View Full Version : FM vs. FM AFC


cpacamper
06-01-2009, 12:23 PM
On these old tube FM/AM radios, why would anyone choose plain FM over FM AFC? Why would manufacturers even have seperate FM and FM AFC choices on the band switch?

steerpike2
06-01-2009, 12:50 PM
If there is a more powerfull transmission on a nearby frequency, that can cause the AFC system to pull the tuner off your wanted station onto the more powerful one. Almost all tuners - modern ones included (excluding quartz synthesizers of course) - have a switch to disable the AFC.

bgadow
06-01-2009, 12:52 PM
My understanding is that you are supposed to tune the station in using the 'FM' setting, then switch it over to AFC to hold it. Doesn't always work great, though.

Jeffhs
06-01-2009, 12:56 PM
On these old tube FM/AM radios, why would anyone choose plain FM over FM AFC? Why would manufacturers even have seperate FM and FM AFC choices on the band switch?


AFC stands for automatic frequency control. This locks in an FM station so it won't drift after you tune it in. Normal procedure for tuning in an FM station on a radio with an AFC position on the band switch is to place the switch in the FM position, tune in your station, then place the switch on FM AFC and leave it there until you change stations again.

Very old FM radios did not have AFC circuitry, so they are very prone to drifting while the set warms up. I don't know when AFC was first introduced in FM sets, although my best guess would be in the early-mid-1950s.

Most FM radios today have what is known as "full-time" AFC which cannot be defeated; the band switch only has AM and FM positions. I personally am not fond of this arrangement because it will cause the AFC to pull the radio to the strongest signal it receives; this can be a problem if you are trying to receive a weak station which is very close to a stronger station on the dial. An example here in my own area, 35 miles east of Cleveland, is a classical music station on 104.9 MHz and a country-western station on 104.7. The latter station is so strong here (the other station is much weaker) that the AFC circuit in any FM radio will lock the receiver onto the stronger signal.

cpacamper
06-01-2009, 01:45 PM
AFC stands for automatic frequency control. This locks in an FM station so it won't drift after you tune it in. Normal procedure for tuning in an FM station on a radio with an AFC position on the band switch is to place the switch in the FM position, tune in your station, then place the switch on FM AFC and leave it there until you change stations again.

Ah-ha! Worked great on a weak station right next to a strong one on the dial. Thank you. Very useful feature like I said :yes:.

slow_jazz
06-01-2009, 03:50 PM
I have an old clock radio with AM/FM - AFC. Still works today...

wa2ise
06-01-2009, 04:13 PM
The AFC switch was probably partially a marketing tool. The fact that the switch is there tells the consumer shopping for a set that this one has this feature, see the switch for it... Vs. if the feature was hard wired "ON" all the time, then the radio looks like one that doesn't have this feature.

marty59
06-01-2009, 07:17 PM
I agree with you above. Plus, most consumer grade radios didn't have much of a "real" antenna for good FM reception or anything directional. AM was always easier to tune in being amplitude modulated. I remember being taught about tuning a station in then "locking" it with the AFC. FM multiplexes were pretty touchy and sometimes hard to keep the stereo signal locked in too.

jeyurkon
06-01-2009, 10:24 PM
The AFC switch was probably partially a marketing tool. The fact that the switch is there tells the consumer shopping for a set that this one has this feature, see the switch for it... Vs. if the feature was hard wired "ON" all the time, then the radio looks like one that doesn't have this feature.

I also wonder if it wasn't easier to design a receiver with AFC rather than trying to make a stable design. I don't have a FM receiver of that vintage other than the radio in my '49 TV console combination. The FM receiver in it seems to be rock solid and doesn't drift. It doesn't have AFC.

I did have an inexpensive solid state clock radio with FM and AFC. You couldn't disable the AFC and it was very strong. It was difficult to tune adjacent stations. You turned the dial and it would keep holding on to the station until it made a big jump.

John

hvlee
06-02-2009, 07:51 AM
I agree with your suggestion that FM radios/tuners could be designed to be stable without AFC.

My Dynaco FM-3 will sit on a station all day and still be there when turned on the next morning. Dynaco said this was achieved by careful design and selection of components.

I also have a Zenith H725 AM/FM radio apparently without AFC which is quite stable. I say "apparently" because there's no switch position labeled "AFC" and when tuning in a station you get the classic "three peaks"; the tuner doesn't reach out and grab the station.

On the other hand, I had a Hallicrafters SX-43 which was my only radio for years and no amount of warm-up would stop the drifting.

Harry
Maryville, Tennessee

Reece
06-02-2009, 01:16 PM
All things being equal, the higher the frequency, the more a receiver will drift with changes in component values due to heating. Just moving your hand near some FM tuning components can make them change stations. I recall some early FM sets that drifted badly.

Reece

amptramp
06-19-2009, 10:58 PM
As well as the problem with pulling toward a stronger adjacent station, AFC reduces the bass response.

AFC determines frequency by taking the DC average voltage from the detector, and it does this by using a low-pass filter at the output of the detector stage. When tuned to the center frequency, the detector output is zero and the AFC circuit attempts to drive this output to zero. But since the low-pass filter also passes the low bass frequencies, any low bass frequencies in the music will be attenuated by AFC. The low-pass time constant could be made longer to reduce this effect, but then it takes longer to take effect and cannot respond to sudden changes as rapidly (such as frequency shifts due to a sudden change in line voltage).

There was a Knight-Kit tuner with "Dynamic Sideband Regulation" which was basically audio feedback at audio frequencies to the AFC stage. This supposedly reduced the distortion by limiting the IF bandwidth requirement. This feature was switched separately from the AFC (which was strictly low-pass). I have one of these tuners and although it is interesting, the pentode RF stage limits the signal-to-noise ratio, so it cannot be considered to be a "premium" tuner.

cpacamper
06-22-2009, 06:03 PM
Amp, some of what you said is above my head but the bottom line is afc reduces bass response. So the questions I have are what bass frequency is the cutoff, and I am wondering if the "higher end" tabletop 1950's and 1960's AM-FM radios such as the Zeniths discussed so often here have sensitive enough speakers where use of AFC would have a significant effect on bass response?