RF / microwave circuit design
Academic projectIn progressRF Frequency-Conversion Chain
Academic project - chain design in Keysight ADS, roughly 2.5 GHz to roughly 5.2 GHz
Identification
- Type
- Academic project, ENSEEIHT / INP Toulouse, SYSCom track
- Tool
- Keysight ADS
- Input
- Around 2.5 GHz
- Output
- Around 5.2 GHz
- Function
- Amplification, filtering, mixing, LO generation
- Reference impedance
- 50 ohm throughout
Target specification
- Conversion gain
- Around 18 dB
- Noise figure
- Less than or equal to 6 dB
- IIP3
- Greater than 20 dBm
- Rejection
- Around 50 dBc
- Stability
- Unconditional over the band of interest
- Status
- Targets, not results. No simulated or measured figure is published yet.
Academic project
Academic project, coursework inside the SYSCom track at ENSEEIHT. The specification is the target the design is held to; no simulated or measured figure is published yet.
The design problem
Take a signal at around 2.5 GHz and deliver it at around 5.2 GHz, through a chain that amplifies, filters, mixes and generates its own local oscillator, while meeting gain, noise, linearity and rejection targets at the same time. The targets are the design: each one is easy alone and they pull against each other.
- Gain against noiseNoise figure is set mostly by the first stage, so the front end wants gain. Gain early costs linearity later.
- Linearity against gainIIP3 above 20 dBm constrains how hard the chain may be driven, and every stage of gain moves the whole chain closer to compression.
- Rejection against complexityAround 50 dBc on the image and the spurious products is a filter problem, and every filter costs insertion loss that the gain budget has to absorb.
- StabilityAn amplifier that oscillates has no specification at all. Stability is checked over the band, not only at the design point.
Chain and functions
- AmplificationLow-noise amplification at the input, where the noise figure is decided, and gain stages placed so that the linearity budget survives.
- FilteringBand selection before the mixer, image rejection around it, and output filtering of the conversion products.
- MixingThe frequency-conversion stage itself, with its conversion loss, isolation and the spurious products it generates.
- LO generationThe local oscillator that drives the mixer, and its own purity, drive level and leakage.
- Matching50 ohm matching at every interface, because a mismatch inside the chain is not just a loss, it changes the response of the stages around it.
Analysis to be carried out
Which analysis is run, and what each one settles.
| Analysis | Settles |
|---|---|
| S-parameter | Matching, gain, isolation and the stability factors across the band |
| Noise | Chain noise figure, and which stage actually sets it |
| Harmonic balance | Steady-state nonlinear behaviour: conversion, compression, harmonics and mixing products |
| Two-tone nonlinear | IIP3 and the third-order intermodulation products |
| Budget analysis | Gain, noise and linearity accumulated stage by stage along the chain |
The two cascade relations that make the chain a single design problem rather than five independent ones. Friis pushes noise responsibility to the front, the linearity cascade pushes it to the back, and the gain distribution is what settles the argument between them.