I'm doing pre-certification testing to ETSI EN 303 372-2 V1.1.1 (2016-04), specifically the Adjacent Signal Selectivity test in Section 6.1.
I am a Systems engineer with lots of experience designing and performing equipment testing to DOD, DOE, FAA and FDA standards. Though I'm not an RF engineer, the test mentioned above makes no sense whatsoever. Here's the relevant section:
6.1 Adjacent signal selectivity
a) Two test signal generators shall be used. Each signal generator shall generate a modulated signal in the IDU input frequency range and thermal noise.
b) The signal generators shall be connected to the IDU input through a splitter (combiner).
c) The symbol rate shall be set to the low end of the IDU's range.
d) The test signal generators shall be set to the frequencies and levels according to table 1. For each row of the table:
a) The IDU shall be set to receiving the signal of the first test signal generator.
b) The second test signal generator shall be set to output signal off.
c) The noise level (or signal to noise ratio) of the first test signal generator shall be varied in order to determine the threshold for quasi error free reception.
d) The second signal generator shall be set to output signal on.
e) The noise level (or signal to noise ratio) of the first test signal generator shall be varied in order to determine the threshold for quasi error free reception.
f) The degradation is equal to the noise level (or signal to noise ratio) determined in step e minus that determined in step c.
e) Repeat from d) with symbol rate set to the high end of the IDU's range.
f) The result is the highest degradation found.
What makes no sense to me is step e) under d): There is no limit to how much the SNR of the desired signal can be increased to counter the influence of the interfering signal!
What I believe is needed is to quantify the effect of the interfering signal on the desired signal, and this step actively cancels that out.
What am I missing here? What aspect of selectivity is this test seeking to measure?
Based on comments below, my understanding is shifting:
For now, I've decided to take an approach that splits the issue down the middle:
- For the "wanted" signal, determine QEF at the receiver, then record both the generator and receiver SNR (they will likely differ by a small amount).
- Add 1 dB SNR to the "wanted" generator, and ensure the same step change is seen at the receiver.
- Add the interferer signal, and ensure the receiver SNR is reduced by less than 0.4 dB
My hope is this test procedure meets both the letter and spirit of Section 6.1 of ETSI EN 303 372-2 V1.1.1 (2016-04). It keeps the "wanted" signal very close to the receiver's QEF while making the effects of the interferer signal clear and unambiguous.
One minor concern is to not push the receiver below QEF during this test, because SNR measurements below QEF may not be completely reliable. That is, I don't want to have to fully characterize the receiver SNR value outside of the fully functional state, as it tends to deviate from the generator SNR at both high (>30 dB) and low (below QEF) values when directly connected.
Also, the spec says nothing about the SNR of the interferer signal: I initially chose to make it equal to the SNR of the "wanted" signal, though I am now revisiting that decision as degraded SNR looks like increased roll-off (alpha), so doing any SNR reduction to the interferer would make it differ from what the spec requires.