Technical Topics > Part 15 AM and FM Station Operation
Talking House Transmitter - some spectrum plots
ChrisSmolinski:
I was curious about the audio bandwidth of the Talking House transmitter, so I did some tests. It is easily 20 kHz, even a bit more. Actually I was generating audio on the computer, so I was limited to 22 kHz. Who knows how wide it really is.
Lots of 60 Hz and harmonics crud around the carrier frequency.
Baseline, TH transmitter off:
Carrier on:
White noise:
1000 Hz audio tone, I am fairly confident that I was not feeding in too high of a level and over-modulating:
Dave Richards:
For a transmitter whose original intended purpose is simply to play recorded loops/announcements for realtors, and similar simple informational broadcasts, I can't say I'm too surprised. It suffices for that purpose and, being Part 15, is not subject to the audio bandwidth restrictions for licensed stations. Anyone who is keen to limit the audio bandwidth could put an LPF or BPF at the audio input. On the other hand, if you are not required to limit your audio bandwidth by law, why do so? I doubt there'd be a perceptible increase in range by doing so.
It would be nice to be able to get rid of some of the 60 cycle hum though.
Brian:
Was that into a dummy load?
I suspect the spectrum will be narrower into an antenna.
ThaDood:
A couple of improvements that can be done to the Talking House / I Am Radio transmitter is to GND the master OSC crystal to chassis GND. That helps to reduce, but not eliminate, the PM (Phase MOD.), coming from that. Then, to smooth out the quality of the modulation and reduce the harmonics, run several GND braids from main PC board GND to the chassis. Mine sounded much cleaner, louder, and fuller, after that. No, it will not put out a more powerful carrier, that will stay a dead-nuts 20-some mW's. But, it will make that TX sound way cleaner. I wish that I could totally eliminate that PM in it. On a C-QUAM receiver, it drives that nuts going in and out of stereo. You can also hear that PM in SSB mode. Other-wise, my I Am Radio TX is a nice sounding AM mono TX.
ChrisSmolinski:
Thanks for the grounding tips, ThaDood!
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