Signal Generator Using DDS

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mrmonteith
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Signal Generator Using DDS

Post by mrmonteith » Fri Feb 05, 2016 7:41 pm

I didn't want to hijack the code snippets discussion on the AD9850. But wanted to share some things I'm working on regarding using a DDS signal generator and some findings.

As a new ham operator I've been thinking of a few projects involving radio, rf, and micros. One of the projects uses a QRP Pixie. The Pixie is a small transceiver that you can get as a kit for less than $4. It works on the 40M band, which is ~7.025-7.125Mhz. Of course for a $4 transceiver you can't expect many features. The one limitation right off the bat is it comes with a 7.023 crystal, which you're stuck with if you solder it in. The 7.023 is not a frequency a Tech can broadcast on here in the states. The other option is mount a 2-in screw terminal and then you can swap out crystals. But then you're limited to a finite set of crystals and could add up in costs and a pain to change frequency.

Another alternative is to use DDS signal generator to feed the circuitry whatever frequency you wish, within the range of the transceiver circuitry. Basically leaving off the onboard oscillator components and feeding the DDS signal directly into the board. This of course gives complete control over the frequency range.

Here are several DDS signal generators I'm looking at:
1. AD9833 - 0-12.5 Mhz
2. AD9850 - 0-50 Mhz
3. AD9851 - 0-70 Mhz
4. Si5351 - 0-150 Mhz
5. Si5351A - 0-290 Mhz

About all of these have breakout boards or you can buy complete units with display, rotary encoders or buttons. There are usually plenty of examples and code out there for all of the devices. I personally have the AD9850, Si5351, and the AD9833 that I bought off E-Bay. the Ad9833 is on it's way.

Right now I've been playing with the AD9850 making sure I can wire it up, get a signal output, and look at the signal. Seems to work great. Without a spectrum analyzer I can't tell the harmonics. My next step is to build my 2nd QRP Pixie kit and leave off the oscillator components and feed it the DDS output of the AD9850 and see the results. This will be fine for testing my theory of using it on the QRP and listen to whatever frequency I want in the 40m band.

A few notes to mention that probably applies to all of these. I've seen some tests on the outputs of a couple of these and due to several reasons they have some bad harmonic output. Bad as in if you were using them in a transmitter you would seriously check the output to make sure you're not splattering other frequencies. The best suggestion is to use low pass filter or filters in your designs to get the levels of harmonics down to a manageable level.

I work 5 days a week and weekends are somewhat limited too. So as I work on these projects I'll be glad to give updates. I also have a project that's been a priority that I need to get done first. Mainly I wanted to share ideas if anyone needed some sort of signal generator for whatever purpose you need them for whether a function generator or a VFO for your radio. But if you do need any information on any of these I keep links on stuff I run across for reference such as sample code, datasheets, special notes, applications, etc. feel free to private message me. I'd rather you have over abundance of information than not enough and struggle.

Michael

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RogerClark
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Re: Signal Generator Using DDS

Post by RogerClark » Fri Feb 05, 2016 8:36 pm

Hi michael

I had a play with the AD9850 a while ago, and even managed to use it as an FM transmitter on the 100MHz band by continuously sending changes to the frequency in response to the ADC input.

Of course, its not supposed to operate above 40mhz, and the output is very weak. But as you are not allowed to broadcast on 100MHz and I had to put the radio very close to pick anything up.

But it worked better than Id expected!

I think you have one of those Tv dongles, which can be used as a spectrum analyser, but you would need an up converter e.g the "ham it up"

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mankan
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Re: Signal Generator Using DDS

Post by mankan » Sun Oct 30, 2016 2:18 pm

I've used AD9850 in a homebrew RF generator, 10dBm output 1-30MHz as well built a AD9851 based PHSNA.

