IQ (quadrature signals) Signal-To-Noise Measurements
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
are you filtering out, with a physcal rf bandpass filter, the "image noise" in the downconversion of the receiver?
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)
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
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
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.