R&S CMU200 is the RF tester used for production RF calibration of FreeCalypso
GSM devices.  The CMU200 can be operated in three ways: manually via the front
panel, programmatically via GPIB and programmatically via SCPI commands over
RS-232.  GPIB is the industry standard, but for FreeCalypso the Mother has
adopted the RS-232 control interface method instead in order to avoid the
exotic hardware and equally exotic drivers and libraries required for GPIB:
using the RS-232 interface requires absolutely no special hardware or drivers
or libraries, just userspace C code without any dependencies talking to a
serial port just like we do when communicating with Calypso target serial ports.

Initialization commands
=======================

Our Test System Interface Daemon for the CMU200 will issue the following SCPI
commands to the instrument on start-up:

*SEC 0
*RST;*OPC?
SYST:NONV:DIS
SYST:REM:ADDR:SEC 1,"RF_NSig"
SYST:REM:ADDR:SEC 2,"GSM900MS_NSig"
SYST:REM:ADDR:SEC 3,"GSM1800MS_NSig"
SYST:REM:ADDR:SEC 4,"GSM1900MS_NSig"
SYST:REM:ADDR:SEC 5,"GSM850MS_NSig"

VCXO calibration
================

When commanded to prepare for VCXO calibration, our TSID will command the
CMU200 as follows:

*SEC 2
RFAN:CHAN 40CH
RFAN:TSEQ GSM5

Command to read frequency offset:

READ:MOD?