USRP2 External Clock

GNU Radio
1

I’m wondering if it is possible to connect the USRP2 to an external
100MHz clock like the USRP was able to be modified so it could connect
to an external 64MHz clock. I don’t see any place to solder on a SMA
connector but there are a set of pins labeled Test CLK. Could that be
used for inputting an external clock or is it just for testing the
clock output?

Thanks,
Kyle

2

On Wed, Oct 22, 2008 at 04:12:07PM -0400, Kyle Pearson wrote:

I’m wondering if it is possible to connect the USRP2 to an external
100MHz clock like the USRP was able to be modified so it could connect
to an external 64MHz clock. I don’t see any place to solder on a SMA
connector but there are a set of pins labeled Test CLK. Could that be
used for inputting an external clock or is it just for testing the
clock output?

Thanks,
Kyle

Note the “Ref Clock” connector on front panel:

http://gnuradio.org/trac/wiki/USRP2

It takes a 10 MHz reference.

Eric

3

I’m aware that the internal clock can be locked to an external 10MHz
reference. I take it that there is no way to use an external 100MHz
clock instead of the internal clock then.

-Kyle

4

Actually, you can use 100 MHz as well, you will just need to change one
line of the firmware to set the R-divider on the PLL.

You can use any frequency for which the following equation has a
solution:

100 MHz / N = Ref / R

for which N and R are integers and smaller than 32K

Matt

5

Thank you. Running at 100MHz is fine for me, I just have a 100MHz
external clock that I would like to use as a reference for the USRP2.
Could you point me in the direction of where that code is in the
firmware? I took a look and I haven’t come across it yet.

Also, could you tell me what the pins labeled Test CLK are? Can they
be used to see the clock after locking it to the external reference?

Thanks,
Kyle

6

Kyle Pearson wrote:

I’m aware that the internal clock can be locked to an external 10MHz
reference. I take it that there is no way to use an external 100MHz
clock instead of the internal clock then.

The internal clock (a very clean VCXO) is locked to the external
reference. You have a lot of flexibility as to the reference frequency,
as I said in the previous message, but you cannot pull the VCXO further
than a few kHz, and you cannot use the reference as the main system
clock. So there is no way, for example, to put in a 120 MHz clock and
have everything run off of that. You are basically stuck with 100 MHz
unless you are willing to solder.

If you are willing to do some soldering, there are pin-compatible VCXOs
in other frequencies which are available. The part is the CVHD950 from
Crystek, and you can get them from Mouser. They come in the following
frequencies:

54 MHz
74.1758
74.25
80
100
120 MHz
122.88
125

and possibly others. Note than anything over 100 MHz may make it hard
to get the FPGA to meet timing requirements. Also, the ADC is only
specified to 105 MHz, but there is a pin-compatible version spec’ed to
125 MHz.

Matt

7

On Mon, Oct 27, 2008 at 12:05:14PM -0400, Kyle Pearson wrote:

Thank you. Running at 100MHz is fine for me, I just have a 100MHz
external clock that I would like to use as a reference for the USRP2.
Could you point me in the direction of where that code is in the
firmware? I took a look and I haven’t come across it yet.

usrp2/firmware/lib/clocks.c

Eric