1. CTF3 Low-Level RF measurements TBTS/TBS lines and 12 GHz testand (Xbox 1)
Stéphane Rey* - BE/RF
Luca Timeo - BE/RF
Ben Woolley - BE/RF
Wilfrid Farabolini - CEA
October 7th, Rev 1.0

2. Calibrating RF measurement paths
Why ?
To ensure each device is rated for the frequency and power applied
To get best measurement accuracy of RF powers
To measure RF path characteristics (insertion & return losses, linearity, reflections, …)
To check and compensate long term drifts
To provide RF systems diagnosis
How ?
Several methods :
Method 1 : Generator + powermeter  S21
Method 2 : Generator + 4 powermeters  S11, S21, S22, S12
Method 3 : VNA 1-port  S11,
Method 4 : VNA 2-ports  S11, S21, S22, S12
Method 5 : TDR (Time Domain Reflectometry) using a VNA  S11 + post-processing (inverted FFT x virtual pulse)  S11, S21, S22
Method 6 : TDR using a Time Domain Reflectometer  S11, S21, S22
Method 7 : TWT + CTF3 Acquisition system and powermeters  S21, S11, S22

3. Calibrating RF measurement paths
Typical RF path at CTF3
Semi-rigid cables
Diodes
ADC
Coaxial
attenuators IQ
N to N
transition
Klystrons Gallery
CLEX
15-35m 3/8’’ coaxial cable
waveguide to N adapters
waveguide ~10 dB attenuators
50 dB
coupler

4. (or Murphy law variation)
The true life
(or Murphy law variation)
Different measurement methods gives different results
Measurement instruments are less accurate than the expected measured accuracy
Devices are never specified correctly… if they are specified !
… and are anyway never used within their nominal specified range
RF Powers are always above expectations
‘Low-phase’ coaxial cables are phase dependent
Fixed attenuators are not
A 5 watts rated equipment exhibits a derating starting from 1 watt and burns at 4 watts
Symmetric devices are not
‘Perfectly matched’ devices have a return loss > -10 dB
… which fluctuates over time
Controlled temperature means ‘almost controlled’
Screwing a connector again with the same torque never gives the same return loss
‘Do not touch’ warning signs are usually inviting people to touch

5. Instruments accuracy (or uncertainty)
Multimeter Agilent 34401A – 6½ digits
(source 34401A datasheet)
Powermeter Agilent E4416
(source E4416 datasheet)
Absolute accuracy = ± 0.04 dB within ± 5°C from its calibration temperature
Absolute accuracy = ± 0.08 dB from 0 to 55°C

6. Instruments accuracy (or uncertainty) 20 GHz VNA Agilent E5071C
6-20 GHz corrected system performances with 3.5mm D SMA calkit (source E5071C datasheet)
Transmission uncertainty for transmission coef.
between +10 and -40 dB = ± 0.2 dB
@ 23°C ± 5°C with < 1°C deviation from calibration temperature
Phase uncertainty for transmission coef.
between +10 and -40 dBm = ± 2 degrees
Magnitude temperature drift = ± 0.04 dB/°C
Phase temperature drift = ± 0.8 degrees/°C

7. Instruments accuracy (or uncertainty) (source DC270 datasheet)
Acquiris DC270 digitizer : 8-bits / 1 GSPS / 250 MHz
(source DC270 datasheet)
Accuracy ±2% Full scale range
Linearity ±1% Full scale range
ENOB (number of effective bits) :
DC-20 MHz : > 6.8
MHz : > 6.4
Instruments last calibration ? Calkits verification ? Probes checking ?

8. What are uncertainties ?
Uncertainties are not only offsets !
They are made of :
Frequency dependence
Gain & Offset errors
Linearity, Hysteresis errors
Gain and offsets drifts over temperature
Noises (thermal, flicker, …)
No easy calibration to implement !
Global error estimation (Gaussian distr.) :

9. Distinguishe DUT from instruments errors
6-sigma method to determine equipment capability
6-sigma : methodology from Motorola (80s) to increase quality and process efficiency by measuring equipment capability (accuracy & repeatability)
Statistic parameters , CP and CPk characterize the error distribution
Estimate the number of parts out of specified limits Lpl + Lpu
First, characterize the measurement system and then the population

10. Distinguishe DUT from instruments errors
6-sigma method to determine equipment capability
Non-conformities vs CP, CPk

11. Generator + powermeter(s)
Measurement method 1
Generator + powermeter(s) ~ ~
Power
meter
Power
meter
Step 1
Step 2
Measure S21 directly
Pros :
Easy to do even without access to the two hands of the cable
Direct measurement of DUT without extra instruments (couplers, splitters, …)
Cons :
Measure only S21
Measurement error if there are bad return losses
Limited possible path attenuation to dB due to generator power output limit (usually +10 dBm) and powermeter noise floor (~ -40 dBm)

