GSM850 band update

Mychaela Falconia mychaela.falconia at
Fri Jan 5 21:58:19 UTC 2018

Follow-up on the examination of the Openmoko-made GTA02 unit of the
rare tri850 variety: inspection under the good microscope has confirmed
that the GSM850 DL band SAW filter part number is indeed B7845 like I
suspected.  The DCS and PCS band SAW filters are unchanged from the
more common tri900 version: B7821 for DCS and B7825 for PCS, only the
low band SAW filter changes: B7820 for tri900 or B7845 for tri850.

The next steps will be as I've been planning earlier: I am going to
buy some B7845 parts from my trusty Chinese supplier and populate one
of them on an otherwise unchanged FCDEV3B in the place of the current
B7820.  Then test it on the CMU200 and see if we get good GMagic
(meaning no extra insertion loss in that Rx path) without changing
L401 and L402 inductor values.

It will probably be another 2-3 months before that experiment, though,
as I plan on sending our CMU200 to R&S Maryland for recalibration
hopefully soon, and the B7845 on FCDEV3B experiment will probably be
after the instrument comes back from this recalibration.

It should also be noted that in the case of the PCS band (independent
of the tri900 vs. tri850 config) we've been using a slightly newer
Epcos SAW filter part than OM did: OM's PCS band SAW filter is B7825,
whereas we've been using the slightly newer B7851.  Comparison of the
two datasheets shows no difference except slightly improved insertion
loss and VSWR specs for the newer part, and all of the CMU200 RF
calibration runs we've done on our FCDEV3B boards confirm that our
choice of part performs just as well as OM's original.

Historical context: Openmoko's schematics show bogus part numbers for
all 3 SAW filters (NOT matching what they actually populated on their
GTA02 boards, neither tri900 nor tri850), and when I was just starting
my Calypso hw building aspirations all those years ago, the most
readily available source of knowledge was an OsmocomBB wiki page about
the Pirelli DP-L10 (also a tri900 phone) which listed these SAW filter
part numbers:

B7820 for EGSM downlink
B7821 for DCS DL
B7851 for PCS DL

Back then I looked these part numbers up on some datasheet search
website (don't remember which one), and found datasheets for all 3 -
hence these 3 SAW filter part numbers became my confident choice, and
ended up being used on our FCDEV3B boards.  Only this last December,
shortly before New Year, I finally looked at an Openmoko GTA02 MB (the
common tri900 version) under the good microscope and confirmed that
their EGSM and DCS band filters match ours (B7820 and B7821,
respectively), with only the PCS band filter being different as just
described.  Then I looked at Openmoko's BOM and the component data in
the PADS PCB file, and both of those documents list the actual parts
that they populated (B7820/B7821/B7825), and not the bogus numbers
visible on the PDF schematics.

Ahh the joys of hardware (reverse) engineering - but now we finally
have good sets of SAW filter part numbers (and the physical parts!)
for both tri900 and tri850 configs.

Hasta la Victoria, Siempre,
Mychaela aka The Mother

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