So I was putting up an FM antenna recently and got the idea that using a signal amplifier might be a good idea, as the output would go to two tuners, one of which is somewhat "deaf" in terms of sensitivity.
The amplifier in question is a Radio Shack 15-321. It's a two-in-one unit, with two pieces. One goes on the antenna and has an adjustable amplification level that is set via a potentiometer on the indoor unit. The other is internal to the indoor unit and offers a fixed boost that can be turned on or off. Power is supplied to both through the indoor unit.
What I've noticed is that some stations just disappeared from the dial after installing the signal amp. This is not a big deal, the amp works well otherwise and I'm not missing the stations that don't come in. The user's guide points this out as being a possible problem, and it says that the only solution is to completely remove the amplifier. What it doesn't say is "why" and that is what I would like to know. By the way, missing stations are not confined to any one area of the FM band.
I suspect a phenomenon often referred to as "strong-signal desensitization", a.k.a. "desense".
It can occur when you have a mix of strong and weak signals coming into the system through the antenna. If you stick an amplifier into the equation, it amplifies both the strong and the weak signals equally.
This can cause problems, if the signals involved are strong enough to overload either the amplifier itself, or the "front end" in the FM tuner/receiver. What happens, is that the strongest signals actually push some stage of the tuner (or the amplifier itself) into saturation or "clipping", and the RF or IF waveform becomes badly distorted. When this happens, the circuitry can no longer respond to the lower-strength signal at all, and the signal effectively "disappears".
By analogy: consider an old-style vinyl record, with the signal being molded into the record in the form of a groove. Consider a signal which has both a low-frequency component (bass) and a high-frequency component (treble) at the same time. As long as they're of roughly the same amplitude, the shape of the wiggles in the groove will be the sum of the wiggling that you'd get from the low-frequency signal along and the wiggling that you'd get from the high-frequency signal alone.
If the high-frequency signal is weaker (smaller wiggle amplitude), things still work OK, as long as the total signal isn't too high.
Now, try turning up the amplitude of the whole signal a lot - say,
10:1 or so (20 dB). The low-frequency wiggle is so broad that it actually slams the playback stylus all the way to the edge of its travel in the cartidge much of the time... the stylus can't go any further. For all of this time, when the stylus is jammed against its limit stops, it can't wiggle at all in response to the weak high-frequency signal. The playback system is now "de-sensitized" to the weaker signal... it's not strong enough to overcome the saturation effects of the stronger signal.
This is probably what's happening in your situation. You have some strong local FM signals, and the amplifier is boosting them up so much that they are "jamming the stylus" in your FM tuner.
In really severe cases, a strong local transmitter can even saturate and desensitize the amplifier itself... clobbering *all* of the desired FM signals. I had this sort of problem with a TV redistribution amplifier... my 2-meter amateur radio transmissions saturated it and wiped out all of the TV signals. My wife was unhappy... and I had to find a specialized 2-meter notch filter to keep the interfering signal out of the TV coax.
This sort of situation is relatively common, and it doesn't always have to involve an amplifier. Some 800 MHz public-safety (e.g. police and fire) radio systems have had desensitization problems, due to strong cell-phone-tower transmissions on nearby frequencies. This has been one reason for the "push" to digital TV... because it allowed the
700 MHz frequency band to be freed up, and some portions of it are being allocated to public-safety users so they can move their frequencies away from the desense-causing cellphone signals.
--
Dave Platt AE6EO
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That's certainly a likely possibility, but I have a different theory or three.
One guess(tm) is that the Radio Shock amplifier is rather noisy. The additional noise generated by the amplifier is stronger than the previously weak signals. If really awful, weak signals will be totally lost in the noise.
The Radio Shock amplifier might be oscillating, thus creating the in-band strong signal necessary to create the blocking effect.
The FM tuner might have an AGC section. This is required for direct digital receivers that have an A/D converter in the front end of at the IF frequency. The AGC is necessary to use the entire range of the A/D. These usually have a limited dynamic (amplitude) range and are easily overloaded. When amplified, this would cause a strong signal to disappear as it runs out of head room.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
It would be interesting to stick a spectrum analyzer at various points in the system -- starting at the unamplified antenna signal and see what's going on.
