Electronics > Beginners

DIY Function Generator

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Kleinstein:
Fixing the output to get closer to a 50 Ohms output is easy: replace the 51 Ohms behind the OP with 100 Ohms and adjust the filter to 100 Ohms. These two resistors are effectively in parallel to determine the output impedance. Having the filter at the output is not such a good idea.

The output amplitude is also limited by the OPs current limit and the maximum slew rate. For the TLE2081 the slew rate limit is about 10 V_ss at 1 MHz. Which would result in 2.5 V_ss to a 50 Ohms load. So 2 V_ss is already rather close to that limit. There is also a current limit: 1 V at a 50 Ohms load already needs about 30 mA from the OP (with 100 Ohms resistors). So it needs an better OP (or two) to get a higher amplitude.

There is some filtering: the one a the output and the limited bandwidth of the OP also acts like a low pass filter. Still only 4th order with low Q and thus a limit to something like 1-2 MHz despite of a limit of about 10 MHz from the DDS chip.

The amplitude adjustment via the reference input of the DAC in the DDS chip is good for fine adjustment, but not working well for low amplitudes, as this will increase the DDS internal DAC errors. Having a 12 Bit DAC to control does not help here. It is just that one should not go down below about 10% of the maximum for the ref input.

So an additional switchable 20 dB attenuators would really help to extend the amplitude range, without adding to much noise / distortion. For a 50 Ohms signal, relays are not bad and at only 1-5 MHz there is no need for special RF relays. With fine adjustment via the ref input one can get away with just 1 or 2 relays (or maybe just mechanical switches) for steps of 20 dB and maybe another 10 or 20 dB. A passive divider at the output is also the best way to get a really low noise and SWR, at least for the low amplitude setting.

rstofer:

--- Quote from: ziplock9000 on January 17, 2017, 09:42:56 pm ---
--- Quote from: Kleinstein on January 17, 2017, 01:12:06 pm ---External RAM to sample data can be rather fast, but this would need something like an FPGA / CPLD to generate the address sequences. To have a good arb.- generator one might also needs a more or less fine adjustable clock one when to update the clock / data - this is different from an DDS where the clock is fixed.

--- End quote ---
Ah balls. It's looking more and more like even a modest 10Mhz range arb gen is out of range and better to just buy one :/

--- End quote ---

Absolutely!

There are some fairly inexpensive AWGs that can produce some very nice results.  Something in the Siglent or Rigol lines will be just fine.  Even the very cheap FG085 can do something useful.  It a very neat gadget up to a 200 kHz (sine).

Within its frequency range of 12 MHz at +- 5V, the Digilent Analog Discovery can do some very impressive work.  For $279, this device is totally underappreciated.
http://store.digilentinc.com/analog-discovery-2-100msps-usb-oscilloscope-logic-analyzer-and-variable-power-supply/


ziplock9000:

--- Quote from: rstofer on January 17, 2017, 10:02:04 pm ---
--- Quote from: ziplock9000 on January 17, 2017, 09:42:56 pm ---
--- Quote from: Kleinstein on January 17, 2017, 01:12:06 pm ---External RAM to sample data can be rather fast, but this would need something like an FPGA / CPLD to generate the address sequences. To have a good arb.- generator one might also needs a more or less fine adjustable clock one when to update the clock / data - this is different from an DDS where the clock is fixed.

--- End quote ---
Ah balls. It's looking more and more like even a modest 10Mhz range arb gen is out of range and better to just buy one :/

--- End quote ---

Absolutely!

There are some fairly inexpensive AWGs that can produce some very nice results.  Something in the Siglent or Rigol lines will be just fine.  Even the very cheap FG085 can do something useful.  It a very neat gadget up to a 200 kHz (sine).

Within its frequency range of 12 MHz at +- 5V, the Digilent Analog Discovery can do some very impressive work.  For $279, this device is totally underappreciated.
http://store.digilentinc.com/analog-discovery-2-100msps-usb-oscilloscope-logic-analyzer-and-variable-power-supply/

--- End quote ---
What I was hoping to build looks similar to the FG085 but with a PC/Windows based front end for versatility. Thanks

Brutte:
I also like the ideas of a 1Gs 8-channel 14-bit ARB, it would be very interesting to have it on my desk but to be fair I do not have $1000 and anything that sticks beyond 1MHz analog bandwidth is beyond my field of interest. I need a true arbitrary waveform generator (and not sin+square), for embedded, testing and debugging. Current micros are clocked in the range of 50-100MHz, they cannot cope with signals from >1MHz bandwidth (even a stupid PID @ 100kHz is a challenge on a 100MHz micro) so I really have no interest in a 5MHz triangle waveform.
 

--- Quote from: ziplock9000 on January 17, 2017, 10:54:10 pm ---What I was hoping to build looks similar to the FG085 but with a PC/Windows based front end for versatility. Thanks
--- End quote ---
FG085 looks too limited in functionality for me but the bandwidth is ok-ish.

I need:
-a sine sweep, from A to B, in logarithmic scale. Two decades at least. 1Hz:100Hz, 2Hz:200Hz, 5Hz:500Hz,..., 5kHz:~500kHz
-random noise generator, with replay (so not that random)
-trapezoidal shape (for servo control)
-ramp with reset output (for integrating opamp with zeroing)
-external output trigger, input trigger and clock (to synchronize that with external gear for debugging, or to cascade/parallel with second arb generator),
-incremental encoder/pushbutton emulator with switching noise (for testing noisy encoder/button interfaces)
-hardware keyboard and standalone operation not required
-USB interface

rstofer:

--- Quote from: ziplock9000 on January 17, 2017, 10:54:10 pm ---
--- Quote from: rstofer on January 17, 2017, 10:02:04 pm ---
--- Quote from: ziplock9000 on January 17, 2017, 09:42:56 pm ---
--- Quote from: Kleinstein on January 17, 2017, 01:12:06 pm ---External RAM to sample data can be rather fast, but this would need something like an FPGA / CPLD to generate the address sequences. To have a good arb.- generator one might also needs a more or less fine adjustable clock one when to update the clock / data - this is different from an DDS where the clock is fixed.

--- End quote ---
Ah balls. It's looking more and more like even a modest 10Mhz range arb gen is out of range and better to just buy one :/

--- End quote ---

Absolutely!

There are some fairly inexpensive AWGs that can produce some very nice results.  Something in the Siglent or Rigol lines will be just fine.  Even the very cheap FG085 can do something useful.  It a very neat gadget up to a 200 kHz (sine).

Within its frequency range of 12 MHz at +- 5V, the Digilent Analog Discovery can do some very impressive work.  For $279, this device is totally underappreciated.
http://store.digilentinc.com/analog-discovery-2-100msps-usb-oscilloscope-logic-analyzer-and-variable-power-supply/

--- End quote ---
What I was hoping to build looks similar to the FG085 but with a PC/Windows based front end for versatility. Thanks

--- End quote ---

For $35 and a couple of hours work, you can just buy the FG085 and leave it laying on your workbench.  No USB cables, no coupling with the PC, etc.
https://www.amazon.com/Function-Generator-DIY-Tech-FG085/dp/B00C5UO8U6

If building an AWG is a learning project with value beyond just making signals, great!  Building something is good.  OTOH, if the goal is to have some kind of signal source for other projects, maybe $35 isn't such a bad idea.  Besides, you still get some soldering practice and that isn't all bad.

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