RE: [Discuss-gnuradio] Unable to tune Tx or Rx with XCVR2450 on USRP2

Hi Matt

Apologies is you got a similar reply about 10 minutes ago, but the webmail logged me out whilst I was trying to send it and it didn't appear in my sent items when I logged back in.

>You should know that we only spec the boards from 2.4 to 2.5 GHz and 4.9
>to 5.9 GHz.  When we test XCVRs before shipping, we make sure that they
>will lock from 2.35 to 2.55 and 4.85 to 5.95 so that there is 50 MHz of
>margin in case of variation due to temperature or other factors.  But
>there is no reason to think that they would lock all the way to the
>edges of the ranges you list since those are well outside of what we
>(and the chip manufacturer) specify.

Thanks. I had thought the low and high limits in the source code were the spec'd ones. Based on your above comment, combinations A, C, and D would seem to be within spec, though I didn't try all the stepped frequencies for case C or D, but just a few (e.g. 2.4G, 2.462G, 5G).

> Combination ID | USRP2 Serial | XCVR2450 Serial | Working
>       A         | 933          | 990             | YES
>       B         | 933          | 988             | NO
>       C         | 1159         | 990             | YES
>       D         | 1159         | 988             | YES
>
> In my testing today, an additional problem was also noticed, as below.

>To simplify testing, you only need to run either RX or TX since they
>both share the same synthesizer on the XCVR2450.

Thanks. I will do this in future.

>Normally I would tell you to send the parts back for me to check them,
>but since you are in AU, it would be expensive and take a long time.
>Instead, we may be able to debug this if you have an oscilloscope.  If
>so, can you look at the signal on R45 and R56 on the XCVR?  Note the
>frequency, and high and low voltages for each of the 4 combinations you
>mention above.  They should look like a square wave in all cases.

We have a borrowed oscilloscope spec'd to 1.5 GHz with no probe. I will try to borrow or buy a suitable probe. Can you confirm R45 and R56 are just digital logic signals, as would seem to be the case from what you state above?

>Assuming the signal you were transmitting was a sine wave with a
>baseband frequency of 0, then what you are seeing here is completely
>expected and normal.  When the clocks are not locked to the same
>reference, there is some frequency error, and the signal received is at
>some frequency other than exactly on the LO of the receiver, and it will
>get through just fine.  However, when the 2 clocks are locked to the
>same reference, the transmitted tone will be received exactly on the LO
>frequency of the receiver.  When this is downconverted to baseband, it
>will appear at DC, and it will be nulled out by the DC offset
>correction, which occurs in both the analog and digital domains.  You
>can turn off the digital one, but not the analog one on the XCVR.

>To demonstrate this, you can run the following commands:

>- To show a good tone being received:
>     usrp_siggen.py -f 5.65G -A 0.1 -x 100k --sine
>     usrp2_fft.py -f 5.65G

>- To show the tone being nulled out:
>     usrp_siggen.py -f 5.65G -A 0.1 -x 0 --sine
>     usrp2_fft.py -f 5.65G

Whoops. Didn't think about the DC offset correction. It was a sine wave at the carrier frequency that I tried. I will hence try your suggestion tomorrow, as it is evening here and I am at home without access to the radios.

Thanks

Ian.