Re: [Openbts-discuss] [USRP-users] WBX daughterboard - transmit LO signal harmonics

[Ralph A. S. d. <ra...@sc...>]

I want to add that very often harmonics are generated in the spectrum
analyzer, when input level is too high. An external attenuator may help.

 

Ralph.

 

From: USRP-users [mailto:usr...@li...] On Behalf Of
Matt Ettus
Sent: Thursday, July 11, 2013 6:36 PM
To: Perper
Cc: usr...@li...
Subject: Re: [USRP-users] WBX daughterboard - transmit LO signal harmonics

 

 

On Thu, Jul 11, 2013 at 9:07 AM, Perper <pe...@o2...> wrote:

Hi all,

I'm want to transmit signal using WBX daughterboard.
I encountered one serious problem: when I transmit signal on a carrier
frequency fc I have also very strong repetition of it on odd multiplies
of fc (3*fc, 5*fc, 7*fc...)
On each of those frequencies I have the same spectrum as spectrum from
baseband frequency.
I suppose it is effect of mixing of the signal with an LO signal that is
VERY far from being sinusoidal and this LO signal have very strong odd
harmonics.

Question for Ettus Research - are you aware of this effect and is it by
design? Therefore WBX daughterboard should always be used with some
lowpass filter that will cut unwanted repetitions that effect from
mixing with harmonics of LO? Can I somehow change LO signal so it will
have better quality (much lower harmonics)?
 

 

This is by design.  You can reduce the harmonics somewhat by reducing power,
but not that much.  The reality is that mixers and amps have harmonics, and
the only way to get rid of them is with filters.  The WBX has such a wide
range of frequencies (50 MHz to 2.2 GHz that we would need 8 or more filters
and that would dramatically increase cost.  The intended use is for you to
do one of the following:

 

- Use external filters that you can switch if you are operating over a large
frequency range

- Replace FIL1 on the WBX grand-daughterboard with an LPF that fits your
frequency range if you are operating over a narrower range (< 1 octave).
Minicircuits makes a whole line of pin-compatible filters that fit there.

 

or

 

- Operate at low enough power or over cables so that it doesn't matter

 

Matt