Analog Front End Design

flux

0:34

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Even though a DDS gives you superb frequency resolution and very fine tuning, in a 5–15 GHz front end you’ll almost always pair it with a clean PLL/VCO stage. Here’s why:
  1. Phase Noise & Jitter
    • DDS outputs typically have higher phase noise and discrete spurs from the digital-to-analog conversion and clock feedthrough.
    • A low-noise PLL/VCO can “clean up” those spurs and reduce jitter on your RF carrier, especially critical in radar or communications links.
  2. Frequency Coverage & Multiplication
    • Most DDS chips top out well below 1 GHz (even with internal ×10 multipliers).
    • A PLL/VCO lets you multiply or lock to those lower-frequency DDS outputs and reach 5–15 GHz directly—without cascading many external multipliers.
  3. Power & Efficiency
    • High-frequency VCOs built into PLLs are optimized for output power and tuning sensitivity out in the microwave band.
    • Driving a passive mixer or power amplifier from a purpose-built VCO usually yields better gain and flatter tuning curves than a multiplied DDS output.
  4. Spurs & Harmonic Suppression
    • A fractional-N PLL can further suppress fractional spurs when compared to a straight DDS ×N chain.
    • Most PLLs include internal loop-filter recommendations to optimize spur performance over temperature and supply variations.
Recommended Topology
  • Use your DDS as a high-resolution frequency source or as the reference input to a fractional-N PLL.
  • The PLL/VCO then generates the actual 5–15 GHz LO for mixing or upconversion.
  • Optionally, use the DDS output directly for low-frequency test modes or calibration sweeps.
In short, the DDS gives you fine tuning and digital control, but the PLL/VCO gives you the low-noise, high-frequency punch and spur performance you need in a true microwave front end.
223 days
andewx

andewx

Great apologies I'll also need some parts for the loop filters in the mixing and downconversion stages, any suggestions for the 5-15GhZ range
223 days

flux

5:20

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It will help to know exactly what “loop filters” you have in mind:
  1. PLL loop-filter for your synthesizer (i.e. the R–C network in the PLL feedback path), or
  2. RF band-pass/low-pass filters in the mixer/down-converter chain (i.e. microwave filters centered at some IF or RF frequency)?
Each requires very different parts:
  • If you mean the PLL loop-filter, please share:
    • Desired loop-bandwidth or pole/zero frequencies (e.g. 100 kHz corner, 45° phase-margin).
    • Filter order (2nd vs. 3rd).
    Then I can pick precise R and C values and link to NP0/C0G capacitors and 0.1 % resistors in 0402/0603.
  • If you mean RF filters around 5–15 GHz, I can recommend off-the-shelf Mini-Circuits modules (e.g. high-pass NHP-6+ and low-pass SHC15+ to build a 6–15 GHz band-pass, or dedicated band-pass filters like BPF-6W6+).
Which of these two (or both) are you looking for?
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[Multi-layer] JLCPCB Constraints

[Multi-layer] JLCPCB Constraints
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Last updated by andewx
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