Why do I have a near perfect spike at 144.000.000 ?

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Murphy625

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I've noticed that some "bands" have signal spikes right at the lower edge of the band.. Is this an internal thing or what?

I have a large spike right at 50.000.000 Mhz, a really big spike at 144.000.000, and another, albeit smaller spike at 420,000,000

Where do these originate?
 

Murphy625

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Its an SDR, but I get the same effect with other radios as well. Not on all bands though.

I've seen the effect(?) lots of times on different radios.. There is frequently a very strong signal right at the edge of a band.. and I don't mean close to the edge, I mean the exact frequency.
 

wtp

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mine

when i turn my computer off most of the even ones disappear, and the rest with the bedroom TV goes off.
so start turning things off and you might find it.
 

zz0468

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Its an SDR, but I get the same effect with other radios as well. Not on all bands though.

I've seen the effect(?) lots of times on different radios.. There is frequently a very strong signal right at the edge of a band.. and I don't mean close to the edge, I mean the exact frequency.

I'd be curious what other radios. I see the signals on RTL-SDR dongles but changing the sample rate either moves them, or makes them go away. So, in that case, I suspect aliasing or some other artifact of the D to A process somewhere.

In a more conventional receiver, the mixing process will produce spurious signals called "birdies". High end receivers will intentionally place the birdies where they don't bother the listener. Scanners are loaded with them because they're inexpensive, and cover wide frequency ranges. The most desirable features of a scanner are mutually exclusive of a clean receiver with no birdies. Some seem to just fall on a band edge, but without a comprehensive understanding of the frequency mixing schemes used, it's just a guess why they fall where to they do.
 

ko6jw_2

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An internally generated birdie will be there whether or not the antenna is connected. However, external spurious signal will go away (or be greatly attenuated) when the antenna is disconnected.
 

needairtime

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I heard someone talk about something like this on the local 70cm repeater, and sure enough when I tune my 2m radio to around 144.000 (actually depends on the area but anything from 143.990 to 144.010) MHz, it breaks squelch compared to another frequency 5KHz up or down from that band. The splatter tends to be +/-5 KHz from the center, at least that's the minimum step of the HT I'm using.

I'm in a suburban area so it may make a difference. However moving 10 miles and the splatter gets shifted 5KHz.

I'm baffled by this too, I've been hearing 144.000MHz is unfortunately a harmonic of a lot of computer clock crystals and hence spew noise everywhere, but I can't prove this to myself...
 

Murphy625

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Well, the fact that no one in this thread has said "Oh ya, that's the government carrier wave which marks the end of the ham band" or something similar, means that it must be a noise signal.. and that's what I wanted to know.
 

needairtime

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Yeah it's very strange that there's that spike of noise at the start of the ham bands, but yeah it's just simply a massive number of computer equipment spewing harmonics that line up with 144.000 MHz.

50.000MHz computers (and harmonics of other frequencies) definitely will make their mark on 6m ham radio, so that's not unexpected either. The 70cm band I don't think there's a lot of common noise sources that can generate that with any appreciable power with a fairly low harmonic that lines up with 420.000MHz, so that probably explains why the peak is weak.

BTW -- What SDR are you using, sounds interesting... I don't have a spectrum analyzer but it sounds that an SDR would suffice as a poorman's spectrum analyzer...
 
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Murphy625

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Yeah it's very strange that there's that spike of noise at the start of the ham bands, but yeah it's just simply a massive number of computer equipment spewing harmonics that line up with 144.000 MHz.

50.000MHz computers (and harmonics of other frequencies) definitely will make their mark on 6m ham radio, so that's not unexpected either. The 70cm band I don't think there's a lot of common noise sources that can generate that with any appreciable power with a fairly low harmonic that lines up with 420.000MHz, so that probably explains why the peak is weak.

BTW -- What SDR are you using, sounds interesting... I don't have a spectrum analyzer but it sounds that an SDR would suffice as a poorman's spectrum analyzer...

Its a $20 RTL-SDR..
 

zz0468

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I don't have a spectrum analyzer but it sounds that an SDR would suffice as a poorman's spectrum analyzer...

