This RF board will likely contain an RF oscillator.
If you’re not a member of the RF design community, then the use of any analog oscillator may seem esoteric. More designers, both at the IC and board levels, should familiarize themselves with RF oscillator components and circuits as newer products continue to push designs to higher frequencies.
Once you get to the GHz range and beyond, SoCs and ICs become a better choice than discrete components for RF oscillators. We’ve compiled some basic oscillators any designer should understand and some important layout tips for designers working at the GHz range.
Designing an RF Oscillator
There are a number of standard oscillator circuits that you can design from readily available components. These circuits typically involve one or more FETs (BJT, JFET, or MESFET), passive components, one or more op-amps, and/or a varactor diode. Getting any of these circuits to run up to GHz frequencies requires working with GaAs (less than ~10 GHz bandwidths) or GaN (greater than ~10 GHz bandwidth) active components.
It can be difficult to get any of these oscillators built from discrete components to run at high GHz frequencies for a number of reasons. The problems involved are due to the availability of very small inductors and capacitors with sufficiently high resonance frequencies, as well as parasitics in a real circuit layout. The costs involved will also be greater than simply using an RF oscillator IC or crystal oscillator. Despite these difficulties, you can certainly use discrete components to build an RF oscillator that runs at 100’s of MHz or a few GHz with COTS components.
Four fundamental oscillator circuits.
If you’re designing a custom signal chain that must operate at high power, and there are no RF oscillator ICs available, you can build any of the above oscillator circuits or a VCO/VCXO/NCO circuit from discrete components. Building any of these oscillators from discrete components requires passives with sufficiently high self-resonant frequency.
The output from your particular circuit may be square, triangular/sawtooth, or exponential (relaxation oscillator). The easiest way to convert your output to a sine wave is to an integrator, differentiator, higher order RC filter, or clipping circuit. As an example, many VCOs generate a triangular wave, which is converted to a square wave with a Schmitt trigger circuit. The square wave output can be converted to a nearly-sinusoidal wave by passing the wave through a third-order (or higher order) RC filter with a cutoff frequency close to the fundamental harmonic, although this becomes quite difficult at GHz frequencies with discrete COTS components. More precise signal conversion methods require op-amps and LC tank circuits, which is a bit beyond the scope of this article.
Bringing an RF Oscillator into Your Signal Chain
Microwave components companies have spent a considerable amount of time developing and perfecting ICs for RF oscillator circuits. These components tend to have quite low phase noise and are typically surface-mount components, especially when designed to operate at high frequencies. These components may use one of the circuits introduced above, or they may use an internal integer/fractional PLL for frequency synthesis up to high frequency. Options are available ranging from a few MHz to 10’s of GHz.
These RF oscillator ICs might also use an NCO or VCO to generate a MHz or GHz signal, respectively. These ICs can also be used as the base oscillator in a PLL feedback loop and used to synthesize much larger frequencies. Be careful with this implementation as any RF oscillator used in this system will have some limited bandwidth. In addition, the loop filter (basically a low pass filter) and phase detector limit the capture and lock ranges to narrow values. The oscillator you use should have a wide enough bandwidth such that it overlaps with the capture/lock ranges.
An RF oscillator IC, any RF oscillator built from discrete components, and all other components in the signal chain should use surface mount components as certain signal integrity problems can arise when through-hole vias are used with through-hole components. With sub-WiFi frequencies, you may not have signal problems with through-hole components as long as you backdrill any via holes and leftover component stubs. However, this increases manufacturing and assembly costs as multiple steps are required to remove via and component lead stubs. Therefore, it is better to use surface mount components at higher RF frequencies.
Connectors, such as this edge launch SMA connector, require precise impedance matching when used with any RF oscillator.
mmWave Oscillator Routing
If you are using an RF oscillator for mmWave frequencies as a stable reference oscillator, you should try to avoid the use of any vias, especially through-hole vias. The issue with through-hole vias in mmWave boards relates to insertion loss and resonance. First, these structures tend to be rather large, thus their geometric resonance frequencies tend to be similar to the output frequencies from an RF oscillator. Any resonating signal in a via will become a source of EMI and capacitive via-to-via coupling.
Second, it can be difficult to match the impedance of these vias to an interconnect to prevent reflections and ensure low insertion loss. With highly stable RF reference oscillators, ensuring signal integrity and preventing distortion is paramount, and vias need to be properly sized to prevent insertion loss. With RF oscillators that are frequency modulated or are used to modulate another signal, your vias need to be precisely constructed to have sufficiently broad, flat bandwidth. As an example, ensuring signal integrity in 5G modulation schemes like filter bank multi-carrier (FBMC), universal filtered multi-carrier (UFMC), generalized frequency division multiplexing (GFDM), and filtered OFDM (f-OFDM) with RF oscillators in HDI boards can be difficult if the via impedance spectrum is not modeled properly.
Whether you are designing an RF oscillator from scratch or you need to bring an existing component into your PCB, you can design, layout, and simulate the behavior of these circuits and your PCB when you work with the right PCB design and analysis software. The simulation tools in Allegro PCB Designer and the full suite of analysis tools from Cadence are ideal for running pre-layout simulation of your RF oscillator circuit. The post-layout simulation tools from Cadence are also ideal for examining signal integrity problems that can arise in complicated layouts.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.