Tj256
Electrical
- Nov 8, 2011
- 4
I think you'll find this is an easy question if you are well versed in taking impedance measurements with a VNA (or even a slotted line).
I believe my setup is flawed in some subtle way. I am able to plot changes on a smith chart as I add/remove components to my PCB using the technique I am about to describe, but sometimes things don't work quite as I'd expect. The parts I'm using are thin film, designed for 5.8 ghz.
I'm building a PA based on the HMC408 and I have to match a 4 ohm output (with maybe a slight imaginary component) with a 50 ohm connector. Mind you, I'm at 5.8 ghz so there is probably little room for slop. I thought I'd use my slotted line to help me measure the impedance to make sure it will work.
My test setup looks like this:
Signal generator---> Slotted line ---> Output connector on the PA board (what I'm working on)
I replaced the MMIC with a 50 ohm resistor on the PCB, and took some measurements with the slotted line. Don't get too hung up on this though. I am easily able to measure SWR (to draw the circle on the smith chart) and see the phase shift from my changes relative to a short I calibrate with. Things move around on the smith chart MOSTLY as I'd expect.
The signal path on the board looks like this :
1)SMA connector
2)50 ohm TX line
3)Shunt capacitor (1.3pf)
4)Inductor (.4nh)
5)50 ohm TX line
6)PA chip (50 ohm resistor at this point)
I have a program called "smith tool" that lets you drive around the smith chart by inserting components into a matching network. If I "drive around" from the center of the chart starting at 50 ohms and part #1 above, I can get to my desired impedance with the above list. The final inductor and TX line rotates clockwise around the chart to hit the desired impedance.
I'm finding that in real life, that final inductor and even the txline has NO EFFECT on the final result. Almost as if it doesn't count. This has been verified as the behavior by not only my slotted line measurements, but by a software program I use called "ansoft designer", if I try to plot S11 on the smith chart on the same network. I can boost the inductor to 1 or 2 nh and still no change in the measured impedance.
In other words, that final txline should be adding a phase delay, and rotating the measured impedance. It doesn't.
Clearly I am missing something here. Is this a valid way to measure my matching network to make sure it's "tuned", and why are these inductors/transmission lines having no effect on the measured S11?
I'm hopeful I can avoid buying a VNA. And again, I don't think this is the problem because the simulator shows the same deal. Try it yourself: Port 1 goes to a shunt 3pf cap. Then through an inductor. Then through a txline. Then to port2, a 50 ohm port. You'll notice that S11 doesn't change as you'd expect if you play with the inductor/txline.
Maybe I am not doing this right...
I believe my setup is flawed in some subtle way. I am able to plot changes on a smith chart as I add/remove components to my PCB using the technique I am about to describe, but sometimes things don't work quite as I'd expect. The parts I'm using are thin film, designed for 5.8 ghz.
I'm building a PA based on the HMC408 and I have to match a 4 ohm output (with maybe a slight imaginary component) with a 50 ohm connector. Mind you, I'm at 5.8 ghz so there is probably little room for slop. I thought I'd use my slotted line to help me measure the impedance to make sure it will work.
My test setup looks like this:
Signal generator---> Slotted line ---> Output connector on the PA board (what I'm working on)
I replaced the MMIC with a 50 ohm resistor on the PCB, and took some measurements with the slotted line. Don't get too hung up on this though. I am easily able to measure SWR (to draw the circle on the smith chart) and see the phase shift from my changes relative to a short I calibrate with. Things move around on the smith chart MOSTLY as I'd expect.
The signal path on the board looks like this :
1)SMA connector
2)50 ohm TX line
3)Shunt capacitor (1.3pf)
4)Inductor (.4nh)
5)50 ohm TX line
6)PA chip (50 ohm resistor at this point)
I have a program called "smith tool" that lets you drive around the smith chart by inserting components into a matching network. If I "drive around" from the center of the chart starting at 50 ohms and part #1 above, I can get to my desired impedance with the above list. The final inductor and TX line rotates clockwise around the chart to hit the desired impedance.
I'm finding that in real life, that final inductor and even the txline has NO EFFECT on the final result. Almost as if it doesn't count. This has been verified as the behavior by not only my slotted line measurements, but by a software program I use called "ansoft designer", if I try to plot S11 on the smith chart on the same network. I can boost the inductor to 1 or 2 nh and still no change in the measured impedance.
In other words, that final txline should be adding a phase delay, and rotating the measured impedance. It doesn't.
Clearly I am missing something here. Is this a valid way to measure my matching network to make sure it's "tuned", and why are these inductors/transmission lines having no effect on the measured S11?
I'm hopeful I can avoid buying a VNA. And again, I don't think this is the problem because the simulator shows the same deal. Try it yourself: Port 1 goes to a shunt 3pf cap. Then through an inductor. Then through a txline. Then to port2, a 50 ohm port. You'll notice that S11 doesn't change as you'd expect if you play with the inductor/txline.
Maybe I am not doing this right...