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Design >> RF Design >> RF transmitter questionss
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Message started by Giorgos on May 18^th, 2013, 7:38am

Title: RF transmitter questionss
Post by Giorgos on May 18^th, 2013, 7:38am

http://i1284.photobucket.com/albums/a578/fovos1/P5181092_zps5ee33fa8.jpg

Hi experts. I am new in the RF-Field. I am trying to make a RF-Transmitter .. at the time i have a block diagram like this one of the picture. There is a problem with the output o first mixer..the input 10khz baseband is so near to L.o freq and the filter cannot cut all the interferences... so maybe i must upconvert the 10khz to higher with one mixer more and the use the mixer ZAD6 ..but i thinkg i will have problem with filters bandwidth... maybe the solution of SSB is better after mixerzad?

Looking forward for your reply! :)

Title: Re: RF transmitter questionss
Post by RFICDUDE on May 20^th, 2013, 6:59pm

The 10kHz IF is a problem because the 30MHz filter does not provide much rejection of the 30.020MHz sum mixer product (fin+flo).

You would, at least, want the sum mixer product to be in the stopband of the 30MHz filter.

So is it possible to have the transmitter put out fc=70kHz and tune the LO to 30.07MHz?

Title: Re: RF transmitter questionss
Post by Giorgos on May 23^rd, 2013, 5:29am

I cannot make the output 70 Khz. i have one output for the DSP and it is an analog 16QAM siganl. Can i use 90 degrees splitter and then a I/Q Modulator.?

Maybe if i have I and Q output signal at my dsp (baseband) what i do next to upconvert it? and I/Q Modulator?> :-/

Title: Re: RF transmitter questionss
Post by RFICDUDE on May 23^rd, 2013, 5:25pm

Hmm, how fast is the DSP?

If it is not fast enough to raise the output to 70kHz then I guess you will have to find a way to split the output and shift one of the two paths by 90 degrees before using an I/Q modulator to implement a SSB modulator.

I'm not aware of a commercial 90 degree phase splitter at such low frequencies. You will have to implement the 90 degree IF phase splitter yourself. A RC polyphase phase shifter should be implementable with discrete circuit components. Although you will need two mixers and a phase splitter for the the LO signal too.

Title: Re: RF transmitter questionss
Post by Giorgos on May 24^th, 2013, 12:02pm

The dsp cannot have ouput up to 15khz. but i tried today and i make I AND q OUTputs for dsp. the outputs are analog baseband signals with bandwidth of 8.5khz. then i will put them in a i/q modulator right?

what would be the result of the modulator? :)