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

Kevin McGuire kmcg3413 at gmail.com
Fri Oct 13 07:29:34 EDT 2017


Derek or anyone,

Do you know if it's a 12-bits complex on the b200 (6-bit I + 6-bit Q) or a
24-bit complex? I always thought it was a 24-bit complex but from what you
said and what I see in the schematics it is plausible things may not be as
they seem.

This is ultimately my question but I have a weirdness of going around the
world to ask it, haha. The exact precision used of the ADC before the FPGA
for comparison is what I am after - but was also after any details too.

Derek,

Your information is most helpful and id assisting me in getting closer to
understanding. I sincerely appreciate your help/wisdom and I am most
thankful for people like you (and others) who voluntarily take their time
to answer questions. I would know nothing if not for the help of people.

I also feel like a lazy dummy because I just realized I could have looked
at the schematic and seen how the, what appears to be LVDS enabling pins,
are configured/connected.

On Oct 13, 2017 5:02 AM, "Derek Kozel" <derek.kozel at ettus.com> wrote:

> 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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