- October 24, 2019 at 7:48 pm #113205
Would this tell me if my transmitter is causing interference on any other bands or frequencies?
It’s new and affordable!October 25, 2019 at 12:51 am #113207RichParticipant
Total posts : 195
Observation: it doesn’t include a receive antenna with a known pattern/gain across its operating range. Therefore it can’t accurately measure the absolute value of the field intensity or power of signals arriving at that receive antenna.October 25, 2019 at 11:37 am #113210radio8zSenior Moderator
Total posts : 247
Not knowing anything about this specific device I will make a general comment about spectrum analyzers applied to Part 15 devices.
For AM transmitters, an analyzer most likely will detect harmonics and spurs on the signal but, as Rich mentioned, the absolute amplitudes are not guaranteed to be accurate, but amplitudes relative to the carrier are possible. For example, I checked the signal from my home brew AM transmitter using an oscilloscope with a FFT function and determined that the harmonics were sufficiently below the carrier (dBc) to meet Part 15 requirements and likely not to cause interference. I also observed an unexpected spur at one half the carrier frequency which should not be there and traced the trouble to a poor PLL filter design which I fixed.
I used a 100 MHz bandwidth digital scope with a properly compensated probe for this measurement and since the AM signal and harmonics were well within the instrument’s bandwidth I have confidence that the relative readings are correct.
That being said, using this scope on the FM band would not be appropriate since the frequency is above the scope’s bandwidth. Despite this, the FFT does show the carrier and harmonics but there is no confidence in the results since the instrument is not being properly used even though harmonics were being displayed.
The best I can do is caution anyone using a scope FFT or spectrum analyzer is to carefully consider that the device is being used properly. Watch out for self resonances in the pickup system and aliasing due to improper sampling frequency as examples which are especially important at VHF frequencies.October 29, 2019 at 6:20 am #113222
A few weeks ago, I proposed the use of an SDR receiver as a quick-n-dirty harmonics detector — it won’t scientifically tell you how many dB down your spurs may be, but it will show if you are slobbering all over the spectrum in places other than your chosen frequency.
Is anyone actually doing this with an SDR; either a low end RTL type or a better unit? I’d be interested to know what kind of success you are having.October 29, 2019 at 7:54 am #113224
Looked at this as I didn’t know what an SDR receiver was and saw that this is a receiver that uses your computer for sound like a scanner and also displays on your computer screen something similar to a spectrum analyzer. Plugs into a USB port. They are also cheap and have an antenna for over the air signal receiving. Maybe this would be the poor mans way of doing it. A better one may display the received frequencies. An item to consider.
But how would I know what I see is coming from my transmitter and not some other signal or interference of some kind?
Are you using this?November 1, 2019 at 7:38 am #113248
>>> how would I know what I see is coming from my transmitter and not some other signal or interference of some kind? <<<
Mark, you can cycle the power on your xmitter and observe the computer monitor. If the suspected peaks disappear on power-down but resume when you re-energize your RF gear, you can see it is being caused by you.
Before firing things up, calculate what your anticipated harmonics might be. It will make it easier to find problems if you know where they may happen. Say you are broadcasting on 1600 AM; watch for spurs at 3200 (your F x 2), 4800 (your F x 3), 6400 (your F x 4) etc, on up the line. Also, watch for unusual spurs to randomly occur elsewhere inside the AM band — it might be your frequency “beating” against an existing station in town. Your transmitter antenna may actually receive that signal, mix the incoming frequency with yours and rebroadcast the product, creating a mutant harmonic that could step on some other station in your market.
If you cant hack the price of a pro spectrum analyzer – and few of us can – this will at least put you in the ballpark.November 1, 2019 at 2:06 pm #113250
Good to know. These SDR things which I never knew about look good for $25-40 dollars.November 2, 2019 at 7:38 am #113252
Shop carefully. Some of the lesser expensive devices started out life as the guts from TV tuners and as such do not start coverage until close to 22 MHz; way out of the range of general communications.
You’re going to want one starting way lower that covers 300 kHz to 30 MHz.
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