[Radio Wrangler]

It's a perfectly serious question and has a simple answer.

On the HF bands, communication is limited by the levels of atmospheric noise and now the amount of man made crap over that. Communications receivers just need to be better than this noise level. Most development goes into getting better large signal handling.

On the VHF bands, noise levels are far lower. An antenna pointing at a quiet part of the sky sees a noise level equivalent to the thermal noise from a perfect resistor cooled to 3 Kelvin. Noise from most man made stuff has rolled off well before these frequencies. So on VHF, there is benefit from getting as low a noise contribution as possible in a receiver front end.

The front end of a VHF receiver needs filtering for two reasons.... firstly to attenuate the mixer image frequency. The image crosses the frequencies of other services you don't want to hear along with the station you've tuned to. Secondly the front end filter reduces the number of unwanted signals hitting your RF amplifiers and mixer. This is needed to be able to work amongst a band full of other signals without suffering crossmodulation and intermodulation and blocking effects.

Filtering is not without its costs. If I make a filter narrower, for a given technology and quality factor of components, its loss increases. Increased loss makes the noise performance of our radio worse, just like adding a normal attenuator on the input. So we have a compromise between noise and overload behavuours.

High Q resonarors like Quartz crystals offer much lower loss for narrow bandwidth, but we can't use them if we want to be able to tune to other frequencies, and it is very difficult making them wide enough to pass an FM broadcast signal. We are stuck with L-C filters. Mechanical variable capacitors give better Q than varactor tuning diodes, so the capacitor-string-and-pointer tuners have an advantage over electronically tuned ones.

So the RF section can't have much selectivity ahead of its first amplifier, without much selectivity the amplifier had better not have much gain. But we've helped the noise a little and got some selectivity in. Time for some more selectivity and then another amplifier.

So the best compromise splits the amplification needed into several low-ish gain stages, and splits the selectivity needed into several mild stages, and interleaves the amplifiers and filters.

So you wind up with expensive FM tuners having many tuned resonators in the RF section and a few RF amplifier stages.

Sitting at home I have a Revox B761 tuner. It is ex-BBC, who used them for two purposes: To monitor transmission quality, or to provide audio to modulate a VHF FM transmitter by receiving signal from another transmitter. This was used for some fill-in transmitters and as a back-up at the bigger sites. So for an FM tuner to work at broadcast quality levels receiving a distant transmission while based at the site of a working VHF/FM transmitter isn't bad going! It uses varactor tuning, several stages and a carefully designed RF amp and mixer. Incidentally, the IF/limiting stages are not all lumped into one chip, there are several discrete stages. It doesn't use ceramic filters, it uses L-C adjustable sections and the discriminator is a pulse-count type. It works rather well and shows good enough performance can be had by taking care without going silly.

David