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So I have recently been interested in Radio electronics, and I have been looking around to buy a HAM radio transmitter,but they tend to be very expensive (and you learn nothing by just buying it). So I came up with an idea, I do not know if it will work with what I have planned but that is why I am asking. my circuit diagram will tell you what you need to know.

The computer does all of the modulating with PureData (AM, FM etc.) at 10 KHz and it pipes the signal out into the mixer, where a signal from a signal generator will up convert the signal to the desired frequency. I know i didn't use the correct symbol for the filters but I didn't want to draw out the whole thing.

Would this work? If not what would it take to make it work?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Yes, it's called software-defined-radio, and there's a lot of information about it out there. \$\endgroup\$ Commented Oct 23, 2014 at 20:47
  • \$\begingroup\$ Get at least a "General Class" license first. \$\endgroup\$ Commented Oct 23, 2014 at 21:00
  • \$\begingroup\$ I am working on getting my license. The transmitter wouldn't have enough power to go beyond the 60m limit imposed by the FCC. Also would the multiplier make a negative freqnecy once the generator passes 10 KHz? and would it make it through? If so is there a way I can stop it? \$\endgroup\$ Commented Oct 23, 2014 at 21:05
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    \$\begingroup\$ There are lots of sound-card based SDR ham rigs out there which solve these problems (mostly using IQ/image reject mixers), many of the designs published if you want to build parts of them or make changes yourself - see arrl.org/software-defined-radio \$\endgroup\$ Commented Oct 23, 2014 at 21:19
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    \$\begingroup\$ Your layout is not feasible. First there is no reason to bandpass filter the output of the signal generator. It is already a single frequency sinewave. Second, the BPF at the output of the mixer must be able to filter out the unwanted components of the mixer. If, for example, your desired carrier frequency is 10 MHz and you create an AM signal at 10 kHz,the filter must separate the signal at 10.01 MHz from the signal at 9.99 MHz. That is not a simple filter. You need to re-think your approach. \$\endgroup\$ Commented Oct 23, 2014 at 21:42

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That would work, as the comments correctly noticed: This is a software defined radio. However, unless you plan to generate IF with your sound card (which you most likely aren't doing, since sound cards, even ones that support 192kHz sampling rate, aren't frequency flat for a large range), you will only be able to generate carrier-symmetrical spectra with a real-valued baseband signal (for the ham in you: no SSB!); what you want is to be able to mix up one signal with a cosine @ f_carrier, and one with a sine @ f_carrier (notice the phase shift); that would be called IQ mixing. You can actually do that, and there are commercial, non-commercial and half-commercial solutions out there that implement that for you.

Now, a sound card's bandwidth is not really large, so as soon as you start investing yourself much in this, you might find it unsatisfactory, but for a start, what you're planning is definitely possible.

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  • \$\begingroup\$ This design is generating an IF (or at least inspired by a hand waving sense of those that do) rather than a baseband signal, but the IF is only 10 KHz. As you note, systems which use IQ/image reject techniques are workable, and some do use low non-zero IF's to avoid sound card issues at DC. \$\endgroup\$ Commented Jan 11, 2015 at 16:19
  • \$\begingroup\$ @ChrisStratton : I interpreted the "generates at 10kHz" as in "at a sampling rate of 10kHz", but then the filters wouldn't make sense... \$\endgroup\$ Commented Jan 11, 2015 at 16:22
  • \$\begingroup\$ No, that part should be taken literally - 10 KHz IF to fit in the bandwidth of a 44.1/48 KSPS system. It helps to have read the same sort of design articles as the poster was clearly inspired by. \$\endgroup\$ Commented Jan 11, 2015 at 16:24

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