Agilent Power Sensor Calibration - Need a Calibration Procedure

J

Joe222

#1
I have a question about power sensor calibration. When you look through service manuals from Agilent, they say that they don't provide a procedure because there are many different ways to test power sensors. Does anyone have a procedure that would be suitable for this?
 
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Jerry Eldred

Forum Moderator
Super Moderator
#2
The two most widely used methods used to calibrate RF sensors are comparing against a "gold" sensor (I'll describe below) and using a calibrator such as a Weinschel system.

The Weinschel system (which I have neither used or priced recently) is more than $100K (I think). This is "THE" method. It is expensive, and you need to have a lot of sensors to justify it. This utilizes a primary standard bolometer sent to NIST for calibration, and an RF sweep generator to sweep the sensor's frequency range, as well as a computer to run the procedure and plot the calibration factors for the sensor.

The second method sometimes used involves 1:1 comparison (less than 4:1 Test Uncertainty Ratio) against another sensor of the same model using either an RF power splitter or substitution method. You sweep through the frequency range of the sensor, and at cardinal point compare each of their power readings to the nominal (vastly oversimplified).

I have used both above methods in years past. The second method must be based on the end user's willingness to accept the reduced uncertainty ratio in the cal.

As far as procedures, if you are a GIDEP member, there are some USN or USAF procedures. If you are not a GIDEP member, they can't be distributed to non-members.

Hopefully my brief explanation above gives you some idea as to whether you wish to pursue power sensor calibrations. If you only have a few sensors, the most reasonable option is to send them to a reputable, appropriate vendor of your choice for cal.
 
R

Ruebenn

#3
Mr.Jerry,

Thanks for the insight and the info.
Yes,the most regularly used method would be the 1:1 method.
If we have a HP436A power meter along with the 8481A power sensor as our working standards, can we use to calibrate the more decorated power sensors from makers such as Rohde and Schwarz/Boonton and Gigatronics?
Or do you suggest to go for a HP437(Digital power meter) or even the HP 438A power meter?
I have been using this method for quite some time now.
And to calculate the VSWR measurements, i use a vector/network analyzer and sweep it across the range and get the eqvt measurements..am i doing it right?
And pertaining the MU budget, what are the normal uncertainties involved in deriving the cal factor calculations?
Appreciate the advise and hope you can provide these infos so that i can use it to come up with the MU when i get audited next month?

Rgds
Ruben
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#4
I have a question about power sensor calibration. When you look through service manuals from Agilent, they say that they don't provide a procedure because there are many different ways to test power sensors. Does anyone have a procedure that would be suitable for this?
The best would be to have access to GIDEP, as they include Navy NA17-20 series procedures, and there is one or more for power sensor calibration.

The procedures I believe are common but aligned along each band (e.g., X band) so the equipment could easily vary.

Hope this helps.

Hershal
 
R

Ruebenn

#5
Dear all,


Pertaining this issue, i stumbled upon a website belonging to IFR and it is entitled Power Sensor Calibration System.
Trust me , it is a good read especially if you are burdened by power sensor calibrations.
I see the formula given to calculate the Cal factor,CF(for a device under test) is given by

CF = [email protected]/[email protected] * [email protected]/[email protected] * C(f)

Whereby:-

PI= Indicated power at frequency f
PR= Reference indicated power at frequecy f
C(f) = Cal data at frequency for standard head(splitter/reference detector combination)
CF= Measured calibration factor for DUT at frequency f

Assuming this formula holds and if i use the standard 1:1 comparison method against a gold sensor(reference standard) , i can somehow get the cal factor but i am a bit confused by the c(f) which is the cal data for standard head( i am using the splitter-11667A) .
I have the cal cert(i sent this to Agilent for an accreditated cal) of my splitter with me and i am blur to what values to look at to plug in the value.
For an example...

I have obtained the values of both the DUT and the reference standard at 50MHz and the corresponding values at 1 GHz which reads 0.99925mW,0.99995mW,1.000mW and 1.0041mW but i am blur of what to do when it comes to putting in the value of c(f).

Referring to my calibration cert of the 11667A, i see only the results for the tests input port SWR and output tracking test only...which values in which specific test should i take as the c(f)?

Furthermore, should i use the values in dBs or in watts to calculate the cal factor?
Also i see the cal factor values are all in percentage valus,any idea how we convert this ?
Appreciate if any of you can guide and shed some light on this problem that i am facing?
Or if any one of you can share some other methods or formula to help me calculate the cal factor, please feel free to share?
Appreciate the help and looking forward to your help.

