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Modern radio

Started by glen herrmannsfeldt May 6, 2015
From Wikipedia [[Amplitude Modulation]]:

   "In modern radio systems, modulated signals are generated via 
    digital signal processing (DSP)."


While this sounds right, how much DSP is really done in radio
transmitters?  Are they now using DSP modulators in place of
mixers for AM and FM broadcasting?

Even more, is there a reference for it?

-- glen
On Wed, 06 May 2015 21:29:08 +0000, glen herrmannsfeldt wrote:

> From Wikipedia [[Amplitude Modulation]]: > > "In modern radio systems, modulated signals are generated via > digital signal processing (DSP)." > > > While this sounds right, how much DSP is really done in radio > transmitters? Are they now using DSP modulators in place of mixers for > AM and FM broadcasting? > > Even more, is there a reference for it?
They surely do. Here's a press release from 1999 from Harris, saying that they had sold two thousand FM digital exciters. <http://harris.com/view_pressrelease.asp?pr_id=163> Some of the early ones were quite bad, e.g. DDS at some low IF, followed by an analog upconversion, typically with poor image suppression. Out of band spurious would actually be worse than that from an equivalently priced analog exciter. You can still buy low cost or kit ones that are like that. Academic references might be hard to find, but I suggest checking the datasheets on the manufacturers' web sites. Disclaimer: I have never worked with AM, and I haven't worked with FM since the '90s. Allan
glen herrmannsfeldt  <gah@ugcs.caltech.edu> wrote:

>From Wikipedia [[Amplitude Modulation]]: > > "In modern radio systems, modulated signals are generated via > digital signal processing (DSP)."
>While this sounds right, how much DSP is really done in radio >transmitters? Are they now using DSP modulators in place of >mixers for AM and FM broadcasting?
Usually there are both. A radio system (which may be more inclusive than just a radio transmitter, depending on terminolgy) might create a modulated digital signal at either baseband, or low-IF, which then is converted to analog through a DAC or DAC's, and then mixed up to RF by one or more analog mixer circuits. When I say "might", I'd guess 90+ percent of the time. Steve
Steve Pope <spope33@speedymail.org> wrote:

(snip, I wrote)
>>From Wikipedia [[Amplitude Modulation]]:
>> "In modern radio systems, modulated signals are generated via >> digital signal processing (DSP)."
(snip)
> A radio system (which may be more inclusive than just a radio > transmitter, depending on terminolgy) might create a modulated > digital signal at either baseband, or low-IF, which then > is converted to analog through a DAC or DAC's, and then > mixed up to RF by one or more analog mixer circuits.
OK, so no digital mixers, then? (Or not so many?) It wouldn't seem hard to do at AM radio frequencies. -- glen
glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:

> Steve Pope <spope33@speedymail.org> wrote: > > (snip, I wrote) >>>From Wikipedia [[Amplitude Modulation]]: > >>> "In modern radio systems, modulated signals are generated via >>> digital signal processing (DSP)." > > (snip) > >> A radio system (which may be more inclusive than just a radio >> transmitter, depending on terminolgy) might create a modulated >> digital signal at either baseband, or low-IF, which then >> is converted to analog through a DAC or DAC's, and then >> mixed up to RF by one or more analog mixer circuits. > > OK, so no digital mixers, then? (Or not so many?) > > It wouldn't seem hard to do at AM radio frequencies.
Aren't there plenty of DACs that run in the hundreds of MHz range these days? Like the TI DAC3282: http://www.ti.com/product/dac3282 Cost doesn't seem prohibitive, either. -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
glen herrmannsfeldt  <gah@ugcs.caltech.edu> wrote:

>Steve Pope <spope33@speedymail.org> wrote:
>(snip, I wrote) >>>From Wikipedia [[Amplitude Modulation]]:
>>> "In modern radio systems, modulated signals are generated via >>> digital signal processing (DSP)."
>(snip)
>> A radio system (which may be more inclusive than just a radio >> transmitter, depending on terminolgy) might create a modulated >> digital signal at either baseband, or low-IF, which then >> is converted to analog through a DAC or DAC's, and then >> mixed up to RF by one or more analog mixer circuits.
>OK, so no digital mixers, then? (Or not so many?)
>It wouldn't seem hard to do at AM radio frequencies.
Agreed. At FM frequencies, it would seem to require a high speed DAC and at least an FPGA for the logic, and then you would need a bunch of analog circuitry anyway to beat down spurs and whatnot. But perhaps the Harris RF "Digital Exciters" mentioned upthread do all this. Steve