How does a signal that is inside the 70 MHz 1st IF passband but not withing the overall filter passband impact the imd?
That's exactly what the 2 kHz spacing IMD DR is all about. And in my example, you would need two signals, spaced 2 kHz apart, and at least 25 dB over S9 to ever even begin to hear/notice the IMD. And that's in the laboratory. With an antenna connected, the noise floor would be 10 dB or more higher than the MDS, at least on the low HF bands. So, you would need two signals far stronger than 25 db over 9 for IMD to be an issue.
I thought the dynamic range wide and narrow spaced was how much a strong signal within those two bandwidths impacts the signal you want to hear, not distortion, but things like AGC action.
Nope. Two-tone tests are static measurements and will not test the dynamic functions of ALC. Did you read the article I linked to? Blocking dynamic range is closer to what you want. From the article:
BDR is the difference, in decibels, between the minimum discernable signal (MDS) and an off-channel signal that causes 1dB of gain compression in the receiver.Still, BDR is a static measurement, so it won't show any dynamic characteristics of the ALC. Further, both of the above measurements mostly show the characteristics of the mixer following the high IF and those preceding it in the signal chain. I'm pretty sure that ALC usually isn't even involved here.
If a signal is within the 1st IF's passband but not in the 3 KHz filter you have in the 3rd IF and it activates the AGC loop in the 1st IF, it HAS TO effect the weak signal you want to copy.
Only if the ALC desensitizes the receiver enough that the signal goes below the noise floor of the receiver. I've NEVER seen that happen, even when working European stations several hours before sunset on 80 meters (yea, the were PW).
If you think the pro series receivers are good, its time to try some modern stuff.
I've used receivers more modern than the Pro series since the 1980s, including SDRs.
I'm not defending the Pros. I just used them as an example since they were on the list. If one is going to use the list, they should know what the numbers actually mean. Yes, the receivers listed above the Pros on the list are probably better. How much is the question, especially for those not operating in contests. Even a hardcore DX, weak signal guy like W8JI has written that 80-85 db 2nd order IMD DR is good enough, most of the time.
Remember that even in my very quiet rural location where I have to "beam" through the east coast to hear Europe, 80dB or so IM3 is probably more than enough. The reason my Drakes are so good is because they were built to duplex on the same band with my own transmitters running!! Unless you are in a situation like that, anything over 80dB is likely wasted.In other words, some of the hype around these numbers is just hype and used to sell receivers.