Is there a reflection in the feeder from mis-match? That would cancel some stations and the null would be quite sharp and deep.
Has anyone previously tried to use the cable to feed two sets and installed a hidden junction box somewhere along the line? Is the cable connected in the correct way to the units, with the screen properly clamped? Is the cable of the correct characteristic impedance for the pre-amplifiers (50-ohm or 75-ohm)?
You could try cutting an inch or two off the feeder (if you have enough to spare) and see if the missing stations come back and others disappear.
Another possibility is a balance/unbalance problem at the head amplifier. If you have a spare ferrite ring, try slipping it over the cable near the input or the output of the head unit and see if that makes any difference.
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~ Adrian Tuddenham ~
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"William I. Walsh" wrote in message news:WfSdnYAlnI3EDC_WnZ2dnUVZ snipped-for-privacy@mchsi.com...
Could the amplifier possibly have and FM trap built in? This is common in many amplifiers designed for TV use. A quick look at the user manual says there is a switchable trap on this model. How is the switch set?
uh, most HP spectrum analyzers had a 35+ dB noise figure, and you are trying to 'see' the NF for RadioShack's amplifier, right, probably 2 to 3.5 dB NF?
So, a quick check that the cause of losing stations was due to increasing the noise floor can be checked by verifying that all the lost stations were weak stations to begin with.
I don't know about the specific number, but I believe that in general spectrum analyzers make quite poor receivers (for weak-signal work at least). They're designed more for bench-work, where signals are relatively strong and there's a real need to ensure that strong-signal intermodulation doesn't occur.
It's not a trivial job to make a receiver front-end with a really low noise filter, enough gain to take advantage of it, the ability to withstand really strong signals without overloading, and a wide frequency range.
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Note the AGC in the front end of the FM section (running a PIN diode). The idea is to keep the output signal level within the maximum dynamic range of the DSP A/D that follows this chip.
NXP also makes an AM/FM front end chip, the TEA5777 (which seems to have disappeared from the NXP web pile.
See section 8.2.1 which proclaims: An RF AGC circuit prevents the mixer and IF filter from overdrive conditions.
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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS
I wondered if something like this might be taking place. But I was not at all sure.
Something curious happened last night. I went over and turned the stereo receiver on. By complete chance, I ended up tuning the receiver to one of the stations that had disappeared from the dial. Its automatic seeking function stopped on the station.
To my complete surprise, the station was coming in perfectly. So too were several other stations that I had previously noted to be gone.
I checked the amplifier, it was still plugged in and seemed to be doing its thing. The HD radio tuner also picked up the stations.
I am unsure what changed...could it have been something in the atmosphere or even space affecting this? Weather has been sunny and warm, last night's sky had a full moon (maybe that's an idea! ) and some cloud cover.
I am not sure how I would know. There is a matching transformer way up at the top of the antenna that goes from its connections to the coaxial cable. This in turn feeds into the input of the amplifier.
Coming out of the signal amp is another length of coaxial cable, this one 25 feet long. This goes to a grounding block and then on into the building with another twenty five feet or so of coax, where it connects to the "power" unit.
I'm not sure if the grounding block is in the "right" place but it is connected and properly grounded. I don't know that it would make a difference. I tried temporarily removing it and noticed no change.
The "power" unit has a built in splitter. A stereo receiver is connected to one end with approximately six feet of coaxial cable. The other output goes to an HD Radio tuner. Both have 75 ohm connections.
Definitely no. I ran all of the cable for this, so there is nothing in it that I would not know about.
I believe I've done this correctly, by following the printed instructions.
The instructions that came with the amplifier don't talk about this in any great detail. The obvious thing to do (to me at least) was to use
75 ohm cable. I would have to think that if some special cable were required, the instructions would talk of it.
Yes, it does. It is located in the outdoor unit and is switchable. I was sure that I had switched it off, so I went up and looked. It's definitely off.
I checked and turning it on basically kills all FM reception, so I'm sure the switch is working.
I have seen this effect. It could also cause multiple frequencies being generated. Sometimes adding core chokes around the in and the out of the amp, can help. And you can reduce gain.
40dB of gain from VHF to UHF TV bands (54 to 700Mhz)? That's a bit much. Oh, the box says 33dB. A bit better, but still too much gain.
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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS
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