Rgds
Ruben
 

Jerry Eldred

Forum Moderator
Super Moderator
#6
This may be an oversimplified answer. But in calculating Cal Factor (which I will call C.F.), it is normally done in Watts this a linear unit (compared to dB/dBm which are logarithmic units).

Cal factor is simply the % error correction (in Watts or milliwatts) from what the sensor reads to what the actual power is. I apologize that I am a "Woodshed" mathematician. That is, I am a very visual person, and do calculations in a way where I am able to mentally visualize the issue.

If you use a power splitter (11667A) which you either assume is perfectly linear between the two outputs, or you correct for those differences, then when you connect a GOLD sensor on one side (one whose accuracies are known and accounted for), and an unknown sensor on the other side, you measure power level at each frequency (typically at 0 dBm/1 milliwatt). At each setting, you adjust for "exactly" 1 milliwatt as read on the standard sensor (including correction for it's Cal Factor). What ever difference is read on the sensor being calibrated is accounted for with its Cal Factor.

Additionally, Cal Factor is the percent correction as referenced at 0 dBm @ 50 MHz (called the REFERENCE CAL FACTOR). AND, NORMALLY, you will compensate in such a way as to assure no cal factor exceeds 100% (as some instruments/software have difficulty in accounting for those higher cal factor). The 8481/2/3/4...etc. series of sensors are relative sensors, in that they are referenced against response at 50 MHz.

For example, if you set the power sensor for 100% Cal Factor at 50 MHz (let's say an 8481A with a range of 18 GHz). Then with the C.F. set for 100% at 10 GHz, and with power level set to exactly 1.000 mW at 10 GHz, you make a reading with the sensor being calibrated, and it reads 0.965 mW. It's Cal Factor is therefore 96.5%.

To recap some details, the sensors above are relative sensors, in that they don't measure absollute power by themselves. They measure relative power against the internal built-in reference at 50 MHz/0 dBm. So their measurements become absolute measurements of power level only when compared against, and referenced to the reference. The cal factor is just the relative error in Watts (milliwatts) as a percent error correction against that reference frequency/amplitude.

Hope this is a little start in answering your questions.
 
R

Ruebenn

#7
Dear all,

Can you tell me what is the SI unit of amplitude modulation?
Also the frequency and phase modulation as well?
I wish to know how to put them in the S.I units?
I know all the units are in Hz..hertz over Hertz cancels off leaving a relative value, true?

For the calculation of amplitude,phase and freq modulation using the 8902S system,
it is stated as 3% + 1 digit (what is the digit here- is it the noise floor,resolution or what?)

Rgds
Ruben
 
R

Ruebenn

#8
Hi all,

My lab was audited recently by an ISO auditors.
I have some old units with me in which i use to calibrate the older version of sensors but now after getting audited by these people , i learnt that i have been making calibrations with wrong instruments.
I would be much obliged if i can get some suggestion on the instruments that i should have for the calibration of power sensors.
I need to suggest some instruments and i would appreciate getting some info through this group.
If i wish to clean the connectors be it the sma or the N type..can i use an alcohol based solution? Plus is it a must that i get a connector pin depth gauge?
Appreciate the help and reply.


I wish to also get a suggestion on how to make accurate frequency measurements using a microwave counter.
I have been using A HP5351 unit to do it till it got faulty and ever since, i have been using the 8563E to make my microwave freq measurements.
Am i doing it right?
How do i calculate the freq uncertainty using the analyzer with an external GPS unit?

Rgds
Ruben
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#9
For the power sensors, see if you can get the factory service manual, as it will likely have the calibration procedure and likely a list of requirements for the reference standards, or may specify the standards.

For cleaning the connector, alcohol and a little brush should be fine. Use compressed air if you have it to blow out the excess which should also take any dirt with it.

For a freq counter sync'd up to a GPS, the uncertainty that normally would come from the oscillator aging rate will now come from the GPS, which is good. Given the normal accounting for any mismatch loss or connector loss (both of which are very small at 10MHz), then the GPS will drive the Type B uncertainty, and may well be in the -12 to -14 range.
 
R

Ruebenn

#10
Greetings Mr.Jerry,

Thank you for the info.
I have a question about the HP8484A which is a power sensor (-Frequency range: 10 MHz to 18 GHz
Measurement range: 100 pW to 10 uW (-70 dBm to -20 dBm) .
I was reading somewhere that this unit was replaced by the HP8481D series but i cant recall the reason.
Could it be due to the sensitivity of the unit?
Moreover if i were to use it to measure power levels, what should i be looking out so that i can minimize the measurement uncertainty?
How about the linearity of the sensor?

Rgds
Ruben
 
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