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question re: definitions of "overload"

P

pianoplayer88key

Hi all...

I'm trying to figure out if I have various types of radio overload correctly defined, or what certain types are.

I've heard/seen references to "front-end overload".  As I understand it, that would be when your receiver begins to desense/block (i.e. a station is not heard that would be heard if the strong signal wasn't on the air, even if no audio from the strong signal is detectable), or a station is being heard on frequencies it doesn't belong.
As I understand it, examples of the 2nd category would be a station showing up 900 kHz below its assigned frequency (on a radio with a single conversion 450 kHz IF), a harmonic (for example 760 showing up on 1520), an intermod mix (several years ago in my dad's mid 70s pickup that radio heard 600 and 1130 mixed on 530), or modulation splatter or IBOC hash spilling over onto the adjacents (for example tuning to 1180 or 1160 and hearing splatter from a station on 1170).  Am I understanding that correctly?

However, when I tune to the offending stations' actual frequencies, the audio, in most cases, is clean, unless the signal is extremely strong.
I had always (or previously) thought that "overload" meant the station was actually distorting / compressing / clipping /etc, on its own frequency.

So, apparently just having splatter, blocking/desense, intermod, harmonics, IF images, etc, is called "front-end overload".
This 36-second clip is what I've thought overload was...

760 KFMB - Tecsun PL-606, adding Select-A-Tenna & groundwire (on utility pole) partway through

The signal is relatively clean for about the first 20 seconds or so, after which I slowly move the PL-606 closer to the inductively coupled dual antennas over the next several seconds, until the PL-606 is resting on the Select-A-Tenna (which in turn is being held up to the utility pole's grounding wire) for the final several seconds or so.  BTW I'm about 7.3 miles southeast of their 50kW transmitter.

So what would be the term for describing that type of overload, if "front-end" is for the less severe type?  I'd like to be able to refer to one or the other (or other types of overload) using a couple words, without having to take an entire paragraph to describe what I mean.



Also sometime several months ago I had the Tecsun PL-380 (same DSP chip, smaller antenna so not quite as sensitive) and Select-A-Tenna outside the antenna farm for 5kW 910 KECR and 50kW 1170 KCBQ, doing the same experiment.
Normally, with those DSP radios (PL-380 and PL-606), when you first turn the radio on, or access certain functions, the audio is usually muted briefly (anywhere from maybe a second when changing a function (although just tuning up/down on the same band only mutes for a small fraction of a second, if that) to several seconds when turning the radio on or off).
However, when I had the PL-380 and SAT up to the groundwire with the SAT tuned to 1170, I could hear distorted audio from 1170 KCBQ at ALL times, from the instant I hit the power button to turn the radio on (bypassing the usual several second delay), through changing bands/functions/etc, even to turning the radio off (the audio wouldn't disappear until the display had changed to showing the clock, AFTER it said "Off")!
Here's an audio clip.
The signal was so strong at that location that its harmonics were showing up near the FM band, and was even overloading a crystal set that otherwise only had a small ferrite bar for an antenna.
That happened at a signal strength well above that at which the previous overload demo clip was exceeded.  (With that previous one, a stronger signal would have less audio come through until it eventually would sound like an unmodulated carrier.)

What's happening when I have that more severe example of overload?

(Also sometime I'll want help figuring out how to calculate the gain I'm getting from those big wires that seem to be acting as antennas, but I'll save that for another topic some other time.)
 
(This was supposed to go in the edit of the previous post, replacing the last mini-paragraph.  15 minutes, IMO, is not nearly long enough of an edit timer. :()

Also, is there a way to figure out how much gain the big antennas are giving?  As an example, though, a month ago I used the same setup to get an overload-level signal from a 50kW on 1580 (KMIK) 300 miles east of me at night (only the radio's display is visible due to the nearly non-existent lighting (I didn't want to draw unwanted attention by using the camera's flash), and another day had to be 90 feet from the tower of 5kW 1290 KKDD to get the same signal level using the PL-606's internal ferrite bar.
As another example, I had to judge it using the 2nd harmonic as the fundamental pegged the meter, but I got that strong signal on 50kW 760 KFMB from 7.2 miles away using the big antennas, but had to get fairly close (maybe several feet or so) to a 2.5kW 590 KTIE tower to get a comparable level.
 
Say you have a 50 kw AM transmitter. With 100% audio modulation the transmitter is putting out 50 kw. If you put in 120% audio modulation the transmitter (thanks to the front end processing) will still be putting out 50kw but the audio will be distorted.

That is not the issue you are talking about. Say the front end of your receiver will start to distort at 100 uv input. At 10 miles from the 50 kw transmitter the signal may be 75 uv (less than 100). You are good. As you move closer the level goes up. Say 1 mile from the transmitter the level is 120 uv (and for you purists, I know it is an inverse square law but I am not about to calculate it for this example). It is now higher that the front end is designed for, and all the nasty things you mentioned (intermod, images, etc.) start to happen.

Move even closer, and the level goes up even more. The level is so high that the transistors in the front end are saturated and no signal goes through the radio at all. The radio starts to 'de-sens' at that point. One of the reasons I can't get my HT to work on Mt. Wilson :)
 
Ahh ok. (And you're right about the fields not being accurate :) - I'm 9 miles from a 50kW and it's probably about 125 mV/m here.)

So about that front-end saturation ... is that what's happening when the audio on-channel begins distorting/clipping (or in extreme cases disappears altogether so it sounds like an unmodulated carrier)? Would that be called "saturation overload", or is there a better / more widely accepted term for it? Also what about when the level is so high that the audio (as distorted/clipped as it may be) actually gets through when it's otherwise (by design of the radio's functions while changing things) supposed to be muted?

As for desense, I thought it was happening before it saturated, at least in my Tecsun DSP (Si4734) radios. In one test, I set up an oscillator at a fairly high level, but not high enough to start producing intermods/images). I tuned in a station (with the oscillator off) that was a moderate strength signal (strong enough to trip seek/scan, but with a little background noise noticeable with the volume cranked up during non-modulation (when those on a talkshow are briefly shutting up for example)) several channels away. Upon turning on the osc, the station significantly weakened (and may have vanished completely, although I don't remember). In another experiment, I noted that I had difficulty receiving 1260 KMZT (then KGIL) at my grandma's house in San Gabriel, CA, 1/3 mile from 1300 KAZN, even though none of KAZN's audio was bleeding on 1260. In Loma Linda, CA, I was able to hear KMZT somewhat better, even though I was much farther away. (1290 KKDD and 1350 KTDD were the pests there, though.)

So if "front-end overload" is when you're getting images/intermod, then what about when you're just getting excessive splatter width? For example if you can detect any hint/trace of splatter on a station's first-adjacent (including IBOC hash), I had thought that was "front-end overload", but if not, what would it be? BTW the splatter was fairly easily noticeable on a Panasonic radio (with a wide filter not much different than the Sony SRF-M37*) on the first-adjacent at midday on 50kW 1070 KNX about 111 miles away before they ran IBOC, and their IBOC can even be detected on the 2nd-adjacent on that radio. Would that mean that KNX is overloading somehow, as they're leaking onto the adjacents? (It's not the strongest signal here by any means, though.)
 
K6JHU said:
If you put in 120% audio modulation the transmitter (thanks to the front end processing) will still be putting out 50kw but the audio will be distorted.
Click to expand...

good transmitters can do 200% positive peak modulation without distorting

I can't get my HT to work on Mt. Wilson :)
Click to expand...

a 2 meter resonant loop isnt out of the question. i use one on a Yaesu for FM BCB selectivity, it's abut 6 inches across
 
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