EMRFD Message Archive 12309
Message Date From Subject 12309 2016-02-09 08:12:09 mosaicmerc Microstrip Impedance question Hi all, I have an RF detector for AGC leveling for the Tek SG504 sig. gen (1Ghz).
If you look at page 11, note 4 you'll see the microstrip transmission line is several hundred ohms of impedance.
Why would this be for a 50 ohm detector?
The author does not know. I asked.
12318 2016-02-09 13:22:08 Andy Re: Microstrip Impedance question Where does it say several hundred ohms?How likely is it to even achieve several hundred ohms in microstrip over common materials?And what are the lengths compared to 1GHz wavelength?I see a little zig-zag trace on page 5 which makes me think some sort of delay or impedance compensation, maybe to offset the capacitance of the solder pads? (just a guess)Andy 12321 2016-02-09 13:34:50 mosaicmerc Re: Microstrip Impedance question Have a look at Note 4 on page 11. Add up the impedance it clears 300 ohms in that 'squiggle' . I verified the calcs based on the dimensions and the ε of FR4 @ 4.2 and Alumina @ 9. 12323 2016-02-09 15:41:19 dixonglennb Re: Microstrip Impedance question I don't think you can add the impedances of those short sections together in that manner. Think of different impedance coaxial cables, all of length much less than a wavelength, spliced in series. The impedance transforms according to the various lengths and impedances, but the impedances do not add. Also, the traces are close enough together that the calculated impedances would seem wrong to me due to mutual coupling.
I suspect the total network was experimentally developed (by someone very clever) to make the input and output impedances and overall response flat over the entire operating range.
12324 2016-02-09 16:08:00 dixonglennb Re: Microstrip Impedance question Also, the ground plane seems to be removed both around and under the network, so this is not a microstrip and the calculations are probably not valid. This pattern is often used on PCBs for time delay, but usually with a ground plane under it and much longer traces. If I had to guess I would say perhaps a frequency varying inductance, though I don't know how that would enhance the frequency response.
12325 2016-02-09 18:39:56 Andy Re: Microstrip Impedance question "Add up the impedance it clears 300 ohms in that 'squiggle' "I don't think those are DC resistances. I sure hope not! Unless that squiggle is something like nichrome.I think they are supposed to represent characteristic impedances. You don't add those to one another.Andy 12327 2016-02-09 19:31:25 Ed Manuel Re: Microstrip Impedance question Think it's an RF choke.Ed 12328 2016-02-09 20:02:51 augustinetez Re: Microstrip Impedance question Looking at the pics in the pdf, that is not a microstripline but an etched inductor.Possibly a filter of some sort. You see this a lot in commercial gear with high speed digital lines mixed with analogue stuff.A microstripline will have (most commonly on double-sided pcb's)) a straight track of constant width, with a ground plane either side of it, the spacing from the track will be about the same as the width of the track and a ground plane under it.(Multi layer boards have the stripline sandwiched between layers).Terry VK5TM 12331 2016-02-10 03:30:00 mosaicmerc Re: Microstrip Impedance question I have the PCB files as well...the 'inductor' is a microstrip it sits over the ground bridge between the two spaces in the ground plane.
So this is like 4 chunks of 'different' impedance coax of a certain length to create both stepped VSWR and phase delay?
12333 2016-02-10 10:05:33 mosaicmerc Re: Microstrip Impedance question It's some kind of low pass filter it seems. Based on this paper.
12334 2016-02-10 13:03:43 dixonglennb Re: Microstrip Impedance question The feature sizes seem much, much too small to have any effect at 1 GHz and below, or even 10 GHz. 1 GHz wavelength is 30 cm. These features seem to be less than 2-3 mm.
Also, you may wish to double-check the PCB layout...a missing ground plane can be seen under the trace in the photos, and the back photo's rectangular keepout is right where the trace is (note the nearby vias).
12336 2016-02-10 19:41:26 Andy Re: Microstrip Impedance question On closer examination, I think that rectangular void in the ground plane is NOT under the squiggle trace. The squiggle trace is much closer to the center line with the RF connector J2, than to the row of vias. I think it goes over the piece of ground plane between the two voids. The larger, rectangular ground plane void appears to be under R1/L1/C1/R2. That makes sense.In the top-side view, what looks like a void is actually a rectangular opening in the green soldermask, exposing the bare PCB. I guess the designer didn't want any solder mask over his squiggle trace.As to whether it is a low-pass filter or something else, who knows? Inductors are found in low-pass filters, and they are found in impedance compensation networks. It could be either. Or something else. It doesn't look right to me for an inductor; it doesn't spiral.I wonder if the designer wanted to match the propagation delay of the original head? In the photo of the original, the trace looks longer. Also note the varying trace widths in the original head. So I'm thinking he copied the original trace widths (or calculated impedances), and kept the same total length of trace but 'accordioned' it to make it fit into the shorter space between J1 and J2. That's my current thinking.Andy 12337 2016-02-10 23:52:39 czelusniakd Re: Microstrip Impedance question
It is called a Meander Line Inductor, and is used to reduce the area occupied by the element.
They have lower inductance and lower SRF (self-resonant frequency) than spiral inductors.
12339 2016-02-11 05:35:40 dixonglennb Re: Microstrip Impedance question Yes, I can see that now. Thanks.