Simple radio alignment question - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: August 19, 2023, 12:43:52 pm
content: Just spent days reading up on am and fm alignment for the first time, but looking at vintage rf generators (got my eye on one to buy) a lot go up to 110MHz or 150MHz whilst overlaying 30% modulation with a 400Hz tone. Why do they go into the FM band and beyond when they only modulate with a.m. ? What good is that?I feel a self-kicking coming on...

author: CaptDon
author_rank: Super Contributor
timestamp: August 19, 2023, 01:14:17 pm
content: How about the aircraft band from 108Mhz to 130 something Mhz? That is an A.M. modulated band. Two meter ham from 144Mhz to 148Mhz was C.W. and A.M. You can still align the R.F. front end and the I.F. of an FM radio with just a carrier and no modulation. You would need modulation for alignment of the final discriminator stage. Any V.H.F. signal generator that can't do F.M. modulation is probably older than the radios you are trying to align, or they are a featureless cheap junk.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 19, 2023, 02:30:15 pm
content: Why AM in the air band?Because you can hear other people transmitting on the same frequency. With FM you just hear the weaker stations on the same frequency as annoying interference.Horses for courses.

author: Quantumplate
author_rank: Regular Contributor
timestamp: August 19, 2023, 02:31:10 pm
content: Quote from: CaptDon on August 19, 2023, 01:14:17 pmCould you elaborate just a little on that please?

author: xrunner
author_rank: Super Contributor
timestamp: August 19, 2023, 03:29:30 pm
content: Quote from: Quantumplate on August 19, 2023, 12:43:52 pmOh you want to modulate FM with x amount of deviation in the FM band? I see. Then spend more money on a better more capable rf generator.

author: CaptDon
author_rank: Super Contributor
timestamp: August 19, 2023, 04:26:33 pm
content: Reduce the sig gen output to stay out of the radio's limiters and align for peak received signal strength going into the discriminator stage. As I said you will need FM modulation to center the discriminator and sometimes the very best fidelity doesn't always occur when the tuning meter is exactly centered. Most FM radios can slope detect AM when tuned off center although the audio quality is poor and usually the closer you get to true center tuning in the discriminator the less A.M. will be recovered. The limiter stages in an F.M. receiver also tend to reduce the apparent percentage of A.M. modulation.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2023, 04:39:19 pm
content: AM on signal generator:Years ago, when I had only a Heathkit RF generator (with AM capability), a good audio VTVM, and a Simpson 260 (in bad condition), I would do simple alignment using an AM drive and a simple Heathkit diode RF probe into the VTVM (military Daven meter), I would tune for maximum on the audio meter, which responded according to the square law for the RF probe.

author: radiolistener
author_rank: Super Contributor
timestamp: August 19, 2023, 04:52:04 pm
content: Quote from: Quantumplate on August 19, 2023, 12:43:52 pmThere is aircraft band between broadcast FM and VHF band, the standard for aircraft band is AM, because FM works very bad for station which moves very fast... Also FM has non-linear properties, while AM is pure linear. So there are pros and cons for each modulation. The bad side of FM is a waste of working bandwidth, bad long distance communications and sensitivity to a station movement speed. The bad side of AM is a waste of working power. There is also DSB modulation which has pros of AM modulation, but it don't have carrier, so it's more power efficient, but also bad for a fast moving station.

author: BillyO
author_rank: Super Contributor
timestamp: August 19, 2023, 06:33:27 pm
content: About the least expensive way to get in the 88-108MHz range and proper frequency modulation is with a TinySA Ultra. They are only about $130US.

author: torch
author_rank: Frequent Contributor
timestamp: August 29, 2023, 03:10:15 am
content: Quote from: BillyO on August 19, 2023, 06:33:27 pmYou don't need the Ultra, a regular TinySA can generate a sine wave to 350MHz with choice of AM, narrow FM or wide FM modulation. Square waves up to 960MHz.The Ultra reaches 5.3GHz, but is twice the price.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: August 29, 2023, 09:30:58 am
content: Quote from: Quantumplate on August 19, 2023, 12:43:52 pmFor at least half a century, signal generators have had carrier, AM and FM. One example is the Marconi TF 995 from 1969. Selling on eBay for around £100, covers to 220 MHz.SJ

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 29, 2023, 01:23:16 pm
content: Bigger numbers sell stuff. Sweep/FM also requires more parts, and parts cost more back then.

author: janoc
author_rank: Super Contributor
timestamp: August 29, 2023, 01:47:31 pm
content: Quote from: Terry Bites on August 19, 2023, 02:30:15 pmThat isn't quite true even for AM. If someone transmits over your transmission (everyone is on the same frequency - not a near close by one, pretty much exactly the same!), everyone hears a garbled mess. Depending on where you are relative to the transmitting stations you hear the stronger of the two signals or some mix of the two. However, often you often get only a loud screech or some other noise.There are plenty of ATC recordings online demonstrating this and there have been even some notable accidents when either the crews or the controllers didn't hear each other because of such "getting stomped on" interference from other planes.It is mostly historical - keep in mind that aviation and systems around it (ATC, navaids, etc.) long predate wide availability of FM radios. AM is simple, cheap and mostly works, so that is what has been used. And once you have the systems in place, it is incredibly expensive and difficult to change them, esp. in aviation where everything has to be certified and is tightly regulated for safety reasons.