Preliminary Technical Data
UG-1828
Rev. PrC | Page 185 of 338
In a communication system, a desired signal is transmitted by the transmitter at RF over the air. Since the clock reference at the
transmitter and the receiver are independent, this may result in the RF carrier frequency offset between the transmitter and the receiver.
This frequency difference is named by the carrier frequency offset (CFO). The CFC in rxnbdem enables the BBIC to remove the CFO
before the channel selection filtering (Rx PFIR) at the receiver side. The correction value applied to the CFC, must be estimated and
further input by the BBIC. The change of the correction value may occur immediately or relative to RX frame boundary.
The CFC is implemented as a Digital Down Converter (DDC), which consists of an NCO and a complex multiplier in the datapath. As
the correction value, the NCO frequency word should be in the range of min (±20k,20% of sample rate).
Figure 170. Output Spectrum of the CFC/NCO as f
S
= 61.44 MHz
Figure 170 presents the spectrum of desired tone and the generated NCO spurs levels relative to desired tone for the CFC NCO at
61.44 MHz sampling frequency. The outlined plots show a typical case (-20Mhz CW) and some worst cases. As shown in Figure 170, the
NCO output spurs are −100 dBc below desired tone across the range of [−f
S
/2, f
S
/2]:
•
−20 MHz CW,
•
𝑓𝑓𝑓𝑓
/16
CW,
•
𝑓𝑓𝑓𝑓
/8
CW, and
•
𝑓𝑓𝑓𝑓
/4
CW.
Please note these four tones’ frequency is not within the NCO range defined above, just for the NCO spurs level demonstration.
Rx Programmable FIR Filter
Rx Programmable FIR Filter in rxnbdem is multi-functional and customizable. This module can be bypassed.
The Rx Programmable FIR supports up to 128 taps. Each tap is 24 bits width with the signed bit included. 4 sets of customized FIR
profiles can be stored at the initialization phase. One of the 4 stored FIR profiles can be switched to be loaded on the fly under the control
of the BBIC.
0
–20
–40
–60
–100
–80
–120
–30
–20
–10
0
10
20
30
dBc
MHz
0
–20
–40
–60
–100
–80
–120
–30
–20
–10
0
10
20
30
dBc
MHz
–20MHz CW
Fs/16 CW
0
–20
–40
–60
–100
–80
–120
–30
–20
–10
0
10
20
30
dBc
MHz
0
–20
–40
–60
–100
–80
–120
–30
–20
–10
0
10
20
30
dBc
MHz
Fs/8 CW
Fs/4 CW
X = –22
Y = –2.388
24159-
128
