Broadband RF noise source/generator

Question

I’m try to make a wideband noise source up to 150MHz, around 0dBm and as flat as possible, based on diode’s avalanche mechanism, plus some buffering and amplification stages. My questions:

1. What is pros and cons to use zener diode or B-E junction of a transistor.
2. In case of zener, what is the impact of breakdown voltage, bias current and junction capacitance (or any other characteristics of diode), on noise bandwidth, level and spectrum.
3. In case of transistor, is should be an RF/low noise one or a general purpose it is adequate?
4. Can I simulate this project on LTspice, since spectrum analyzer currently it is not available?

Answer 1

For \$150\mathrm{MHz}\$ bandwidth, you need an input \$\tau\$ of \$1\text{ns}\$. Thus \$1\mathrm{k}\Omega\$ and \$1\mathrm{pF}\$ suffice.

With \$1\mathrm{k}\Omega\$ \$R_\text{noise}\$, producing \$4 \mathrm{nV/\sqrt{Hz}}\$, the integrated input noise from the resistor is $$ \begin{split} \sqrt{\frac{ \pi \cdot 150\mathrm{MHz}}{2}} \cdot 4\mathrm{nV_{RMS}} &= \sqrt{220 \mathrm{MHz}} \cdot 4\mathrm{nV_{RMS}}\\ &= 15,000 \cdot 4\mathrm{nV_{RMS}} = 60 \mathrm{\mu V_{RMS}} \end{split} $$ broadband, any amplification.

A gain of \$1,000\times\$ (using cascode CommonEmitter amplifier, cascaded for low \$C_{in}\$) produces \$60\mathrm{mV_{RMS}}\$, or about \$400 \mathrm{mV_{pp}}\$ at \$1\mathrm{PPM}\$ level (\$6.2 \sigma\$ or RMS).

Is that adequate? to get gain of \$1,000\times\$ in one bipolar stage, you need at least \$26\mathrm{V}\$ power supply for resistive load. Give you need \$0.5 \mathrm{ns}\$ collector-node \$RC\$ to have hope of achieving \$150\mathrm{MHz}\$ bandwidth, you should plan on at least 2 stages, gain of \$30\times\$ in each stage. And you can do this with \$V_{DD}=5 \mathrm{V}\$.

You will need to find bipolars with \$C_{ob}\$ of \$\mathrm{0.5pF}\$, at \$I_C=2\mathrm{mA}\$ and \$V_{CE}=2\mathrm{V}\$. If you use 2N3904, with about \$C_{ob}=10\mathrm{pF}\$, expect about \$\mathrm{10MHz}\$ noise bandwidth.

^{simulate this circuit – Schematic created using CircuitLab}