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From: | Matt Ettus |
Subject: | Re: [Discuss-gnuradio] USRP and RFX2400 block diagrams |
Date: | Wed, 30 Aug 2006 11:15:09 -0700 |
User-agent: | Mozilla Thunderbird 1.0.8-1.1.fc4 (X11/20060501) |
Here are a couple of tweaks: The digital down-conversion should show a single complex multiplier instead of two real multipliers.Thanks. I'll make the change. A complex multiplier is just a more efficient digital version of what I've drawn, right?
Not exactly. What you show in the diagram is that Iout is a function of only Iin, and the same for Q. In reality Iout is a function of both Iin AND Qin. The same goes for Qout.
Think of it as a complex multiply -- (a + bi) * (c + di) = (ac-bd) + (ad+bc)i There are actually 4 multiplications happening.We implement this all in a CORDIC structure, which is much more area efficient than using 4 multiplies. In fact, it is smaller than a single multiply.
Yes. This is actally one of the major differences between the upconverter path and the downconverter path--Same comment on the digital up-conversion. We've got "Block D" enabled in the AD9862, thus it's functioning as a complex multiplier.Okay. Out of curiosity, is "Block B" enabled too?
In the downconverter (in the FPGA), we run the complex multiply at the full sample rate.
In the upconverter (in the AD9862, from Figure 3 in the 9862 docs), they split the complex multiply into 2 parts -- coarse and fine. The fine part (block D) runs at 1/4 of the sample rate. This means that it can only move the freqency 1/4th as far. The block is then followed by the 4x interpolation (Block C), and then the coarse modulation, Block B. Block B only moves the signal +/- fs/4 or fs/8.
Matt
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