in the Field (1) Person 1

25. Alignment in the Field (2) Or Spectrum Analyzer with Video Out Function Or Baseband Output of Analyzer into Modulator

26. Combining Network Node TP TP Out System Sweep Transmitter 3SR System Sweep Transmitter 3SR Stealth Sweep Stealth Sweep help FILE FREQ abc def ghi 1 2 3 status AUTO jkl pqr 4 5 mno 6 CHAN alpha yz stu 8 vwx 9 ENTER 7 SETUP x light . +/- space CLEAR FCN 0 PRINT SWEEP LEVEL SCAN TILT SPECT MOD C/N HUM 10 40 Alignment In the Field #3

27. Node-Hub Return Link • Set up link to carry max (example) 23 (QPSK) ch • OT drive spec for 2 Video channels  10 - 20 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

28. 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 +2dBmv total = -12 dBmV/ch +24dBmv total = +10dBmV/ch

29. Based on Channel Bandwidth (42-5)/1.6=23 channels 10*log(23ch)=13.6 dB 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 2ch.@ 18dBmV = 24dBmV • ( 20*log(2)=6 ) • QPSK Channel width =1.6 MHz 22-13.6= 8.4dB per Channel (22dBmV value 2dBmV below QAM Clip)

30. Specified level into forward TP is 39dBmV Node adjustment Test point sticker level is level for video carriers => for digital, target is TP level is 8dB Corresponding input level is 19dBmV (20dB)

31. 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.

32. Combining Network Node Optical Receiver System Sweep Transmitter 3SR System Sweep Transmitter 3SR Stealth Sweep Stealth Sweep help FILE FREQ abc def ghi 1 2 3 status AUTO jkl pqr 4 5 mno 6 CHAN alpha yz stu 8 vwx 9 ENTER 7 SETUP x light . +/- space CLEAR FCN 0 PRINT SWEEP LEVEL SCAN TILT SPECT MOD C/N HUM Stealth Reverse Sweep Optical Transmitter TP Out Optical Receiver 3ST Reverse Sweep Displayed on 3SRV 3SRV

33. Reverse Sweep Reference

34. Reverse Sweep

35. Return Path Requirements Signal Levels Passive Values Unity Inputs

36. 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?

37. 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.

38. 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 = -50 + -18.75 = -68.75dbc -68.75 = X + 59 – 5 -68.75 = X + 54 X = 54 – (-68.75) X = 14.75 dB(15) Determine Return Input Levels • What return amplifier inputs are required?

39. Return path alignment example

40. 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

41. Ref Head End Reference 18dBmV 49dBmV Note Reference levels at Headend and retain for rest of amp chain (Start with longest link)

42. 15dBmV L “X” Return Amplifier Set-Up Headend Ref “X”dB “X”dB Level applied to return amp input (Take into account The test point loss and the Amplifier embedding loss) Output Equaliser (per map Design) Output Attenuator (per map design)

43. Return Amplifier Set-Up Headend 15dBmV Ref “X”dB Set Equaliser to get equal signal levels at both frequencies in Head End

44. Return Amplifier Set-Up Headend 15dBmV Ref Set Attenuator to get correct signal level in Head End

45. Example of losses at 40MHz Drop -2.1 dB Splitter -3.5 dB RG59 -0.8 dB Splitter -3.5 dB RG59 -0.8 dB ============= Total -10.7 dB Tap 150' RG-6 -2.1 dB -3.5 dB 50' -0.8 dB 50' -0.8 dB -3.5 dB In-Home Signal Losses We will use -10dB as the typical in-house and drop loss.

46. RF Plant Passive Losses Relative to Return Amp Input 7 dBmV embedding loss +15 dBmV at amp input Cable Losses +23 dBmV +25 dBmV @870 MHz +28 dBmV @40 MHz -5 -1.0 -7 -11 -2.0 -3.0 26 23 20 17 +22 dBmV needed at Input to Housing +45 dBmV into tap port +15 dBmV at amp input +48 dBmV into tap port A= Closest to node High tap Value B= Farthest from node low tap value -10 dB internal and drop loss -10 dB internal and drop loss +55 dBmV Modem output +58 dBmV Modem output

47. +22 dBmV +15 dBmV Plant Actives - Type AmpsRelative to Return Amp. Input +42dBmV H H 0 L L H 0 L 5-LER-91 H 0 L -2 dB -7 dB Network Amplifier

48. H H 0 L L +17 dBmV 5-LER-91 +15 dBmV -2 dB -2 dB Line Extender Plant Actives - LERelative to Return Amp. Input +47dBmV +37dBmV

49. -30dB TP @ +52 dBmV +35 dBmV +35 dBmV H H H 0 0 L L L H H 0 0 L L H H +15 dBmV 0 0 L 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 +47 dBmV + 35 dBmV 23 H H 0 L L +17 dBmV 40 dBmV +15 dBmV Return Set Up relative to Return Amplifier Input -20dB TP @ +42 dBmV H +22 dBmV L +22 dBmV 9 Pad 5 EQ. +15 dBmV L 2 Pad 5 EQ. To TX Input at Node 15 dBmV Input Level 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.

50. Common Mode Distortion • 6 MHz Beats • Cause • Location