An SDR can make a usable spectrum analyzer, but it's loaded with all sorts of things that can fool you. They overload easily, and there's the problem with aliasing, false signals created as an artifact of the A/D process. Nothing that can't be dealt with, though.
 

needairtime

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Sorry for this if it's starting to be off topic, but I seem to read that there are band gaps, or are there "choice" SDRs out there that could do a very broadband scan, even if slow, without gaps, and even down to HF frequencies.

I suppose I want to see if my transmitter experiments have spurious emissions in more than just the ham bands... last thing I want is the FCC knocking on my door because I have spurious emissions interfering with LMRS because I didn't account for something in my transmitter design...

Oh, and in Linux :)
 

zz0468

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Sorry for this if it's starting to be off topic, but I seem to read that there are band gaps, or are there "choice" SDRs out there that could do a very broadband scan, even if slow, without gaps, and even down to HF frequencies.

More bits, more bandwidth = more $$$. As with anything, you get what you pay for. A $20 SDR dongle is an excellent way to get started, an amazing piece of technology, and one of life's little bargains. A $300 SDR is all that, only more so.

I suppose I want to see if my transmitter experiments have spurious emissions in more than just the ham bands... last thing I want is the FCC knocking on my door because I have spurious emissions interfering with LMRS because I didn't account for something in my transmitter design...

Well, I wouldn't use an SDR receiver/ spectrum analyzer for that. They're just too prone to generating spurious signals to be useful for FCC compliance. They're good for seeing what signals are out there to be monitored.
 

kj6psg

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More bits, more bandwidth = more $$$. As with anything, you get what you pay for. A $20 SDR dongle is an excellent way to get started, an amazing piece of technology, and one of life's little bargains. A $300 SDR is all that, only more so.



Well, I wouldn't use an SDR receiver/ spectrum analyzer for that. They're just too prone to generating spurious signals to be useful for FCC compliance. They're good for seeing what signals are out there to be monitored.

They usually don't have enough dynamic range to check for harmonics, anyways. A spectrum analyzer will give more meaningful results as SDRs aren't designed for transmitter qualification and can be finnicky about how RF gets into the receiver. For FCC-grade tests, you measure directly at the transmitter output plus whatever attenuators are needed to keep the signal in testing range, and specans handle that type of input much better. Linearity matters when all other noise sources are (hopefully) isolated.
 

Murphy625

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More bits, more bandwidth = more $$$. As with anything, you get what you pay for. A $20 SDR dongle is an excellent way to get started, an amazing piece of technology, and one of life's little bargains. A $300 SDR is all that, only more so.



Well, I wouldn't use an SDR receiver/ spectrum analyzer for that. They're just too prone to generating spurious signals to be useful for FCC compliance. They're good for seeing what signals are out there to be monitored.

The little $20 SDR has done more to amaze me than my ICOM 746 or any of the HT/Mobile 2/440 radios I have..

Being able to see the 3khz bandwidth coming from transmitters at 40 or 80 meters just hooked me instantly.. And don't even get me started on how cool it is to be able to download actual weather maps directly from a NOAA satellite..
Among some of the neatest things I've seen is the 100 hz PL tone being emitted from those talking through local repeates and learning how to play with filters and being able to actually see the filter working.

I know a lot of the newer $2000+ radios do this stuff too.. but doing it for $20 is just fascinating to no end.

And now I'm learning how to decode the digital modes and listen to those as well.

I still can't believe this thing was only $20.. My neighbor spent more than that on a USB 3.0 cable..
 

needairtime

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Yeah, I am not expecting real spectrum analyzer quality dumps, but I'd think it will go a long way in finding all the spurs that may be generated. I wouldn't expect to get everything completely clean with an SDR, but it should go a very long way to telling how bad my filters are.

I'm also not a chatterbox and that transmitter won't be frequently used, but it would be great to get a first pass look at the emission without buying an expensive real spectrum analyzer, when it comes time I should sell these units, then it's time to invest.

--

I've been seeing "cheap chinese sdrs" (CCS instead of CCR?) that people complain do not receive ATSC in the US market, are these the ones to buy, or should there be ones that come with ATSC software and also work as well as the ones you're using?
 

K8EAA

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Spike could be caused by cable tv leakage or even LED home lighting. I have had both over the years.
I also get a spike on 146.76 from my mobile station, Seems to be coming from my Honda accord ECU.

Don
K8EAA
 
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