Re: The point of antenna amps


kwetzel2001
 

--- In FMtuners@..., "Al Germaine" wrote:
Well, it would be nice if one of the RF engineers
here would comment.
Maybe I can shed some light here, responding as a semi-retired RF Engineer.

The part of the tuner most vulnerable to 'overload', or the creation of IM spurious signals, is the mixer. The more gain that precedes the mixer, the more IM the mixer will create. The IM created in the input RF amplifier of the tuner is generally insignificant compared to what's created in the mixer. Overload in the IF amplifier of an FM tuner will not be noticeable except with laboratory instrumentation.

RF engineers use the "third order intercept point" (IP3) as a figure of merit of an RF component's susceptibility to creation of IM. The higher the IP3 number, the better. I've measured the IP3 of a few tuners, and found a wide variation. An ST-9030 had an IP3 of +6 dBm, while a KT-7500 had an IP3 of -21 dBm, with other good tuners falling between those two. Those values are for undesired signals 400 kHz apart (alternate channel spacing). For signals with a spacing of 1 MHz or more, the IP3 performance improves dramatically.

As far as antenna amplifiers go, I haven't measured consumer-grade units, just a couple of communications-grade units. The IP3 of an FM head-end preamp was +20 dBm, and the IP3 of a GaAs pHEMT preamp was +11 dBm. I would guess that a consumer-grade antenna preamp would be in the range of 0 dBm.

The bottom line: any gain added before an FM tuner will degrade its IM performance. For signals closely spaced, IM performance will be limited by the tuner, degraded by the added gain of the antenna amplifier. For signals widely spaced, IM performance will be limited by the (consumer-grade) antenna amplifier.

So we understand what we're talking about, the effect of inadequate IM performance of an FM tuner will be the creation of signals at the wrong frequencies across the dial. Those false signals are usually very distorted. If the frequency where a false signal is created happens to be your favorite low-power college jazz station, the effect will be elevated background noise, especially in stereo.

Ken

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