IQ (quadrature signals) Signal-To-Noise Measurements

[NirAlon]

Hello all,

I have a question regarding SNR measurement when using IQ Modulation:

In my design, I'm using a IQ Demodulator + IQ Modem.

The modulation type is 4QAM, at 4.8k/symbols. The receiver noise figure is 4dB, and the modem SNR requirement is 4dB in order to achieve a PER of 1%

The RX Bandwidth of 4QAM, 4.8K/symbols is 4.8KHz

The expected RX sensitivity is:

RX(sns) = -174dBm/Hz + 10log(RX-BW) + NF +SNR ==> -174 + 10log4800 + 4 + 4 = -129dBm

The actual sensitivity that I'm getting is somewhere between -126, -127dBm...

I started searching for the missing 2dB.

The first thing I checked is the SNR performance of my RF chain.

At -129dBm, I'm measuring 5dB SNR at the "I" or "Q" inputs of the modem, which seems OK, but the modem's I and Q inputs actually have a bandwidth of 2.4KHz each, not 4.8KHz...

If the "system SNR" requirement is 4dB, The "I" or "Q" inputs should have 4+3dB of SNR? (because of the half RX-BW?)

Thanks guys,

Nir

 

[biff44]

are you filtering out, with a physcal rf bandpass filter, the "image noise" in the downconversion of the receiver?

 

[NirAlon]

Hi
Yes, I use a crystal filter that connected right before IQ Demodulator. The filter bandwidth is +/-3.75KHz (the total IF bandwidth is 4.8KHz)

 

[mtwieg]

In my design, I'm using a IQ Demodulator + IQ Modem.

You'll have to explain your measurement setup more clearly. Is it RF synthesizer > RF LNA > IQ demod (RF to IF) > IQ Modem (IF to RF) > Spectrum analyzer? Are you using an IF or is this direct conversion? How are you measuring sensitivity?

If you're using a quadrature detector properly, an image rejection filter before the demodulator should not be necessary.

 

[NirAlon]

You'll have to explain your measurement setup more clearly. Is it RF synthesizer > RF LNA > IQ demod (RF to IF) > IQ Modem (IF to RF) > Spectrum analyzer? Are you using an IF or is this direct conversion? How are you measuring sensitivity?

If you're using a quadrature detector properly, an image rejection filter before the demodulator should not be necessary.

My first set up is the IF chain alone, later on I plan to test the entire RX chain (BPF>LNA>BPF>Downdonverting mixer>BPF>AMP>IQ Demod>Modem)

Here is my current setup:



Thanks

Nir

- - - Updated - - -

In order to measure the SNR, I'm injecting a CW at the center frequency with 1KHz offset, which generate a 1KHz tone at the output I or Q.
The tone is then measured in the SNR meter

 

[mtwieg]

What is your "SNR meter" in this case? And how are you defining SNR/sensitivity?

There's a lot going on which could impact SNR which isn't accounted for in your sensitivity calculation. Filter attenuation, demodulator/modem noise figure, etc.

Ideally the SNR of the combined IQ signal should be 3dB above either the I or Q signals alone, assuming good image rejection.