Signal/Function Generator

[lk]

Greetings,

i found this schematic for a signal/function generator, it looks very straight forward to build. But i have some questions/thoughts  about it.

would it make sense to play around with the values of R7, with a pot to change the frequency, perhaps with a multiswitch for and Caps in Parallel for C3, for further control?


And maybe a CMOS 555 for higher frequency.

http://www.overunity.com/6857/diy-simple-signal-generator/


Or does someone have a better idea? I don't really have any requirements for this type of device, but this one looks fun to make.

-lk

[nctnico]

While searching for the venerable XR2206 I came across this chip (AD5932):
http://www.farnell.com/datasheets/67758.pdf

[poorchava]

I've done a project based around XR2206. Above around 200kHz sinewave's distorted. Sometimes you can also spot disturbances on the positive and negative peaks of the sinewave (guess: internal sine shaping circuitry).

Very interesting option are DDS chips from Analog Devices. They are often very simple to use. You can buy signal generators based on those on ebay for a few bucks (eg. http://www.ebay.com/itm/New-AD9850-DDS-Signal-Generator-Module-0-40MHz-Test-Equipment-/170783661135?pt=BI_Signal_Sources&hash=item27c37fdc4f).

I've also been experimenting with http://www.intersil.com/en/products/timing-and-digital/dsp/dsp-synthesizers/ISL5314.html, although interfacing is a bit of a PITA because it's parallel + serial or parallel only (serial only can't be used).

[Rerouter]

personally i have simply been planning to buy an alanog devices DDS module, hook it up to an arduino module, and then add my front end and power supply and be done with it,

however before that i settled on building a voltage controlled oscilator, as its much easier to get wider frequency ranges over them, the pot degrading effects it less, and the number one key thing, the amplitude does not vary with frequency , it gives you square and triangle, then using a multistage triange to sine converter to get the same constant amplitude out of it, (only diodes and resistors) one can get good square tri and sine at constant amplitude over a decent range with only a quad op amp (2 for VCO, 1 for output buffer and other possibly as input buffer or secondary output) with the tri to sine being the largest component wise thing of the bunch

if you start using some of the integrated ones you will find hell from integrators killing your amplitude and having to use all manner of correction circuitry,

[Spawn]

Or does someone have a better idea? I don't really have any requirements for this type of device, but this one looks fun to make.

I got some schematics from Elector (Elektuur in Dutch version) but those are copy righted I believe, so I can’t post them. Does anyone know what Elektors policy is about these schematics?

But there is also one on ebay, it is pre build and 10MHz it also can be driven by a PC with USB. It looks really nice but I am not sure how clean the signals are, it has good specs if I may believe what they say there:
AE20125 10 MHz Wobbel-DDS-Funktionsgenerator

[poorchava]

and then add my front end

Heh, in my experiments with ISL5314 it turned out that building a digitally controlled funtional frontend which has linear characteristics up to 10MHz and swings +/-10V is at the least NOT EASY.

you need a well designed filter (preferably multipole Butterworth PI-type), voltage/digitally controlled amplifier (if your DDS chip doesn't have internal amplitude adjustment), voltage amplifier (at this frequency probably 2 stage), DC offset circuitry, and buffer.

I've used LMH6505 variable amp for amplitude control and LM7372 / LM7171 for amplifiers. Sadly, for flat amplitude characteristic you need to use much higher GBP amplifiers than it would seem to operate in flat area of gain/frequency graph.

[nctnico]

Gain bandwidth product is often for small signal purposes. Another number you'll need to look for is slew rate. Since the slew rate of a sine wave is at its maximum in the zero crossing you can determine the slew rate for a given amplitude A (peak) and frequency (Hz) with the following formula:

SR= 2*pi * f * A

So if you want to build a function generator which can output 10Vp-p in a high impedance load then A=5 f=10MHz ->
2 * pi * 10M * 5=315MV/s. To make numbers more managable the slew rate is often expressed as V/us so in this case you'd need to look for an opamp capable of 315V/us.

[ampdoctor]

The bigger question is does the OP want something that just works or something that works well.  As one of the previous posters mentioned, designing a high quality signal generator is NOT trivial.

Having said that, if you just want to slam something together that works so you can understand oscillator circuits and maybe use it for very basic testing, I might suggest doing a bit of digging for weinbridge and quadrature oscillators.  Also check out sound.westhost.com   There are some very good designs there that might be far more useful than that 555 circuit in the original post.

[notsob]

I built this one, and put it into a jiffy box

http://www.electronics-lab.com/projects/test/013/index.html

had a couple of shorts on the pcb ( I probably under etched it ), but reasonably easy to make