# why is 3rd order intermodulation for 2110-2155 = 1710-1755?

Trying to calculate 3rd order intermodulation for a frequency range (2110 to 2155). I know the answer is 1710 to 1755, but I don't know how to get there.

attempts:

f1 = 2110
f2 = 2155
2*f1 - f2 = 2065
2*f2 - f1 = 2200


this doesn't seem to follow the equation. help!

update:

My source of the frequency range info is from the website of a manufacturer of PIM analyzer equipment, Kaelus - (formerly Summitek): https://www.kaelus.com/en/test-measurement-solutions/bench-pim-testing/iba-series

If you go find their iBA-1921B analyzer, you will see these frequency ranges in their specs.

This was also confirmed on another site: https://awt-global.com/wp3/resources/pim/intermodulation-products/. From this other site, it appears as though the 9th im product is the problem, but the Kaelus PIM analyzer (iBA-1921B) returns im frequencies 1710-1755.

My question has not changed, but the reason I ask this is because I am trying to calculate how many data points I can expect to get back based on the sweep frequency ranges for carriers 1 & 2 and the receive band for the PIM analyzer. What is odd here is that the 3rd order IM products are not inside the receive band of the PIM analyzer, at all, and yet the analyzer returns 1710-1755 when taking measurements.

• Your equation agrees with literature sources describing IMD3. Please tell us the source for your information that it should be 1710-1755. Apr 26 '19 at 1:42

## 1 Answer

The TX ranges supported by those units are the downlink ranges of recognized cellular allocations and the RX ranges are the corresponding uplink ranges — e.g. LTE Band 2 has downlink from 1930-1990 MHz and uplink from 1850-1910 MHz, and LTE Band 4 has downlink from 2110-2155 MHz and uplink from 1710-1755 MHz. Together they make up the ranges supported by the 1921B. Likewise the 1800B covers Band 3, the 0901B covers Band 8, etc.

So they're not designed around looking for specific IMD products wherever they occur, but around looking at the corresponding uplink range and detecting whatever IMD from the downlink happens to land in that range. Presumably out of concerns of a transmitter raising the noise floor / desensitizing its own receiver. In particular, they have nothing to do with the specific placement of IMD3 products.