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:
- PLL. Settling time cannot be met
- DDS: The best I found is the AD9914/AD9915 with fclk=3.5 GHz (i.e., fmax=1.75 GHz)
- 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
- 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!
- 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:
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?