Electronics > RF, Microwave, Ham Radio

SAW filter design cascading SAW filters

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RFDx:

--- Quote from: GGMM on May 08, 2024, 03:07:04 pm ---I made a spectrum analyzer (SimpleSA) according to the design of M0Wid.
The analyzer can also do signal generator and also tracking.
The observation is that the spectrum analyzer works, but not the generator (and surely tracking).
--- End quote ---

Show the whole schematic.


--- Quote from: GGMM on May 08, 2024, 03:07:04 pm ---In fact, the assembly only works nice  if I shunt the 2 filters. (one by one has no effect).
So I wonder if the filter design is good.

Filter are ECPOS B3555
https://www.alldatasheet.com/datasheet-pdf/pdf/188917/EPCOS/B3555.html
!!!see the adaptation to 50ohm and how it's made on the Simplesa schematic. I thing strange  (?)
--- End quote ---

The matching network for the filter is 3.3p shunt-C + 39nH series-L. If you account for additional 1...2pF parasitic C (depending on PCB layout) in parallel to the filter input and output, you get the aproximate matching network (6.8pF + 33nH) shown in the datasheet.


--- Quote from: GGMM on May 08, 2024, 03:07:04 pm ---I wonder if waterfall design is a problem because it is different from the one I find on a Tinysa that uses the same principle.
Attached files the SimpleSA 433.92 filter and the rf part of TinySa, you can see also the two saw filter (Not completly the same)
--- End quote ---

If you cascade two filters directly they will affect each other. Some isolation between them, like a small 50 Ohm attenuator or an impedance inverter, would be preferable.

Gerhard_dk4xp:
For the B3555 filter, matching components to 50 Ohms are 33nH + 6p8 from the data sheet.
There are too many 50-ohm comments in the schematic and on the wrong places.
It may be worth a test to connect the 2 filters directly without filt1-> 50 ohms and 50 Ohms->
filter2. OTOH the LC should add selection and the high impedance may radiate.


I had 2 400 MHz filters back to back in a clock multiplier 100-> 400 MHz. That kinda worked,
but the far-off suppression was less than I had hoped for. No real improvement to 1 filter.
maybe the 2 filters were too close, mechanically,   or better shielding required.

I also tried 900 MHz filters by Yageo and got completely irreproducible results.
Maybe I burnt them with the hot air.  I should retry with vapor phase soldering.

GGMM:
Hello  Gerhard,

Thank you for your reply.
I tested one of the inductance and one of the capacitor of the bags of pieces received.
The "Chinese" tester is not able to read a very low inductance of 33 NH and rather gives a resistance of less than one ohm.
For the capacitor of 6.8PF, it is the same does not manage to detect it, which proves at least that its value must be low.

I can do a 33nh/ 6.8pf resonant circuit and see at nanovna.

Anyway, I agree with you, it’s so small to weld the filter , only hot air works. I don’t know if they can be damaged, it’s more "mechanical", but I received 20 for a few euros. So I can try to change them.
cdt

GGMM:

hello,

I came across this interesting conversation about SAW filters.
There may be things to see about the adaptation for the input, the output and the connection between the 2 SAW filters.
The theoretical scheme LC 33Nh/6.8Pf  for filter B3555 may need to be revised (?)
Interesting thing about the component layout
Same for calculations, but it’s not clear to have what it is done. We are talking about ADS space with S parameters files
I don’t know what that is.
some help on this subject.

see here:
https://www.edaboard.com/threads/inconsistent-impedance-matching-network-of-a-saw-filter.408321/

cdt

GGMM:
I think ADS is like an equivalent of LTspice.
Don't known is one is better than the other.
As I have never practiced these softs, I think it gets complicated.
In short you have maybe a ready model of SAW filter.
to try ?

cdt

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