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FPGA

Probably understanding what goes on the USRP FPGA is the most important part for the GNU Radio users. As shown in the figure below, all the ADCs and DACs are connected to the FPGA. This piece of FPGA plays a key role in the USRP system. Basically what it does is to perform high bandwidth math, and to reduce the data rates to something you can squirt over USB2.0. The FPGA connects to a USB2 interface chip, the Cypress FX2. Everything (FPGA circuitry and USB Microcontroller) is programmable over the USB2 bus.

The standard FPGA configuration includes digital down converters (DDC) implemented with 4 stages cascaded integrator-comb (CIC) filters. CIC filters are very high-performance filters using only adders and delays. For spectral shaping and out of band signals rejection, there are also 31 tap halfband filters cascaded with the CIC filters to form complete DDC stage. The standard FPGA configuration implements 2 complete digital down converters (DDC). Also there is a configuration with 4 DDCs but without halfband filters, which allows 1, 2 or 4 separate RX channels.

In the 4 DDC implementation, in the RX path we have 4 ADCs, and 4 DDCs. Each DDC has two inputs I and Q. Each of the 4 ADCs can be routed to either of I or the Q input of any of the 4 DDCs. This allows for having multiple channels selected out of the same ADC sample stream.

The figure below shows the block diagram of the USRP digital down converter.

Now let's see the digital down converter (DDC). First, it down converts the signal from the IF band to the base band. Second, it decimates the signal so that the data rate can be adapted by the USB 2.0 and is reasonable for the computers' computing capability. The complex input signal (IF) is multiplied by the constant frequency (usually also the IF) exponential signal. The resulting signal is also complex and centered at 0. Then we decimate the signal with a factor N. The decimator can be treated as a low pass filter followed by a down sampler. Suppose the decimation factor is N. If we look at the digital spectrum, the low pass filter selects out the band [-Fs/N, Fs/N], and then the down sampler de-spread the spectrum from [-Fs, Fs] to [-Fs/N, Fs/N]. So in fact, we have narrowed the bandwidth of the digital signal of interest by a factor of N.

Regarding the bandwidth, we can sustain 32MB/sec across the USB. All samples sent over the USB interface are in 16-bit signed integers in IQ format, i.e. 16-bit I and 16-bit Q data (complex) which means 4 bytes per complex sample. This resulting in a (32MByte per sec/4Byte) 8Mega complex samples/sec across the USB. Since complex processing was used, this provides a maximum effective total spectral bandwidth of about 8MHz by Nyquist criteria. Of course we can select much narrower ranges by changing the decimation rate. For example, suppose we want to design an FM receiver. The bandwidth of a FM station is generally 200 kHz. So we can select the decimation factor to be 250. Then the data rate across the USB is 64MHz / 250 = 256 kHz, which is well suited for the 200 kHz bandwidth without losing any spectral information. The decimation rate must be in [8, 256]. Finally the complex I/Q signal enters the computer via the USB. That's the software world!

Note that when there are multiple channels (up to 4), the channels are interleaved. For example, with 4 channels, the sequence sent over the USB would be I0 Q0 I1 Q1 I2 Q2 I3 Q3 I0 Q0 I1 Q1, …etc. In multiple RX channels (1,2, or 4) , all input channels must be the same data rate (i.e. same decimation ratio).

At the TX path, the story is pretty much the same, except that it happens reversely. We need to send a baseband I/Q complex signal to the USRP board. The digital up converter (DUC) will interpolate the signal, up convert it to the IF band and finally send it through the DAC.

The digital up converters (DUC) on the transmit side are actually contained in the AD9862 CODEC chips, not in the FPGA (as shown in the figure below). The only transmit signal processing blocks in the FPGA are the CIC interpolators. The interpolator outputs can be routed to any of the 4 CODEC inputs.

In multiple TX channels (1 or 2) all output channels must be the same data rate (i.e. same interpolation ratio). Note that Tx rate may be different from the RX rate. The USRP can operate in full duplex mode. In this mode, transmit and receive sides are completely independent of one another. The only consideration is that the combined data rate over the bus must be 32 Megabytes per second or less.

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