I first experienced this hill when a friend of mine got his ham radio operator's license and lived only a block from the site. There is a hill not too far from the author where radio receivers are sorely tested. Carr, in The Technician's EMI Handbook, 2000 INTERMOD HILL: A TALE OF WOE ![]() A typical two-way radio installation is in a signal-rich environment, so when dozens of signals are present the number of possible combinations climbs to an unmanageable extent. But consider the fact that the two-tone case used for textbook discussions is rarely encountered in actuality. There are a large number of IMD products from just two signals applied to a nonlinear medium. Indeed, only the third-order is normally used in receiver specifications sheets because they fall close to the RF signal frequency. However, in practical terms, because each successively higher order IP is reduced in amplitude compared with its next lower order mate, only the second-order, third-order, and fifth-order products usually assume any importance. The IP orders are theoretically infinite because there are no bounds on either m or n. Other third- and fifth-order products may be within the range where interference could occur, especially on receivers with wide bandwidths. Note that one third-order product is on the same frequency as the desired signal, and could easily cause interference if the amplitude is sufficiently high. If the receiver were tuned to 5 MHz, for example, a spurious signal would be found from the F 1 −F 2 pair given above.ĩ,595 kHz (Δ F = 15 kHz) 9,610 kHz (Δ F = 0 kHz) (on channel!)ĩ,590 kHz (Δ F = 20 kHz) 9,615 kHz (Δ F = 5 kHz). This effect is seen often when two strong signals ( F 1 and F 2) exist and can affect the front end of the receiver, and one of the IPs falls close to a desired signal frequency, F d. One such problem is the emergence of “phantom” signals at the IP frequencies. ![]() ![]() If any of these are inside the passband of the receiver, then they can cause problems. The second-order IPs are 5 and 25 MHz the third-order IPs are 5, 20, 35 and 40 MHz and the fifth-order IPs are 0, 25, 60, and 65 MHz. When an amplifier or receiver is overdriven, the second-order content of the output signal increases as the square of the input signal level, while the third-order responses increase as the cube of the input signal level.Ĭonsider the case where two HF signals, F 1= 10 MHz and F 2 = 15 MHz, are mixed together.
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