Michael, let me know the status of your build and I'll can provide more info if needed.
/Marcus

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Pito
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Re: Signal Generator Using DDS

Post by Pito » Sun Oct 30, 2016 6:43 pm

PHSNA
What UI for the PC side do you use??
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mankan
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Re: Signal Generator Using DDS

Post by mankan » Sun Oct 30, 2016 7:09 pm

PHSNA.exe downloaded from the Yahoo group. It does its job but can be improved a lot.
/Marcus

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mrmonteith
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Re: Signal Generator Using DDS

Post by mrmonteith » Mon Oct 31, 2016 1:28 pm

I didn't get to play with it a whole lot before I ran across that sI5351. I bought the VFO Signal Generator from QRP Labs that is based on the Si5351. It has a larger frequency range. I just need to box it all up in a case with power. But work has been crazy. Last week I was in Chicago for training, got some this week locally, then back to Chicago again. So haven't touched any projects lately.

Michael

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Re: Signal Generator Using DDS

Post by mankan » Mon Oct 31, 2016 8:22 pm

Si5351 is a nice little module (I have the Adafruit ones, very useful as LO in various places since mixers likes square waves. I've just played a bit with them, this time I just wanted to test that I2C worked on my blue pill. To use as a multiband HF CW transmitter (I've built one of those as well with a AD9850) or a RF gen the DDS are a better choice.
/Marcus

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mikikg
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Re: Signal Generator Using DDS

Post by mikikg » Fri Mar 10, 2017 3:22 pm

Here is one useful screen shot which show frequency response of AD9850 module:
DS2_QuickPrint1-2.png
DS2_QuickPrint1-2.png (68.63 KiB) Viewed 1330 times
Si5351 is very very interesting small device, I build one SDR receiver with it and standalone VFO version both controlled via USB:

http://yu3ma.net/wp/?p=496
http://yu3ma.net/wp/?p=794

Both constructions utilise 8bit PIC MCU, first one with PIC18F14K50 and VFO use PIC16F1455 which is somehow specific and very useful for HF construction because that MCU do not require Xtal for USB connection so it is much "quieter" in RF spectrum than other MCUs (or FTDs) which requires Xtal.

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Pito
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Re: Signal Generator Using DDS

Post by Pito » Fri Mar 10, 2017 5:08 pm

Why do you use single sided pcb?
For $9.90 you may get 20-30 doublesided pcbs with your design at Seeed..

I built a few softrocks in past, also with Si570. Rx comparable with big rigs.
Btw, the latest softrock designs use opto-couplers between Si570 and the mcu in order to reduce noise.

The response of the 9850/1 and others is given as (sin x)/x, imho (no low pass filter).
Then add the response of the low - pass filter.
Then add the response of the o'scope. In your case DS2072 I guess, so flat till 300 :)

As the first poster has indicated the 98xx output is not great (10bit DAC), so for high end I would use 9951 or 9912 (14bit).

The original VFOs output signals with those 7.023Mhz Pixies and RockMite clones are by factor worse than the 98xx ones, you can easily use 98xx.

The only question is the amplitude - the 985x output is something like 200mVpp, the NE612 needs a little bit more, the diode mixer even more, and the PA pre-amplifier much more. Thus you need to amplify.

Long time back (15y ??) I got these modules:
http://midnightdesignsolutions.com/dds60/
They got an onboard 50ohm capable amplifier handy (4Vpp at 50ohm).
I got the modules much cheaper as I had several 9851 in my junkbox already.
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mikikg
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Re: Signal Generator Using DDS

Post by mikikg » Sat Mar 11, 2017 9:45 pm

At least for SDRs based on quadrature de/modulators it is better choice to use oscillators with square wave output, that will produce much precise timing required for /4 divider.
Sinusoidal wave in this case will have some drawbacks because we mix analog and digital signal and input stage of divider (usually 74AC74) will not interpret that signal perfectly well, some times will switch logic at one level, next time on other level (introduce additional jitter) and that produce some problems later in receiver.
This rule also apply for DBMs (diode mixer), with square wave we can switch internal diodes much faster than using sinusoidal wave.

On other side, sinusoidal wave is required for various RF measuring equipment likes VNAs or NAs where we must use it and as much as possible "pure" in wide range of frequency spectrum.

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