Preamp IP3

[KV4PM]

On my SDS200, I am using a Mini Circuits LNA with admittedly too much gain (~27 dB but I have an attenuator on it) and the IP3 is about 30 dBm. I see MC also sells a 13 dB LNA with an IP3 of around 40 dBm. I have read posts by a few others here who seem quite knowledgeable on the subject, so I am hoping one of them reads this and replies to this question: Would the increase in IP3 dBm make a noticeable difference in the "real world?" The preamp I use now does improve my reception, but at times I have to add even more attenuation when I notice a lot of birdies, especially on the 800 MHz band, when doing a search.

 

[prcguy]

30dBm IP3 would be a wee little preamp for use without a good narrow band pass filter in front of it. With 30dBm IP3 I would expect the IP1 or the 1dB compression point to be around 15dBm. With 27dB of gain a single input signal of -12dBm would already put it into saturation, however multiple signals much weaker will start creating IMD or ghost signals within the preamp raising the noise floor and causing problems. IP1 or 1dB compression and IP3, which is a calculated number go hand in hand and if a particular amplifier has a larger IP3 it usually has a higher IP1.

Yes on IP3 making a noticeable difference in the real world. For example, I'm in a big city with lots of huge signals across all bands receivable at my location. With many years of playing with preamps I've only found a few that sort of work on a wide band antenna like a Discone without mucking up my noise floor or making reception worse. These few preamps have low gain in the 10-15dB max range and IP3 numbers well above 40dBm like 46dBm with a corresponding IP1 of 26dBm or just under 1/2 watt for 1dB compression. Anything less here is not useable on a broad band antenna with no filtering.

If I tried to use a 27dB gain amp with 30dBm IP3 here on a Discone (and I have) it would completely ruin my reception.

 

[Ubbe]

IP3 are measured at the output, so you have to subtract the gain from it to be able to compare amplifiers input IP3 level. If the amplifier receives two strong signals it will create a third signal if it overloads and works like a mixer in a radio. When that third created frequency are as strong as the other two then that are where the IP3 are set, and at that point the amplifier have been useless at much lower signal levels.

IP3 rises by the cubic so if the input IP3 are +0dBm then it will be at -40dBm to -50dBm where that third mix signal are gone in the noise floor. That level of IP3 would create big issues in a receiver.

The best amplifiers like PGA103+ have +20dBm IP3 at the input that equals -20dBm to -30dBm when that mix signal are gone and are what most antennas never produce even without any kind of filters, if you don't have an antenna close to a broadcast tower or 2-way radio towers. If you have one broadcast signal at -40dBm and another broadcast channel also at -40dBm then their combined level are 3dB higher at -37dBm. The more signals in the air the higher their combined signal level.

/Ubbe

 

[KV4PM]

Thanks, mates. I knew I could count on you two to share your excellent knowledge on this subject (I'm an EE but work in the power world, so I know just enough electronics to be dangerous). Anyway, I have ordered a ZX60-P103LN+ and found a low (well, lower) noise power supply for it. I've been using a crummy wall wart and I'm sure that's not helping my reception. I'll let you know how my new setup works. Thank you again for your input.

BTW, since I don't listen to anything below UHF, I use a MC high-pass filter with a 3 dB cutoff of about 360 MHz.