receiver filter selectivity

Can someone help me understand the scaling and axis' of the attached Channel Filter selectivity graphs?

The graphs plot blocking vs. offset. The other graphs plots selectivity vs. offset.

So for figure 5 (attached) does 0 = 868MHz and when you deviate +/- away from 868MHz the blocking and selectivity capabilities of the filter changes? Does a more negative blocking, selectivity parameter equate to bettter performancer in blocking and selectivity

Does blocking mean adjacent channel rejection? What exactly does selectivity refer too? Please help if possible......

The graph on the right is the transmission coefficient of the filter. These are usually plotted in a way that would flip your graph vertically.

The blocking graph is how intense an out of band signal has to be to prevent demodulation of the desired signal.

You are correct that the +- from zero is really +- from the frequency the receiver is tuned to.

There is something wrong with the numbers. The IF is less than half the bandwidth and so some of the signal is at negative frequency. This would be a problem in an all analog receiver, but with DSP negative frequencies can be distinguished from positive frequencies if quadrature methods are used.

what is "transmission coefficient"? The graph on the right is selectivity vs offset. I thought this was related to the receiver filter not "transmission".



You are saying that the IF should not be less than half the bandwidth for analog? So if the filter BW is 812kHz, the IF should be no less than 406kHz? In figure 5(attached) the IF is 355kHz (less than half the bandwidth) but how does that correlate to negative frequencies?

In other words my question is how does an IF of less than half the bandwidth equate to "some of the signal is at negative frequency"?

I have recognized the plots, they are coming from CC1101 or CC2500. Please consider that these plots are the same plot, the secoond one just a zoom of the first one.
They do not represent a simple filter transfer function, but an overall response of a complete receiver. These test are done injecting an interferer at a certain offset and increasing its strenght up to a certain degradation of BER or PER (useful signal is 3 dB above sensitivity).

So, for example, the cochannel rejection is roughly -15 dB.

The IF is 355 kHz just because the signal BW is not 812kHz, but just 700kHz. There is a margin in RX filter to deal with difference between TX and RX carriers (and flatulent says correctly that we are dealing with IQ RX with DSP processing)

Hope it can help.

Mazz

But was does cochannel rejection of -15dB actually mean? Does it mean that the cochannel signal has to be -15dbm or less (-16dBm,-17dBm,-18dBm......) in order not to interfere with the wanted signal?

Lets use an example. So At 0 offset selectivity is -15dBm, so does that mean if the wanted channel and the adjacent/cochannel are both located at 0 offset that the adjacent/cochannel can only be -15dBm or less without interfering with the wanted channel?

How about when the offset is -0,5. Does this mean that when the adjacent/cochannle is -500kHz from the wanted channel that the adjacent/cochannel can only be -11 or less without interfereing with the wanted channel?

Please advise........

All these tests with interferers are related to an sefuls signal that is large enough to give BER=0, sensitivity level+3 dB (for example if sensitivity is -80 dBm, put your useful signal at -77 dBm), then rejection is an C/I ratio (in this example is -92 dBm for cochannel.
When the rejection is >0 dB the interferer is higher than the useful signal (and this happens as the interferer goes away from useful channel).

Hope it can help.

Mazz

p.s.: take a look at the very good book of Qizheng Gu about system analysis of integrated transceivers

So the filter BW is 812kHz and the signal BW is 700kHz? How are you getting 700kHz for the signal BW?

Added after 2 hours 49 minutes:

Ok I think I got what you are saying. If the useful channel is -77dBm the co-channel must be -15dBm less (-92dBm) than the useful signal to ensure the best BER (BER=0) at 0 offset. If the co-channel amplitude increases changes from -92dBm to -85 dBm then that will affect the BER of the useful signal..........please confirm.....

Or are you saying that if the wanted and cochannel signal are both at -77dBm that the digital filtering of the cc1101 will reject the co-channel signal by -15dBm?

I have some pretty god books that help already Intro to Radio Frequency Design by Hayward

It is just the calculation you can do of an GFSK 500kbps with a certain freq deviation.

For cochannel, this is correct.

I hope it can help.

Mazz