gain/phase error from not having reflection/transmission kit  - Page 1
Forum section: electronics / general

author: CopperCone
author_rank: Super Contributor
timestamp: July 27, 2018, 11:22:10 pm
content: Don't know why I did not put this in the metrology forum first but:Thought this might be a better forum since its pretty specific to test equipment:So I have a VNA that has the capability to measure reflection/transmission but it does not have a build in directional coupler or whatever the 87512A is. The manual says to simply use a power splitter. I tracked down that there is a option to use a 87512A with it.I guess taking reflection measurements only using a power splitter to supply the reference channel does not make sense... but how much error does the reflection cause for a typical gain/phase measurement? The VNA is a E5100A and it goes to 300MHz.I mean the reflection goes into the reference port through the external power splitter. It seems really bootleg. the 87512A has 40dB directivity. How can it make any meaningful measurements if it gets that reflection?Is there something I am missing in my understanding of how this works? Are reflections just really small typically? I don't really know what % of the signal power is lost in a reflection.All someone wrote so far is that"The error should be entirely dependent on the short cal accuracy. For an accurate reflection measurement you also need an accurate open cal."Can someone explain this more?Most filters reflect most of their energy right? Unless its a specially built disipative filter. Doesn't this bug the reference port? Especially considering the phase is random?I know with my microwave equipment I need to put isolators or attenuators inline with filters to prevent damaging the generators. All that VNA has is 6dB attenuation from the power splitter. I thought the reference port requires a pure signal, what goes into the DUT. Won't the reflection possibly phase-cancel or add to the reference and cause errors? Or is the reference input reject the non dominant frequencies some how? I thought no, since the operation requires an accurate power splitter.

author: pigrew
author_rank: Frequent Contributor
timestamp: July 29, 2018, 05:25:23 pm
content: (I was waiting for others to reply, but they didn't, so I'll give my opinions.... Perhaps this would have been better to post in the RF forum. My below response is quite hand-wavy, probably not quite right for the "metrology" forum, but may help you ask better questions )Looking at the manual, it seems to describe only a transmission measurement, but not a reflection measurement. I don't think that with the single internal splitter, you would be able to measure the reflection. Or... you're adding an external power splitter?The E5100A can measure the gain/phase between any of its four inputs (R,A,B,C), due to its use of a DSP to do the phase/gain acquisition (instead of an I/Q demodulator like "analog" VNAs). Based on its block diagram, it downconverts the input signal to 12.5 kHz, and then bandpass filters it. This filters out most interference (so a "pure" sine wave is not required). It seems that there is no image rejection filter before the mixer, so it could be confused by signals the reference frequency +/- 12.5 kHz (unless they do have image rejection and just didn't show it on the diagram). The ADC's frontend does have an anti-aliasing filter, so the DSP can pretty accurately discriminate input frequencies.Most power splitters (such as Wilkinson) have reasonable directivity (~20 dB), so the reflected signal doesn't influence the "R" port as much as you may expect (hopefully your DUT is already well matched. With a well matched DUT, the reflection IS small.). Yes, they will be input to the RF source, which isn't so good, but the splitters have enough loss that I don't think the amplifier would be damaged. VNAs only output low source powers, anyway (<10 dBm).Yes, filters must either reflect or absorb the undesired frequencies. With the 3 dB splitters, the signal's power will be halfed during transmission and reflection, so the power incident into the PA will be no less than 6 dB lower than its output power. Most PA are perfectly stable with this. The phase is not "random". The phase of the reflected signal is a function of the filter and is stable... it doesn't change.In the case that you are using both the internal and an external splitter, you're using the external splitter as a 3 dB directional coupler. It still will have decent directivity (> ~20 dB), so things are not as bad as you think. In addition, the VNA's calibration (with a SOLT calibration), is able to determine the and calibrate-out the directivity of your splitters/couplers. I'm bad at the math (and not looking it up at the moment), but having both the open and the short is helpful to figure out the directivity/coupling factors. The correction model you're looking for is "enhanced-response", which is a one-port calibration, plus response (thru) cal. This would use four standards (short, open, load, and thru), and I think is an 8-term error model.Finally, to answer your question about reflections, we usually like |S11| to be > 10 dB, which is 10% of the power being reflected. When you look at precision things, the reflection is usually specified as VSWR, like 1.9:1 (which is around 10 dB). Good connectors will have VSWR ~=1.05:1, which is 0.06% power reflected.

author: CopperCone
author_rank: Super Contributor
timestamp: July 30, 2018, 07:54:48 pm
content: Quote from: pigrew on July 29, 2018, 05:25:23 pmI have a model without a internal splitter. I made a splitter using 12.7 ohm thin film resistors that were hand picked, tiny ones, I think they are like 1/6th of an inch long, leaded, basically soldered right up to each other to make the smallest lead length, then 50 ohm pamona BNC connectors soldered onto each resistor, then this is soldered to a copper triangle that acts to connect all the grounds and work as a shield, the triangle is bent to make a full enclosure and solder wick and solder was used to seal any holes that remain from the crude construction. The soldering was kind of tricky because of how I tried to cut all the leads.Are you saying my 3 resistor splitter has 20dB directivity? Or do you mean when you combine the splitters (which I don't have a real one) you basically retain the wilkinsons directivity?If so, should I build a different splitter then the three 12.7ohm resistor one I am using now? My understanding was that it isolates the R port from the reflection by 6dB only. To my understanding if you reflect back into the output amplifier you get extra heating and drift of the circuit. I am still unclear what happens when the reflection reaches the R (reference) port.

author: CopperCone
author_rank: Super Contributor
timestamp: July 30, 2018, 08:10:44 pm
content: also my VNA can output 24dbm, I have the model with the power amplifier.