Generate signal 1Hz to 1GHz (or maybe even 2GHz) - Page 1
Forum section: electronics / projects

author: perdrix
author_rank: Frequent Contributor
timestamp: August 28, 2025, 11:07:02 am
content: I'm considering building a levelled sine generator with a frequency range from DC (maybe 1Hz) to (say) 1GHz or maybe 2GHzLeaving aside the output stage(s) and levelling stuff what's my best option these days to generate a signal like that?The ability to sweep the output frequency would be great (think wobbulator).I've no RF experience, so assume nothing! My initial thoughts are to use a DDS of some sort, but I'm not clear that such wide a frequency range can be done trivially...Guidance much appreciated.David

author: Picuino
author_rank: Super Contributor
timestamp: August 28, 2025, 11:19:02 am
content: In addition to DDS technology, there are PLLs with digital dividers, which can generate radio frequency in small increments.https://www.analog.com/en/resources/analog-dialogue/articles/pll-synthesizers.htmlhttps://www.analog.com/media/en/technical-documentation/data-sheets/ADF4206_4208.pdf

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 28, 2025, 10:55:51 pm
content: Quote from: perdrix on August 28, 2025, 11:07:02 amDepends entirely on your budgeteg ADI have DDS parts with clocks up to 12GHz, but they are spenny.You have an enormous range there, so more practical may be to use two solutions.DDS can manage sine from 1Hz to whatever you decide is a good budget point.Above that point, RF / VCO synthesisers and bandpass amplifiers get into GHzeg ADI show parts like MAX2871, table says 23.5MHz~6GHz for sub $8/1kAliExpress have DDS modules like AD9851 180MHz DDS at low cost, so that has 7.7 samples per cycle at 23.5MHz crossover point.With a bit more budget, the AD9910 DDS is also common on AliExpress and has 1GHz clock giving you more range overlap.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 29, 2025, 12:24:21 am
content: I would second the dual-range approach. Doing it all with a single DDS / DAC solution will be pretty expensive, though we don't know what your budget is.

author: ftg
author_rank: Frequent Contributor
timestamp: August 29, 2025, 09:43:12 am
content: The DDS+synth approach also makes it possible to drive the synthesizer with the DDS output to get very precise and small frequency steps, while running the PLL with a high PFD for low spurs.But the DDS is unlikely be as clean as a good crystal oscillator.But all MAX2871, MAX2870, ADF4350, ADF4351 or similar parts do the lower frequencies with dividers.So the output will be rich in harmonics and will require a filter bank.One can also mix two of those running in the VCO fundamental and then have less harmonics to deal with, the expense of the mixer and a more complex structure.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2025, 01:43:31 pm
content: Octave range VCOs are available, so use a PLL and VCO to synthesize 2 to 3 GHz, and then mix with a 2 or 3 GHz signal to generate 0 to 1 GHz.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: August 30, 2025, 02:51:18 pm
content: This in Keysights current signal source catalog.https://www.keysight.com/us/en/assets/7018-03356/technical-overviews/5990-9956.pdfOPs desire to go down to 1Hz, or into the audio frequencies suggests audio work, that is not reconcilable with an RF generator. If you need both, you need a DSS based with 0.1Hz-20MHz coverage, possibly with arbitrary waveform. And another one from 10MHz-2..6GHz. Both very easily attainable with chineese dev boards and can be bought online. For example: AD9854 for dds, LMX2594 for PLL synthesizer.Those are two different instruments, with different output stages and adjustability.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 30, 2025, 05:01:56 pm
content: The only sensible way to generate audio frequency sine waves over a wide bandwidth is DDS. So you need a DDS.Most of the high frequency DDS chips don't have great low frequency performance, so it's probably better to pair an rf synthesizer like the LMX2491 or any of the ti synthesizers.All those wide band synthesizers use dividers to generate lower frequency outputs so they will have significant harmonic content. If that's a problem you will need filters. But it's relatively easy to have filter banks to remove third harmonic distortion for signals from 100 MHz to 2 GHz rather than 1 Hz to 100 MHz. Also for many RF applications the harmonic content is not a big deal.Using mixers instead of dividers can make the filtering problem somewhat easier. But it has its own complications.Assuming you have a DDS and a synthesizer, one neat trick you can do is use the DDS to create the reference for the synthesizer. This allows you to get finely variable output frequency with an integer N PLL instead of fractional, and avoid fractional boundary spurs. It's a common approach in high performance synthesisers.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: August 30, 2025, 09:51:11 pm
content: @ejeffrey, that's an awesome point. There is one implementation (no longer online):https://web.archive.org/web/20180314165031/https://www.ke5fx.com/synth.htmlAnd Tek owns the patent:https://patentimages.storage.googleapis.com/7a/cd/a9/a6b1cb8cda42d4/US7155190.pdf

