Empathy List Archives

P

Perper

Thu, Jul 11, 2013 4:07 PM

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 (3fc, 5fc, 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)?

Or maybe my diagnosis is wrong and it is effect of something different?

Attached please find measured spectrum of a sinusoid transmitted on
230MHz and LFM signal with bandwidth of ~20MHz transmitted on carrier
frequency 230MHz. You can observe very strong repetitions on 690MHz,
1150MHz, 1610MHz...

--
Piotr Krysik

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)? Or maybe my diagnosis is wrong and it is effect of something different? Attached please find measured spectrum of a sinusoid transmitted on 230MHz and LFM signal with bandwidth of ~20MHz transmitted on carrier frequency 230MHz. You can observe very strong repetitions on 690MHz, 1150MHz, 1610MHz... -- Piotr Krysik

ML

Marcus Leech

Thu, Jul 11, 2013 4:20 PM

Try turning the RF gain down about 10dB, and also reduce, a little, the magnitude of the signals coming out of your flow-graph, to keep the mixing process, and RF gain chain linear.

on Jul 11, 2013, Perper <perper@o2.pl> 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)?

Or maybe my diagnosis is wrong and it is effect of something different?

Attached please find measured spectrum of a sinusoid transmitted on
230MHz and LFM signal with bandwidth of ~20MHz transmitted on carrier
frequency 230MHz. You can observe very strong repetitions on 690MHz,
1150MHz, 1610MHz...

--
Piotr Krysik

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ME

Matt Ettus

Thu, Jul 11, 2013 4:36 PM

On Thu, Jul 11, 2013 at 9:07 AM, Perper perper@o2.pl 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 (3fc, 5fc, 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

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

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WBX daughterboard - transmit LO signal harmonics