Radiometrix Ltd

MTX2 Data Sheet

page 5

Channel Programming

It is useful at this point to describe how an MTX2 defines it's operating frequency:

At the heart of the device is a fractional N synthesizer locked to a high stability VCXO. The minimum step

size of this PLL is (approximately) 12.4Hz

The data required by the PLL consists of two coefficients: the integer and the fraction. Output frequency

relates to these values thus:

Output frequency (in MHz) = 6.5 x (integer + (fraction / 2^19))

For correct operation, the component (fraction / 2^19) must have a value between 1 and 2 (in other words,

the "fraction" coefficient varies between 524288 and 1048576) while the value of "integer" is limited by the

operating frequency range of the device (422.5 to 525MHz) so will lie between 64 and 79

So how do you calculate it by hand ?

Take the frequency, divide it by 6.5

Take the whole number part remaining, subtract one from it, and that's "integer"

Take everything right of the decimal point, add one and multiply by 524288, and that's "fraction"

Example: 430MHz divide by 6.5 equals 66.15384615

so "integer" = 65

and fraction = (0.15384615 +1) x 524288 = 604948

and to confirm: 6.5 x (65+(604948/524288)) does indeed equal 430MHz (plus 3.9Hz)

In interface terms, these coefficients are expressed as a 32 bit binary word (eight hexadecimal digits) where

the most significant byte comprises the integer value, and the remaining three bytes (24 bits) make up

"fraction"

When programming the MTX2, keep in mind that the unit maintains (in RAM) the current values of all

programmable values (current frequency, band of operation, RF power and frequency offset adjustments

and the sequential table values) and that toggling the EN pin does NOT erase or corrupt them.

These values are only loaded from eeprom at cold start power-up (but not when the EN pin is cycled) or

when the unit is commanded to execute eeprom reads by certain serial functions (such as the

"@PRG_00000000" command)

There are no (or very few) "write a value to eeprom" commands. It is usually necessary to load the relevant

current operating RAM value(s) and THEN issue a suitable command to write the RAM value to eeprom.

The MTX2 eeprom stores a set of frequency coefficients for each parallel channel plus a fifth set for the

sequential table start. It also stores power level, frequency offset, sequential table step size and sequential

channel, and the band select constant

Programming a value or coefficient over the serial bus over-writes the previous value and implements this

change on the PLL immediately, but does not change the eeprom contents until a relevant "program eeprom"

command is issued

In general, the most recent stimulus received by the unit will decide the operating frequency and mode

(although if sequential mode has been selected (serial channel not equal to zero) a change to the parallel

port or a direct frequency program command will not initiate a frequency change).