Electronics > RF, Microwave, Ham Radio
Test points/connectors for matching
okw:
I am reading and reading about matching circuits between equipment and antenna, but can't say I become more confident. Maybe some of you gurus here can help me?
I want to match a GPS module with an active antenna, with a SAW filter (SAFFB1G56KB0F0A) in-between them. I already had a few prototype boards made, and have a big service kit with both capacitors (0402) and inductors (0402), so I'm ready to experiment. In my prototype design, I put a pi-filter between the SAW filter and receiver, and a combo pi/T filter between the SAW and antenna (ANT1 IPEX connector). Don't mind the L/C choice and values, they are just all 0402 placeholders (I didn't know if pi or T makes most sense for a GPS, and with which base/starting values).
My question is, where do you guys place test points, connectors, etc. for matching? Both for prototype and production versions?
And with RF switches, which orientation (to break the circuit) is correct at which placement?
I placed two RF test switches (MM8030-2610) along with an IPEX connector (ANT2) near the receiver RF input. It's the first time I've used RF test switches, and they seem difficult to use without the appropriate probes ($$$).
I tried with my PS100 Antenna analyzer in the ANT2 connector (between receiver and SAW filter), and measured S11 with a really nice dip. -23.3dB @ 1512MHz, sadly not at the GPS frequency (1575MHz) where I read only -1.8dB. Only C1 (33pF) was in in place. I added L1/L2 = 2.2nH, and this moved the center a bit, now it's -40dB @ 1512MHz, -1.89@1575MHz.
First, would the PS100 be appropriate to measure like this (not antenna as end-point, but rather in the middle of to devices)? And secondly, is this point good for measuring between the saw and receiver? What about between the SAW and antenna?
Should I also be worried about the output level of the PS100, considering the GPS receiver is probably very sensitive (not blowing the input stage or something)?
I'm considering buying a NanoVNA, or even an old lab analyzer, e.g. HP 8753.
Any help is greatly appreciated.
yl3akb:
You should reduce the length of Your RF cascade to find where the offset in resonance frequency originates.
1. Measure only the antenna + bias-T. Did You power up the active antenna when measuring?
2. Measure only the SAW filter, but with 47..50Ohm resistor at its other side (Resistor at antenna side for example in place of L3 and with C2 open).
That SAW filter should not need additional matching to 50 Ohms.
That ANT2 connector and 'GPS MODULE RF IN' label is confusing. Is there receiver connected in parallell with ANT2 connector?
That antenna analyzer should be okay for reflection coefficient measurement like this.
okw:
Thanks for your reply.
--- Quote from: yl3akb on May 16, 2023, 05:26:27 pm ---You should reduce the length of Your RF cascade to find where the offset in resonance frequency originates.
--- End quote ---
Keep in mind I'm completely new to this :) How do I reduce it?
--- Quote from: yl3akb on May 16, 2023, 05:26:27 pm ---1. Measure only the antenna + bias-T.
--- End quote ---
Bias T, would that be to remove everything except C2/C3/L4 (and L5/C4 to power the antenna of course)?
--- Quote from: yl3akb on May 16, 2023, 05:26:27 pm ---Did You power up the active antenna when measuring?
--- End quote ---
I didn't. I'll do new measurements with power.
Should I change values, remove, or replace any components first?
I have 10 fresh boards with C1/C2/C3 (33pF), L5 (68nH - not 47 like the image) and saw filter assembled. Everything else, including the gps module, isn't assembled yet (I have full RCL 0402 service kits).
--- Quote from: yl3akb on May 16, 2023, 05:26:27 pm ---2. Measure only the SAW filter, but with 47..50Ohm resistor at its other side (Resistor at antenna side for example in place of L3 and with C2 open).
--- End quote ---
This would be to measure in the ANT2 (left most) connector? Or solder into any other point?
--- Quote from: yl3akb on May 16, 2023, 05:26:27 pm ---That SAW filter should not need additional matching to 50 Ohms.
--- End quote ---
The datasheet says it's unbalanced. Doesn't that mean I need to match it to 50ohms? On both sides? Or isn't a saw filter looked at like a device (like an LNA, which needs matching on both sides(?).
--- Quote from: yl3akb on May 16, 2023, 05:26:27 pm ---That ANT2 connector and 'GPS MODULE RF IN' label is confusing. Is there receiver connected in parallell with ANT2 connector?
--- End quote ---
Yes, I know. GPS MODULE RF IN indicates the receiver input (arrow should be oriented the other way). ANT2 connector was only placed there to have a snap-in connector for the antenna analyzer at that point.
yl3akb:
--- Quote from: okw on May 16, 2023, 06:49:39 pm ---Keep in mind I'm completely new to this :) How do I reduce it?
--- End quote ---
By reducing I mean first verify each sub-block separately and then measure the whole cascade
--- Quote from: okw on May 16, 2023, 06:49:39 pm ---Bias T, would that be to remove everything except C2/C3/L4 (and L5/C4 to power the antenna of course)?
--- End quote ---
Yes, leave C2, C3 and L5,C4, but leave L4 open
--- Quote from: okw on May 16, 2023, 06:49:39 pm ---I didn't. I'll do new measurements with power.
Should I change values, remove, or replace any components first?
--- End quote ---
I think first measure just the antenna with power applied through bias-T. Do not connect shunt inductors.
--- Quote from: okw on May 16, 2023, 06:49:39 pm ---This would be to measure in the ANT2 (left most) connector? Or solder into any other point?
--- End quote ---
Yes, measure at ANT2, but without GPS module connection of course.
--- Quote from: okw on May 16, 2023, 06:49:39 pm ---The datasheet says it's unbalanced. Doesn't that mean I need to match it to 50ohms? On both sides? Or isn't a saw filter looked at like a device (like an LNA, which needs matching on both sides(?).
--- End quote ---
The unbalanced is what You need here, it doest mean it needs matching. See 'measurement circuit' in datasheet, it says 50 Ohms. Also VSWR is stated ~1.4 which corresponds to good enough match.
--- Quote from: okw on May 16, 2023, 06:49:39 pm ---Yes, I know. GPS MODULE RF IN indicates the receiver input (arrow should be oriented the other way). ANT2 connector was only placed there to have a snap-in connector for the antenna analyzer at that point.
--- End quote ---
In general I would not connect RF connector parallel to transmission line like this, because it can act as open stub and cause mismatch. Probably at this frequency it does not matter, but depends how long is the stub.
okw:
Thanks!
Are the RF test switches (MM8030-2610) necessary for testing in mass production?
And where should I put them?
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