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Re: [Discuss-gnuradio] USRP and RFX2400 block diagrams


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.

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?
Yes. This is actally one of the major differences between the upconverter path and the downconverter path--

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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