Electronic – Fast (high) frequency hopping with off-the-shelf components

ddsfrequency-synthesishigh frequencypllRF

I am looking for the easiest solution to

  • synthesize 1.6 – 2.4 GHz (qudrature)
  • in steps of 4 MHz
  • settle within 10-50ns
  • precise timing control when to switch frequency (e.g., at rising edge of an external signal, frequency should be updated)
  • total integrated phase noise (jitter) between 1 MHz and infinity < 400fs rms
  • Cost and power is irrelevant (in a meaningful range) to the extend that they do not add significant complexity or add other problems (like thermal issues)

In considered the following solutions:

  1. PLL. Settling time cannot be met
  2. DDS: The best I found is the AD9914/AD9915 with fclk=3.5 GHz (i.e., fmax=1.75 GHz)
  3. DAC/RFDAC: There are many options (AD9161 at 12 Gsps etc). However, they require that the digital signal is generated externally and high-speed samples pumped into the DAC. A high-end FPGA could implement DDS logic and spit out the samples. However, this has tremendous complexity. Besides, these high-speed RFDACs are 200 ball BGAs requiring 12 PCB layers, serdes and a careful design that would take many months to complete
  4. DDS+Mixer. Let's take the AD9910 as an example followed by a mixer with fc=2 GHz. The issue is that the mixer needs to be an image rejection mixer and hence I need two DACs for each I and Q channel – a total of 4 DDS and 2 Quadrature!
  5. DDS+I/Q Modulator: Basically the same as 4 but the output of the DDS systems is shared between the modulators (2xDDS, 2xIQ Modulators). This would be "ok" but it cannot control the phase difference between the I and Q branches for I/Q compensation.

Is there anything I forget? The AD9956 is a 400 MHz DDS system but claims:

The AD9956 uses advanced DDS technology, an internal high speed, high
performance DAC, and an advanced phase frequency detector/charge pump
combination, which, when used with an external VCO, enables the
synthesis of digitally programmable, frequency-agile analog output
sinusoidal wave- forms up to 2.7 GHz

Two application circuits are given in the datasheet:

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but I do not understand them. What would be the switching time from, say 1.6 to 2.4 GHz for these?

Is there any other hybrid DDS+PLL or DDS+Mixer system?

Best Answer

One option is using a DDS that can reach 1.2 GHz, with a frequency doubler.

Doublers are essentially just some nonlinear circuit to produce harmonics with some filtering to pick out the preferred 2nd harmonic at the output, so they don't require any lock-in time when changing frequency (aside from that implied by the bandwidth of the selection filter).

Doublers tend to allow at least a bit of the input frequency (perhaps 20 dB or so below the 2nd-harmonic output), and also its 3rd harmonic, through to the output, so some careful filtering, or even an adjustable filter, might be required if you need a very pure output frequency.

Doublers also tend to be a bit fussy about the power level at the input, and produce an output attenuated from the input level, so you may need some additional amplification and/or attenuation to get the scheme working well.

Two application circuits are given in the [AD9566] datasheet... What would be the switching time from, say 1.6 to 2.4 GHz for these?

Those are both essentially PLL schemes. The switching time will be limited by the bandwidth of the loop filter. I'd expect it to be difficult to get it below a several 10's of ns. Although 50 ns doesn't seem totally out of the question if the loop bandwidth can be as high as 20 MHz. (This also applies to your proposal of a straight PLL solution)

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