GSM850 band update

Mychaela Falconia mychaela.falconia at
Fri Jan 5 02:34:16 UTC 2018

Hello FC community,

Today I picked up the 850 MHz FreeRunner which I bought just before
New Year and started examining it.  First I read out its Calypso NOR
flash and examined its FFS - the factory calibration performed by OM
looks the way I expected it to, just like the more common tri900
version, but with the 850 MHz band calibrated and the 900 MHz band
left uncalibrated, i.e., the opposite of the tri900 config.  The Rx
and Tx calchan subbands are exactly the same as what our own
fc-rfcal-tools create, based on TI's LoCosto calibration document.

The OM-calibrated GMagic number for the GSM850 band is 202 - it means
that the GSM850 band Rx path is no worse than its 900 MHz sibling on
the tri900 version in terms of the insertion loss.

Then I disassembled the case, extracted the motherboard+display set,
then removed the WLAN daughterboard and the shieldcan cover over the
GSM section.  At first visual inspection everything looks the same as
in the more common tri900 version: 3 Rx band SAW filters after the
antenna switch that look the same to the naked eye, and the population
of the "RF magic glue" components between the SAW filters and the Rita
transceiver's LNA inputs is also the same as in the tri900 version:
series inductors at L401-L406, while the shunt capacitor footprints
C405-C410 are left unpopulated just like in tri900.

The following remain to be determined:

* The main part: some time in the next few days I will take this
  deshielded tri850 GTA02 motherboard to the hardware lab at my day
  job and use the powerful microscope there to read the part number
  marking on what should be the GSM850 downlink band SAW filter at

* The series inductors at L401 and L402: are they the same 7.5 nH
  value as in the tri900 version, or different?

Depending on what the SAW filter part number turns out to be, we may
or may not need to measure the L401 and L402 values on this OM tri850
specimen.  If the SAW filter is a B7845 or equivalent in terms of the
expected output impedance like I suspect it to be (matching B7820 in
the tri900 version), then the simplest course of action will be to buy
some B7845 parts from the same wonderful Chinese folks from whom I got
all of our current Calypso+RF components, put one of them on an
FCDEV3B, and measure the resulting Rx path performance with the
CMU200 - will we get the same good GMagic as OM's without changing
L401 and L402, or not?

We shall know more as soon as I get an opportunity to look at the
deshielded tri850 GTA02 MB under a good microscope - stay tuned!

Hasta la Victoria, Siempre,
Mychaela aka The Mother

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