12. Generator + couplers + load + powermeters
Measurement method 2
Generator + couplers + load + powermeters
Power
meters
Power
meters ~ ~
Power
meter
Step 1
Step 2
Measure all parameters
Pros :
Measure all S-parameters : S11, S21, S22, S12
Cons :
Longer calibration process : calibration of couplers with VNA (uncertainties)
Needs several instruments
Add extra equipment in the measurement path (source of uncertainties)
Limited measurable insertion losses to dB due to generator power output limit (~ +10 dBm) and powermeter noise floor (~ -40 dBm)

13. Measurement method 3 VNA 1-port VNA Measure S11 Pros : Cons :
Measure only S11
S22 or any intermediate return loss effects seen might be seen on S11
With high attenuation paths, not possible to see a bad output return loss
VNA uncertainties

14. Measurement method 4 VNA 2-ports VNA Measure all S-parameters Pros :
Cons :
Impossible if no access to the two hands of the DUT
With high attenutation paths, not possible to see an intermediate bad return loss (damaged cable)
VNA uncertainties

15. TDR using VNA 1-port + post-processing
Measurement method 5
TDR using VNA 1-port + post-processing
VNA +
Measure S11
Pros :
Enable to see intermediate return losses and identify damaged cables
Enable to measure distances (or times)
Cons :
Accurate only if return losses are negligible
No real time measurement : very slow process. Two measurements needed (loaded and shorted)
With 30~40 dB attenuation paths, not possible to measure a bad output return loss
VNA uncertainties

16. Measurement method 6 TDR TDR Measure S11 Pros : Cons :
Enable to see intermediate return losses and identify damaged cables
Real-time
Higher dynamic range, resolution and accuracy than VNA 1-port method
Enable to measure distances (or times)
Cons :
S22 or any intermediate return loss effects seen might be seen on S11
With high attenuation paths, not possible to see a bad output return loss
VNA uncertainties

17. TWT+ couplers + powermeters
Measurement method 7
TWT+ couplers + powermeters
Power
meters
Power
meters
TWT
Measure all parameters
Pros :
Measure all S-parameters : S11, S21, S22
Measure the actual behavior with RF power as in application
Cons :
Longer calibration process : Needs several instruments
Add extra equipment in the measurement path (source of uncertainties)
TWT output impedance is varying vs RF power making S11 measurement uncertain

18. Measurement methods Synthesis
All the methods are providing different results. The choice of the method depends on what exactly need to be measured.
For S21 the generator + powermeter gives the best accuracy but this require to check first with a VNA or TDR that return losses are correct
For S11 measurement, the VNA is easier to use but provide a less accurate measurement. But is there really a need for accuracy in RL measurement ?
For time measurement as well as to check quickly quality of RF Path, TDR is the most suitable.
In order to fully characterize correctly a RF path it might be necessary to combine several methods.

19. CTF3 measurement results
Layout
Diodes / Log
ADC IQ
Coaxial S11 ~7-15 dB
Klystrons Gallery
CLEX
Coaxial S11 ~7-15 dB
Attenuator S22 ~10dB
Coupler S34 ~0dB
Attenuator S11 ~20dB
Coupler S14 ~50dB
Uncertainty ?

20. CTF3 measurement results
Coupler + attenuator + coaxial cable
Calibrated
Not Calibrated
WR90-N Adapter S21=~0dB = 98W
+ Attenuator S22=-10dB (10% = W)
- Coax S11=7-15dB (3-20%=3-19.6W)
= 78.4…95W
= MW
= MW
=98MW
Attenuator S11
+ WR90-N S11
+ Coax S11
- Attenuator S22
= MW
= 3.8 to 18.6 %
Attenuator S21=-10dB = 99,0W
- WR90-N adapter S11=-20dB (1%=0.99W)
= 98W
=0.1MW
=99MW
Coupler S31=-50dB = 1 KW
- Attenuator S11=-20dB (1%=10W)
= 990W
=1MW
=100MW
RF power = 100 MW

21. CTF3 measurement results
Coupler + attenuator + coaxial cable
Possible solution :

22. CTF3 measurement results
IQ Crates
BW = ~150 MHz !

23. CTF3 measurement results
Diode Crates

24. CTF3 measurement results
½ Diode crate chanels cross-coupling
3dB
3dB
CAL in
CAL out
6dB
6dB
Group similar amplitudes on same 6dB splitter
To channels
5-8
3dB #1
3dB
3dB #2
3dB
RFin
3dB #3
3dB
3dB #4
3dB

25. CTF3 measurement results
XBOX 1 calibrations – method 7 with TWT

26. CTF3 measurement results
TBTS calibrations – method 7 + method 4
S11 varies with power :
TWT output impedance or RF path drift

27. CTF3 measurement results
Temperature logging
MKS 03 temperature :

28. CTF3 measurement results
Conclusions
Layout modifications for detectors dynamic optimization & general improvement
Full RF paths characterizations with several methods giving different results are needed
instruments & layout mismatches leads to uncertainties
Next ?
Methods validations in lab on reference cable
Continue equipment characterizations (automatic tests bench, climate room) :
Replace our detectors technology
Current designs :
Down-mixing detector (design for XBOX2)
New log detectors crate
Investigate on direct acquisitions closer to the machine
Investigate on RF & clock distribution quality (phase noise, spurs)