262khz vs 455khz IF? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: September 03, 2021, 09:47:23 pm
content: what's the reason manufacturers switched from 262khz to 455khz IF frequencies in AM radios?and what if any benefits does the 262khz IF frequency have?and what is the reason for conversion, multiple IF frequencies?also wouldn't the beat frequency be offset by the IF frequency?

author: TimFox
author_rank: Super Contributor
timestamp: September 03, 2021, 10:24:56 pm
content: The higher IF gives you more difference between the desired frequency (fLO-fIF) and the image frequency (fLO+fIF). This assumesfLO>fdesired. Both the desired and image frequencies heterodyne to the same IF, so the unwanted image must be rejected by the input tuned circuit(s) before the mixer. "LO" denotes the local oscillator.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 03, 2021, 11:20:22 pm
content: A lower frequencies simplifies the IF ampifier a little ( less GBW needed or more gain from the same GBW). Also fiters with a given Q have less BW. With LC fitler a higher Q was a bit more tricky. With later ceramic filter this was less of an issue /they have a high Q anyway) and a higher frequency filter is smaller and possibly cheaper.

author: Manul
author_rank: Super Contributor
timestamp: September 03, 2021, 11:54:43 pm
content: By doing conversion, receiver has some big advantages. It is more convenient to have amplifying stages tuned fixed. Imagine if there would be 3 or 5 LC stages and all of them you need to tune when you want to receive different frequency. It is better to tune just the frontend (basically for image rejection) and then mix and use fixed tuned IF stages. And a few stages might be necessary to achieve close to ideal bandwith. For typical AM usually +-4.5khz, so 9khz, idealy it should be close to rectangular, so the receiver can reject other signals which are very near in frequency. Also it is usually a benefit that amplification takes places at two different frequencies - some at station frequency and some at IF. It might be problematic to have multiple stages tuned for the same frequency and high gain. This risks unwanted parasitic feedback causing instability. Also, usually, the IF frequency is chosen to be quite low and hence easier to amplify. Espeacialy in the old days it was important. Some high-end receivers do double conversion and have two IF frequencies.

author: bob91343
author_rank: Super Contributor
timestamp: September 03, 2021, 11:55:28 pm
content: Multiple IF has the advantage that the image can easily be moved out of harm's way and generally isn't the problem that occurs with single conversion. Additionally, the first local oscillator can be crystal controlled, avoiding most of the drift. When that is done, the tuning dial can be the same for all frequency bands.Comparable benefits come in transmitters.

author: CaptDon
author_rank: Super Contributor
timestamp: September 04, 2021, 12:42:15 pm
content: 262Khz was the standard for car radios years ago and they were not 455/262 doubleconversion. They were 262Khz all the way to the detector. 455Khz was the standardfor home (line powered) radios. I worked for an AM 5kw station on 910khz in York Paand we got complaints all the time "Why does your station whistle"? Most poorlydesigned radios had enough I.F. leakage that it would hetrodyne back into the receiverfront end and produce a beat frequency. On really terrible radios the beat frequencyvaried with the audio content mainly because the audio power output stage causedthe power supply voltages to vary which then made the L.O. vary. B.T.W., old car radioswere extremely sensitive!! I used one as a receiver placed after a Gonset shortwaveconverter!!

author: TimFox
author_rank: Super Contributor
timestamp: September 04, 2021, 02:09:05 pm
content: Integer harmonics of the IF are known to cause "whistles", at 910 kHz and 1365 kHz for the 455 kHz IF. Also, if the input signal is very strong at approximately twice the IF, intermodulation gives a spurious response that varies with tuning. See p 646 of F E Terman "Radio Engineers' Handbook" (McGraw Hill, 1943). The second harmonic of 262 kHz is just below the US AM band, but there are more harmonics in the band than with 455 kHz.