GRAYCHIP, INC.

- 7 -

JUNE 22, 1998

GC3021 MIXER AND CARRIER REMOVAL CHIP

DATA SHEET

This document contains information which may be changed at any time without notice

permits the user to capture the high speed mode’s even outputs in the snapshot RAM, or to use them in the

phase error lookup circuit.

DC components before or after the mixer can be removed by adjusting the DC_I_IN, DC_Q_IN,

DC_I_OUT and the DC_Q_OUT values. These values are added to the input and output data as shown in

Figure 5. The outputs from these adders are saturated to plus or minus full scale (12 bits) if overflow is

detected.

2.6

SYMBOL ALIGN

The symbol align circuit is used in the carrier removal mode to process offset (or staggered) QPSK

signals. The offset is removed by delaying the SI sample by one clock cycle so that it is paired with the next

SQ sample. Every other SI and SQ pair is held for two clock cycles, effectively decimating the sample rate

by two. The symbol offset circuit is controlled by the OFFSET and OFFSET_HOLD control bits described

in Section 4.1 The symbol offset circuit is synchronized by the OFFSET_SYNC described in Section 4.2.

2.7

PHASE ERROR RAM

The the I and Q samples are passed to the phase error RAM. The phase error RAM uses the upper

7 bits of the I and Q values to look up the sign of the phase error for the sample. The phase error RAM

contents are downloaded through the control interface. The phase error RAM outputs a 1 or a 0 depending

upon whether the phase angle of the (I, Q) complex pair is greater than or smaller than the nearest decision

point for the signal’s constellation pattern. A “0” means that the phase angle needs to be increased to match

the decision point and a “1” means it needs to be decreased. A block diagram of the phase error RAM circuit

is shown in Figure 6.

Figure 6. PHASE ERROR RAM

The round circuit rounds the 12 bit I and Q samples into the upper 6 or 7 bits, or, if rounding is

disabled, truncates them. If the quadrant map (Qmap) mode is enabled the circuit rounds to 7 bits, otherwise

it rounds to 6 bits. The rounding is performed using the round to even technique.

2. The input samples of QAM signals have an extra sign bit before the carrier is removed. The extra sign bit is needed because the

square QAM constellation is spinning. Once carrier has been removed the extra sign bit can be removed by the gain circuit. This

increases the resolution of the phase error RAM and simplifies the symbol mapping.

ROUND

UNSIGNED

MAGNITUDE

Q

QMAP_ROUND

Enable

6/7 Bits

Enable

Mode

QMAP_INVERT

12 Bits

7 Bits

6 Bits

sign

QMAP_ENABLE

I

12 Bits

6 Bits

sign

2 MSBs

4 LSBs

256 BY 16

RAM

16 TO 1

MUX

16 Bits

R[0:1]

R[2:7]

C[0:15]

A[0:7]

A8*CS*R/W

W[0:7]

WE

PHASE

ERROR

From

Symbol

Offset

QMAP_ENABLE

ROUND

UNSIGNED

MAGNITUDE

Enable

6/7 Bits

Enable

Mode

7 Bits