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 31, 2025, 01:10:07 am
content: The OP has not returned to define what precision/stability/steps size etcthey actually need, but parts like AD9528 may offer a third solution band.See a middle ground solution, between full sine tables of DDS and just square waves of Clock generators :https://www.ti.com/lit/pdf/sboa550That uses a shift register as a twisted ring counter or clocked delay line, to feed a simple DAC that greatly eases the filtering needed.You can get pricy ECL shift registers up to 2-3GHz, (MC100EP142) but I see the AD9528 has user defined delays that look to be VCOCLK related, and there is an external VCO in up to 1250MHz.Perhaps that can do the same task, with the multiple outputs set as phased delays then externally summed as per the TI note.Addit: the TI LMK04828 & LMK04808 families look to have a similar VCO in mode, up to 3100MHzLMK04828: fFin Clock input frequency for external VCO or distribution mode AC coupled 0.001~3100 MHz and digital delay range = 4 to 32 VCO cyclesLMK04808: 5-512.5 VCO cycles

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 31, 2025, 01:28:45 am
content: Assuming you want a perfect sine wave over the entire frequency band:1. Get 2 MAX2871 ICs. Set one to oscillate at a high frequency (like 6000 MHz). Use a microstrip filter to knock out all of the harmonics. Designing the filter is doable but is probably going to be difficult without RF experience and also a VNA. Luckily, a VNA isn't that expensive. There are also many filter design tools on the internet, but you'll probably have to cut and try a few times to get it perfect. The other device will oscillate at 5000-6000 MHz, and it will get mixed down. All harmonics will get mixed down to higher frequencies (like 10000 - 6000 = 4000 MHz) so you can just filter them out. You also might want to add a voltage variable attenuator or a step attenuator MMIC to allow you to control the output amplitude. You can choose a lower frequency like 4000 MHz too, but the unwanted frequencies will be at 2 GHz instead of 4 GHz, which means it is harder to filter them out from the 1 GHz signal.2. Find a suitable mixer. I think the SKY-60 would work. Make sure the IF covers every frequency you want to generate.3. Feed the 6000 MHz output into the RF port of the mixer, and then feed the 5000 - 6000 MHz output into the LO port. You will probably need an amplifier to get to 7 dBm for the LO port.4. Filter the IF with a lowpass filter. You can either use a microstrip or a conventional LC filter. Again, you'll need a VNA to make sure the frequency response of your filter is right.5. Use a diode detector to detect the output voltage, and level it. It will be tricky to detect the amplitude at 1 Hz using a diode detector though, so you might need a different method to level it at low frequency. Designing an amplifier that works at 1Hz and 1 GHz is not trivial at all. Your choices are to use a high frequency relay or switch to select between a low and high frequency amplifier, or to just get a super high bandwidth op amp and deal with all the issues that arise from that.If you do actually decide to do this, please put all of the components on a separate board and use SMA connectors to connect them. It will make your life way more easier. Also, get a TinySA and LiteVNA at the very least (or a boat anchor spectrum analyzer). Using mixers is cheaper than all of the other alternatives (like DDS or PLL + filter bank) but you do pay a penalty with phase noise.

author: Bud
author_rank: Super Contributor
timestamp: August 31, 2025, 04:13:35 am
content: Quote from: perdrix on August 28, 2025, 11:07:02 amWow, just wow

author: perdrix
author_rank: Frequent Contributor
timestamp: August 31, 2025, 08:32:35 am
content: I did say considering - application is 'scope calibration...The more I look the harder it seems.David

