Diy spectrum analyzer circuit questions - Page 1
Forum section: electronics / projects

author: sahko123
author_rank: Frequent Contributor
timestamp: October 14, 2016, 09:34:32 pm
content: Hello and thank you for taking your time for checking out this question.I'm creating a spectrum analyser which will be made out of junk bin parts and an oscilloscope(which will use XY mode as a display). I would be using it to test for harmonics in signals and other things. My problem is that I cant find a good VCO circuit to use for the swept tune receiver at the front end. All the other circuit components are ok and will suit my project perfectly. If any one out their can recommend a good VCO(voltage controlled oscillator) circuit the help will be much appreciated. I have done some testing with the most popular VCO circuits with no results.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: October 14, 2016, 10:29:11 pm
content: I have basically the same experience with VCOs: They're hard to build and especially in the lower frequency-area they are really narrowband.If you want to build one, I think it is best to build one that operates at over 1GHz and then use a prescaler to divide the frequency down.Or you use one of the VCO-Modules from ebay. There are some rather good ones available which can be modified to reach much higher frequencies.Another thing to consider: Don't use transistors with a too high Ft. They tend to put out RF-Power all over the place except where you want it - I realized that when I tried to build a 1GHz VCO with a BFP420 (25GHz Ft). The thing was sometimes working, sometimes a noise-source... (see my Spectrum-Analyzer Thread in the Open Source Hardware-Subforum here).You could also use an ADF4351 Module from ebay and create the ramp-signal for the oscilloscope with a PWM-Signal from the controlling microcontroller.

author: sahko123
author_rank: Frequent Contributor
timestamp: October 14, 2016, 10:42:47 pm
content: Quote from: SaabFAN on October 14, 2016, 10:29:11 pmI was thinking more having the ramp signal going into the threshold of an oscillator. in other words as the capacitor charges an opamp measures the voltage and when it reaches a certain voltage there is an output which in turn resets the capacitor back to ground though I am yet to try this method in practice.Thanks for the response.

author: sahko123
author_rank: Frequent Contributor
timestamp: October 14, 2016, 10:46:01 pm
content: sorry but I forgot to mention that the ramp will be the voltage that will be compared against the capacitor but again testing is required

author: Kleinstein
author_rank: Super Contributor
timestamp: October 15, 2016, 10:00:08 am
content: There are many possible variants of spectrum analysers. The classical superhet version uses a relatively high frequency for the VCO. There may be also more than one.However there are also other variations, especially if doing parts in the digital domain.What frequency range and general setup are you planing ?

author: sahko123
author_rank: Frequent Contributor
timestamp: October 15, 2016, 12:07:26 pm
content: I plan to go from as low a frequency as possible(1-10khz) to 200 MHz or so. I plan to have more then one VCO with a different bandwidth to have different ranges. the if filter, attenuator and mixer along with other building blocks are already done. all I need is a versatile VCO to use as the local oscillator.

author: Kalvin
author_rank: Super Contributor
timestamp: October 15, 2016, 12:28:51 pm
content: Alternative to all analog solution would be to use a digital oscillator, like **AD9851-based DDS which can generate sine wave up to 70MHz. Just take an Arduino module and let the Arduino generate the frequency sweep using the AD9851. Use one of the Arduino's PWM output as 10-bit DAC and feed that signal to oscilloscope's X-channel. Then you can easily change the sweep frequency range (like zooming and panning), sweep rate, and perform easily linear and exponential frequency ramps.** Edit: You may want to use some other DDS-chip which can go to higher than 70MHz frequencies as you need to perform up-conversion to the suitable IF frequency. Something like ADF4351 as SaabFAN suggested, or ADF4350. The AD43510/1 will produce square wave output but it should not be a problem with the mixer: You just need to perform low-pass filtering to the input signal before the mixer and bandpass filtering at mixer output anyway.

author: sahko123
author_rank: Frequent Contributor
timestamp: October 15, 2016, 02:16:02 pm
content: I'm making an exclusively analog design and am not considering digital design for the front end.thanks for the response

author: Kalvin
author_rank: Super Contributor
timestamp: October 15, 2016, 04:23:16 pm
content: Here are a few analog spectrum analyzer projects:http://www.qsl.net/n9zia/wireless/appendixF.html#9For example, there is link "A Spectrum Analyzer for the Radio Amateur" 50 kHz to 70 MHz which should help you get going with your own design.

author: sahko123
author_rank: Frequent Contributor
timestamp: October 16, 2016, 11:12:21 am
content: Quote from: Kalvin on October 15, 2016, 04:23:16 pmthanks for the reply i will be sure to check this link outthank you.