GC5016 Datasheet, PDF (15/88 Pages) Texas Instruments – WIDEBAND QUAD DIGITAL DOWN CONVERTER/ UP CONVERTER

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13.2.4 Receiver Desensitizing
GC5016
SLWS142G − JANUARY 2003 − REVISED NOVEMBER 2005
In a few circumstances, it is necessary to reduce the receiver sensitivity, which can be done by adding noise to the
signal. The GC5016 allows this to be done digitally by adding pseudo random noise to selected bits in the input data
stream. The noise power is added by bit wise xoring the input data stream with a Pseudo-random Noise (PN)
sequence. The user has control over the noise power by programming which bits get the noise added. The noise
power can go from −3 dBFS (0xffff) to −99 dbFS (0x1). This is programmed using rcv_noise_A (or B, C, or D). The
noise uses the PN generator that is also used for diagnostics. The generator must be enabled for this feature to work
by setting cksum _sync.front to 0.
13.3 Receiver Diagnostic Selection
The Receiver RINF can select the counter (ramp), zero, a constant, or the PN sequence as the DDC channel real
input. The 0x4000 constant is used with the NCO setting to generate a known complex tone for output testing. The
rinf_sel and rinf_diag controls are used to select a diagnostic input for a DDC channel. See the Diagnostics section.
13.4 Receive Input Selection
In each channel an input selector exists at the input to the mixer. This selects I and Q data from one of four receive
input formatters. The field mix_rcv_sel allows selection of the rinf bus. Full rate real or 1/2 rate complex inputs are
selected with the mix_rcv_sel value as the input port. Special mix_rcv_sel values are needed for full rate complex,
and double rate processing. See Table 1.
13.5 Mixer
The DDC application of the mixer uses the selected RINF and RSEL with the NCO sine and cosine values. The Mixer
equations are:
Imixout = Iin × cos(Phase_NCO) − Qin × sin(Phase_NCO)
Qmixout = Qin × cos(Phase_NCO) + In × sin(Phase_NCO)
Each of the four multipliers (I x cos, I x sin, Q x cos, Q x sin) can be programmed in one of four modes (off, receive,
cross transmit, normal transmit) (see Figure 5). A programmable inversion is provided for each I or Q data source.
Programming Q x sin to be inverted corresponds to a mathematical view of down-conversion (mix with negative
frequency tone to get a positive spectrum). Programming I x sin to be inverted corresponds to a radio view (tune to
a frequency to get the signal at that frequency). The fields involved are mix_icos, mix_isin, mix_qcos, mix_qsin, and
mix_inv_icos, mix_inv_isin, mix_inv_qcos, and mix_inv_qsin. The cmd5016 software automatically programs these
fields assuming a mathematical view.
Selected RINF and RSEL data is accepted into the mixer as 16-bit data, placed into the upper bits of an 18-bit word,
and inverted if programmed. The 18bit input is multiplied by a 20-bit NCO word, summed with the output of a second
multiplier creating a 21-bit output. The Mixer output in the DDC application is sent to the CIC.
This means there is a 6dB attenuation going through the mixer. In other words, there is a 1-bit growth on top to allow
for the extreme case of both real and imaginary inputs at full scale being multiplied by an NCO word that is at 45
degrees. For real inputs, the attenuation is 6 dB, so the CIC can safely be programmed to have 6-dB gain. For
complex inputs, the attenuation is 3dB peak. The cmd5016 software includes this attenuation in its gain calculations
when gain is set using the overall-gain keyword.
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