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 31, 2025, 08:53:04 am
content: Quote from: perdrix on August 31, 2025, 08:32:35 am'scope calibration is more likely to look something like this :https://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/

author: perdrix
author_rank: Frequent Contributor
timestamp: August 31, 2025, 11:46:30 am
content: A fest edge generator tells you about the ultimate bandwidth (or a least a proxy for it in terms of rise-time), but nothing about in-band flatness.David

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2025, 01:37:23 pm
content: Quote from: perdrix on August 31, 2025, 11:46:30 amThat does not seem right. The frequency (and phase) response can be computed from the impulse response, and the impulse response can be calculated from a fast edge.I am sure that the response from my fast edge generator was modified when I fiddled with the midband compensation. How could it not?Did you mean from only the fast edge without the flat tail of the pulse, like with a tunnel diode pulser?

author: perdrix
author_rank: Frequent Contributor
timestamp: August 31, 2025, 04:11:47 pm
content: Can be computed - sure if you know the mathematical model of the impulse response you can work back to the in-band response.But just looking at the impulse response only gives a "feel" for the in-band behaviour (i.e. approximately Bessel or whatever). A levelled sine generator tells all.David

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 31, 2025, 05:55:47 pm
content: Quote from: perdrix on August 31, 2025, 08:32:35 amIt is usually done with 1kHz square wave signal. It can have enough harmonics (if you do it right) even for calibrating 100-200 MHz scopes.

author: Bud
author_rank: Super Contributor
timestamp: September 01, 2025, 12:10:34 am
content: Quote from: EggertEnjoyer123 on August 31, 2025, 01:28:45 amIs it going to have 1 Hz or better resolution?

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2025, 12:25:05 am
content: Quote from: perdrix on August 31, 2025, 04:11:47 pmIt is not that difficult. A fast edge is easier to generate than an impulse, so it is used and then the result is differentiated before applying an FFT. Tektronix and LeCroy have old application notes discussing this method, but modern oscilloscopes usually do not support it.QuoteA leveled sine wave generator is very handy for the analysis, but the phase response tells a lot also. I once had an oscilloscope designer tell me that they used a leveled sine wave source to check bandwidth, but during the design to also check for unwanted resonances.Quote from: perdrix on August 31, 2025, 11:46:30 amThe book The Art and Science of Analog Circuit Design, edited by Jim Williams, has a chapter which discusses this. There is a direct correspondence between ripples in the time domain response, revealed by a fast edge from a reference level pulse generator, and the frequency domain. Shown below are a couple of pages showing the relationship. Compare the last two to see the subtle difference in the edge response.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 01, 2025, 01:47:28 am
content: Quote from: perdrix on August 31, 2025, 04:11:47 pmOf course in principle the time and frequency response are fourer transforms so are equivalent.In practice, the step generator directly gives you the information that is generally most important for an oscilloscope. The amplitude response is usually not super important. What you care about is the phase response. In particular you want flat group delay and fast and uniform settling time. These are present in a sine wave sweep as long as you do a phase sensitive measurement, but it's read right off the step response.A frequency sweep where you collect only amplitude and not phase is mostly useless actually. And it's not entirely trivial to make a levelled sine wave generator with a linear phase. None of the approaches above except for DDS will automatically do that. Anything with multiple signal paths, switchable filters, or divider chains will give phase jumps and dispersion. It's all possible to calibrate and correct but it's a significantly different project than a simple signal generator.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 01, 2025, 03:06:50 am
content: Quote from: Bud on September 01, 2025, 12:10:34 amHmm, I didn't realize that these can only have a denominator of at most 4095...Also the phase noise is probably going to be horrible if you downconvert from 6 GHz to 1 Hz. Perhaps having a DDS generate the low frequencies is a better idea.

author: Bud
author_rank: Super Contributor
timestamp: September 01, 2025, 05:23:18 am
content: DDS would be the least of a concern in such project. RF components and circuits such as mixers, amplifiers, power detectors, typically have limited frequency range. Then comes nightmare with wideband impedance matching, filtering, it has to be a bunch of frequency range specific switchable modules, and for what - scope calibration? Plus the designer must learn how to program the ICs, how to select clock sources, design loop filters, matching circuits, find proper tools, and bunch of other things. With no prior RF experience I'd say forget it, or embark on a long beautiful jorney to learn RF, which may only take a few years, lol.