Electronic – Where to Measure S11 with a VNA

impedance-matchingRFtransmission linevna

I have someone else's design that I have been asked to choose matching components for.
shown here

The antenna is a chip antenna and the filter is a band pass SAW filter in a single package. The SAW filter is a DC short to ground, so at DC, all the components are tied to ground, but at it's operating frequency, it acts like a proper transmission line network.

My question is, since this is on a PCB and I've got an Agilent/HP 8753 VNA with S11 only, what is the proper way to measure and select matching components for this?

More specifically, I have read from some App notes from TI that the way to calibrate the 8753 is to do OPEN, SHORT, and LOAD measurements on the actual board itself…But, what do you remove from the board to test the load? Where should the VNA measurement point be? Do I have to do each filter separately? Can I leave the SAW filter in or should I replace it with a 0ohm resistor?

I've made some component changes (namely only using L1, L2 and C2) and have got a reasonable match by measuring at the L1,C2,BPFilter node and ground (~-27dB@fo), but my range of my antenna is still spotty. I'm not sure what to compare it to or if I'm doing this measurement right.

Thanks!

Best Answer

If you have -27 dB S11, then the match is definitely not your range problem! However, SAW filters often have a good bit of loss, so if there is no amp between it and the antenna, I might be a bit suspicious of that.

Related Solutions

Electronic – Measure RF impedance of PCB or IC using a VNA Network Analyzer

I'll answer the parts of your question where I understand what you're asking:

Can I use a VNA to measure the impedance of the MCU?

If the MCU is powered on and transmitting, it could produce spurious readings from the VNA.

If the MCU is shut down, it will likely not have the same equivalent impedance as when it is operating.

If it can be turned on and put in a no-transmission mode, you might be able to get a good measurement.

Am I going to break my VNA if I plug a constant TX device into it?

Not if the output power is below the maximum input level for the VNA.

But again, the signal coming from the TX could cause erroneous readings.

I currently don't have the calibration tool and it costs $6K... For this exercise what is the consequence of no calibration? Can I make my own coarse calibrator?

There are alternate cal kit vendors, but I doubt you'll find a decent kit for under $1000.

You could try measuring some known devices to see how accurate your VNA is without calibration.

It'd be unlikely you could get a good measurement of the \$S_{11}\$ of a well-matched load without calibration (but you could probably verify the load is not totally broken and behaving like an open or short).

Electronic – Qucs model for 2.4 GHz matching circuit

The FR4 you quoted in comments has a specified \$e_r ~~a.k.a.~~ D_k\$ for 1MHz is not rated for 3e9 (3GHz)! ... let alone 1GHz...

for example. The thickness and the dielectric constant of the generic prepreg glass styles for Isola 370HR material are given in the chart.

Prepreg Styles %Resin Content  (mils) Nominal Thickness
                   Dk at 5-10GHz  (100% copper)   
 106 76% 2.3 mils 3.54
1086 63% 3.1 mils 3.78
1080 66% 3.0 mils 3.72
1080 68% 3.2 mils 3.68
1080 71% 3.6 mils 3.63
2113 59% 4.0 mils 3.86
2116 56% 4.8 mils 3.92

When there are several types of dielectric materials (having different di- electric constants) between the signal layer and the reference plane, it becomes necessary to calculate the effective dielectric constant of this composite dielectric material. A simple way is to calculate the weighted average of the dielectric constant as stated below:

Cu Signal   for controlled imepdance
Prepreg     Dk1
FR4 core    Dk2
Cu Gnd plane

for thickness X1 of material Dk1 and X2 of Dk2

\$ Dk= \dfrac{(X1 * Dk1) * (X2 * Dk2)}{ (X1 + X2)}\$

Some brand names to consider for Controlled impedance for high speed signals in GHz range;

Isola FR408HR
Isola I-Speed
Nelco 4800-20
Panasonic Megtron6
Rogers (like RO4350, RO3003, etc)

Anecdotal

I recall in mid'90's when I ran an R&D proto PCB shop as Ops Mgr in mid'90s my colleague PhD RF designers were designing 3 Revs of Getek and neglecting this fact that Er or Dk was (for their design )to 3.5 than 4.5 at 1GHz until they got it right. I got 48hr turns so it took 1 week.
It would have saved time and money if they had specified the Zo impedance on each track of their filter and pay for IMpedance testing to calibrate the dielectric and they modify the D codes to get <10 % tolerance using TDR test coupons.

update

Using the QUCs application which supports the more accurate Hammerstad and Jenson formulae for COntrolled Impedance using Er~4.35

Compare your results with this.

I'm not certain why Effective Er reduces with rising f when the Mfg specifies that it changes to a smaller degree from their test results with 3 sigfigs. when I always assume it was 5~10% tolerance !!

I suspect the skin effect, eddy currents and conductor thickness plays a role. just a heads up and probably why Impedance Analyzer tests and TDR tests need to be done to get it perfect but with everything connected at antenna port when conjugate matched for s11,then s22 and s12,s21 with appropriate jigs.