Spectrum analyser oscillator choice

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Anonymous
Spectrum analyser oscillator choice 2017-03-15 09:49:18 Post No.
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Spectrum analyser oscillator choice Anonymous 2017-03-15 09:49:18 Post No. [Report]

Hello,
I am currently doing a school project which is making a spectrum analyser (700MHz to 2500MHz). For this, I chose to make a double superheterodyne receiver with the first IF filter of 86.85 MHZ and the second of standard 455kHz. The first oscillator will be made with a DDS and I don't know how to make the second one as it must be very precise in frequency which is 86.395MHz (no more than +/-5kHz in the wors case). I planned to use a Colpitts oscillator with a quartz of a frequency which would be close to the 86.395MHz but I couldn't find any (I live in Europe). Would you happen to know any solutions to my problem?

My circuit is exactly the same as the one which is posted on the picture but I don't have any amplifier which is after the first IF filter.

I bet that there isn't any other way of getting rid of image frequency than using a double superheterodyne. In my case the spectrum is so large that anyway I will have to deal with it.

Anonymous 2017-03-15 09:53:08 Post No.1145340
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Anonymous 2017-03-15 09:53:08 Post No.1145340 [Report]

>>1145336
If you use a tcxo combined with a pll you can get in the 5khz range of accuracy at that frequency.

Anonymous 2017-03-15 09:57:32 Post No.1145346
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Anonymous 2017-03-15 09:57:32 Post No.1145346 [Report]

>>1145340
In fact I think a tcxo isn't even necessary to get into that accuracy range. But why don't you use a more easily dividable frequency? Like 70Mhz or something? Also if you don't know this link yet you'll like it: http://lea.hamradio.si/~s53mv/spectana/sa.html

OP 2017-03-16 05:15:40 Post No.1145719
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OP 2017-03-16 05:15:40 Post No.1145719 [Report]

>>1145346
>>1145340

The problem with an oscillator is that I don't know how to adjust the frequency so that it is the desired one (86.395MHz). I chose this FI frequency because I found a filter with a very good quality factor and this is very important when you build a spectrum analyser.

OP 2017-03-16 05:17:49 Post No.1145720
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OP 2017-03-16 05:17:49 Post No.1145720 [Report]

Also, I forgot to say that in the circuit (before the first amplificator) I have put a high-pass filter to get rid of a part of Image Frequencies.

Anonymous 2017-03-16 07:15:50 Post No.1145768
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Anonymous 2017-03-16 07:15:50 Post No.1145768 [Report]

>>1145719
Like he said, use a PLL. Or another DDS. Or shift your second IF.

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