
VITA-49 Radio Transport Protocol
ordered in the identical order of their occurrence in the Context Indicator Field.
See
for the definition and associated value of each field.
Table 17:
Digitizer Context Field Values
Bit Name
Context Field
# of Words in Field
Period of Validity
I
Context Field Change Indicator
0
N/A
B
Bandwidth
2
Persistent
O
RF Frequency Offset
2
Persistent
R
Reference Level
1
Persistent
Context Field Change Indicator
The Context Field Change Indicator is used to indicate when some context value of the
system has changed. One or more of the other bits in the indicator field will be also set,
indicating which values have been changed and have their updated values in the context
fields that follow. It is possible that a context packet may be sent where the Context Field
Change Indicator is set to 0, indicating that no change has occurred.
Bandwidth
The bandwidth is used to indicate that the amount of spectrum that is currently viewable
due to decimation settings that have been enabled.
The Bandwidth field shall use the 64-bit, two’s-complement format as shown in
This field has an integer and a fractional part, with the radix point to the right of bit 20 in
the second 32-bit word. This gives the RF Reference Frequency a range of ±8.79THz
with a resolution of 0.95μHz.
Table 18:
Bandwidth Word Format
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
Integer Bandwidth (43..12), Hz
Integer Bandwidth (11..0), Hz
Fractional Bandwidth (19..0)
Reference Level
The Reference Level indicator provides a power level reference so that the magnitude of
the received data can be calculated by a user. The reference level provided in the context
packet is adjusted according to the RFE's gain setting. However, the reference levels
must also be adjusted accordingly relative to the antenna (the Reference Point) and to
any other conditions.
The absolute power level P (in dBm) is then computed using the following formula:
P
=
R
+
20
∗
log
(
IQ
measured
)
with
IQ
measured
=
√
(
I
fft
2
+
Q
fft
2
)
where:
R
= the reference level provided in the VRT context packet, dBm
ThinkRF WSA5000 Wireless Signal Analyzer Programmer's Guide
33