How do digital function generators generate precise frequencies?

Question

I was designing an electronic circuit that requires precise frequency control when I realized how much I don't know about the frequency synthesis itself.

For example, when I take my low-end 2-channel arbitrary waveform generator, I'm able to set one of the channels to a sinewave with a frequency of 6.00000000 MHz and the other one to 5.99999999 MHz. When I multiply those two signals and filter out the higher frequency what I get is a precise sine signal with a frequency of 10 millihertz. So the period difference between the two waveforms should be the astonishing 278 attoseconds. That would require a 3.6 PHz of bandwidth just to distinguish the difference over one period! And yet my AWG is able to generate any frequency from 0 to 6MHz with 10mHz precision and with the thermal and phase drift accuracy of a crystal oscillator. (I've checked with an oscilloscope and the frequency is stable up to a 6-digit precision, can't check any further).

So it happens to be beyond my understanding how this can be accomplished (especially under $60). Using frequency division/multiplication would be unthinkable at that bandwidth and any VCO I know of has far more jitter and will eventually drift too much.

Am I missing something? Does any of you engineers know what's happening inside such cheap digital generators?

Answer 1

That sort of generator uses DDS, or Direct Digital Synthesis

It keeps track of the phase of the required output in a register, and outputs the cosine of the phase.

To use nice round numbers, let's say you have a 10 MHz clock, and want to generate 1.000000 MHz. Each 100 ns clock cycle, your phase register gets incremented by 0.1 of a cycle. If you want to generate 1.000 000 001 MHz, then each clock you'd increment by 0.100 000 000 100 of a cycle.

Frequency resolution is cheap, you just give your phase register enough LSBs. In this case, mHz resolution with a 10 MHz clock, you'd need 10 digits if the sums were done in decimal, or as more likely at least 34 bits if they were done in binary. With binary arithmetic, sometimes the master clock will be a nice 'binary' frequency to get nice Hz resolution. With cheaper generators the designers often just throw another load of LSBs at the phase register, and have very fine resolution of a nasty binary Hz fraction, which is 'fine enough'. My low cost generator has μHz!

Only the top few MSBs, typically 10 to 16 depending on the quality (cost) of the output, get to drive the phase -> cosine converter. As a result, the phase at your output is never more accurate than a few parts per thousand. However, the output phase error is non-cumulative, and over time, the waveform will keep the correct average rate of change of phase (aka frequency) with respect to the clock, and to any other channels being generated by the same clock.

So the frequencies are precise, the phases approximately so. It's this approximation to phase that means that DDS outputs have phase noise spurious outputs. These cannot be seen on an oscilloscope, but will be visible on any modest spectrum analyser.

Answer 2

You can teardown your cheap function generator, I am certain you will find a DDS chip or a FPGA doing the same. However it is possible to increment/decrement frequency in range of milihertz but the precision is relative to XTAL precision. So you are able to get a sinewave of mHz when you subtract two generated frequencies from the same XTAL. If you would have a separate function generator, you would notice difference in frequencies of two sources.

The solution is to have a reference clock as the input of multiple function generators. GPSDO (GPS Disciplined Oscillator) can be used for precise synchronization.

Answer 3

Neil and Marko have already responded in great detail. From my superficial point of view: the generator contains a DAC (or several DAC channels) and performs PCM wave playback. The design tradeoffs are approximately:

• how stable a reference clock you can get
• DAC resolution (bit depth) and sampling rate are factors (and a partial mutual tradeoff)
• computing power - how many cosines (or sines) a second you can calculate, or approximate using a look-up table, down to what amplitude-wise resolution
• how smooth (and adaptive) an analog filter you can build for the DAC output

Obviously money is the overall envelope in all those items.

Answer 4

That would require a 3.6 PHz of bandwidth just to distinguish the difference over one period!

You misunderstood that. I can set an average frequency to a very high precision without a high bandwidth. For distinguishing frequencies precisely, just do the exercise which you did over 1000 seconds, and suddenly the requirement goes down. What you need to look at is the phase noise or jitter of your source, whic you did not specify - it is actually very likely that, would you measure the jitter of your cheap source you actually would end definitely up above the 100ps range - I once use a super high-end AWG (think about a one family home in price) and got after careful calibration an estimated jitter of 20ps.

You for signal synthesis have 2 ways:

• DDS (like @neil_UK excellent answer) for low frequencies, high relative bandwidth and arbitrary Waveform generation - remark: DDS is also pretty cool for fast generation of sine signals which change the frequency instantaneously, but depending on the application the filtering can be tricky.

• PLL for high frequencies, lower noise and sine waves and/or genration of harmonics: in such a setup precision you always use your VCO in a phased locked loop. Here drift and long-term noise don't matter. Example high-bandwidth synthesizer: https://www.ti.com/product/LMX2820

Both methods use a fixed frequency reference (which in turn may be locked to an external signal like GPS or the 32kHz time signal in France)

Edit: typically achieving a jitter in the ps range fro the reference signal is possible with of-the-shelf components: e.g. see the specs of: https://www.orolia.com/products/atomic-clocks-oscillators/gxclok-500-gps-gnss-ocxo-clock-module