[USRP-users] ADC + DAC Compare of N200 and B200

Derek Kozel derek.kozel at ettus.com
Fri Oct 13 06:02:20 EDT 2017


Hello Kevin,

No, the N200 and B200 do not change the electrical transport modes based on
the frequency or bandwidth requested by the application, there is no need.
The 12 and 14 digital bits are available at all frequencies. The actual
effective number of bits out to the host depends on the ADC performance and
the decimation in the FPGA DSP.

I have not looked in depth at the design decisions that went into selecting
the interfaces but I'm confident that the hardware designers verified the
signal integrity.

You are correct about the use of the two output DAC. For most
daughterboards they are used for I and Q. For the others such as the LFRX
and LFTX the two channels can be used either as a complex pair or two real
valued channels.

Regards,
Derek



On Fri, Oct 13, 2017 at 5:21 AM, Kevin McGuire via USRP-users <
usrp-users at lists.ettus.com> wrote:

> This is connected to me investigating power level but the question is
> specifically about the number of bits per RX and TX channel between the
> N200 and B200. However, I fell into the rabbit hole that Alice went into
> and I seem to be stuck for the moment in determining what I am missing.
>
> I looked at the datasheet for the B200 and the AD9361. The AD9361 is
> advertised as using a 12-bit ADC and DAC. However, I see that it can
> operate in a 6-bit mode using the inputs as differentials - i suspect that
> is useful at ever higher frequencies. I counted 24 data pins.
>
> Now, the N200 uses this ADS62P42 which is a dual channel ADC. When looking
> at the datasheet it has 14 output ports per channel and two channels. I
> think this chip can also run in LVDS (differential) or CMOS (single-ended)
> modes. I am guessing once again LVDS gives only 7 effective bits per
> channel and CMOS gives 14 effective bits per channel.
>
> It seems that the AD9361 has two data ports where each port uses 12 of the
> 24 pins. Each of the data port pins can be an input or output. And, from
> what I can read it seems like these data ports can be combined in various
> ways, such as:
>
> 12-bit RX channel + 12-bit TX channel [FDD]
>    I = 6-bit and Q = 6-bit (single data rate)
> 24-bit RX channel + 24-bit TX channel [TDD]
>    I = 12-bit and Q = 12-bit (single data rate)
> 12-bit RX1 + 12-bit RX2 + 12-bit TX1 + 12-bit TX2 [FDD]
>    I = 6-bit and Q = 6-bit (double data rate)
> 12-bit RX1 + 12-bit RX2 + 12-bit TX1 + 12-bit TX2 [TDD]
>    We can have only two RX or TX active at the same time. (double data
> rate)
>
> Is this correct? And, which modes are actually used?
>
> The N200 I believe I have a handle on because it appears more straight
> forward. I can see how the RX provides two independent DAC outputs and each
> are 14-bit. If I have a handle on it then one should be the in-phase and
> the other the quadrature or two separate in-phase depending on the daughter
> board.
>
> Then, I also wonder, does the FPGA/firmware switch between CMOS and LVDS
> on either the B200 or the N200 in order to improve quality at higher
> frequencies?
>
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>
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