[SI-LIST] Re: which one is better
- From: Javier del Valle <jvalle@xxxxxxxxxxxxxxxxxxxxxx>
- To: "'rajiv_gupta@xxxxxxxxxxxx'" <rajiv_gupta@xxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Fri, 8 Nov 2002 12:06:40 +0100
Hello,
I would choose the first one (50 Ohms microstrip line). I prefered not to
change of layer if I didn't need it in my previous experience as RF
engineer. Sometimes, the antenna connector is isolated itself from the rest
(the transceiver, etc.) through a land to GND in the top layer, so you need
to use strips. I have also seen some TDR measurements showing that when
going from ustrip to strip, the impedance does fall to ~30 - 35 Ohms (the
via) before recovering again to 50 Ohms in the stripline section. Infact, if
the stripline it too short going from ustrip-strip-ustrip can result in that
the stripline section does not recover to 50 ohms producing quite mismatch
(only if the strip is too short). I would also consider, instead of the pi
network, if the transceiver is internally matched to 50 Ohms, a series
capacitor + an inductor to GND + another series capacitor (high pass
filter). The idea is to compensate the capacitance of the antenna connector
(matching purpose) and also provide some ESD protection. Anyway, it is my
personal opinion and it would be better you try to simulate and measure the
different options (you could also open some mobile phones and see the
connection between the Transceiver part and the antenna connector ;-) ).
I hope it helps,
Best regards,
Javier
-----Mensaje original-----
De: Rajiv Kumar Gupta [mailto:rajiv_gupta@xxxxxxxxxxxx]
Enviado el: jueves 7 de noviembre de 2002 7:08
Para: si-list@xxxxxxxxxxxxx
CC: Subrata Saha
Asunto: [SI-LIST] which one is better
Hello
I am working as Sr. RF Design Engineer at MindTree consulting Pvt. Ltd,
Bangalore India
At present I am working on design of Bluetooth (2.4GHz -2.5GHz) products viz
PDA, Headset,Lan access Point
I just want to clarify one issue related to transmission line:
I am considering two alternatives w. r.to transmission line between antena
and transceiver for a Bluetooth Headset,
I just want to know which one is better
1.
A 50 ohm microstrip line of about 10 mm in the top layer with a pi-
matching
network between antenna and Transceiver.
2.
Antenna
- 50 ohmmicrostip line of about 1-2 mm with pi matcing network in top layer
(as discrete components for pi- matching network can be placed in the top
layer)
then a via
then 50 ohm strip line of about 8mm in layer 3 with ground plane in layer 2
&
layer 4 above and below the 50 ohm strip line
then a via
- microstrip line 1-2 mm
and then Transceiver.
Secondly I read some gidelines about RF area from SiliconWave which is as
follows:
Routing of the RF Output Path
(a) It is important to keep the RF path from the Transceiver RF
port to the antenna port at a minimum distance to mini-mize
the signal loss.
(b) In addition, because of component
solder pads around the match into the Transceiver, keep the
stray capacitance to a minimum by using small solder
pads and clearing out the ground copper on all layers di-rectly
beneath the pads. Solder pads for the bandpass fil-ter
should also be kept as small as possible with all
copper removed from beneath these pads.
Since pi- matching network (discrete inductor -capacitor-inductor) are on
the
microstrip transmission line itself.
and microstrip line requires a ground beneath the microstrip line.
so part (b) will effect the very structure of microstrip line
and I feel that it will cause more reflection due to impedance mismatch.
Please comment.
Regards & thanks
Rajiv
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