How to find the right Varactor Diodes AI guide
Written by an AI model from its trained knowledge.
What the AI looks for in Varactor Diodes
- Capacitance at a stated reverse voltage
- Match the datasheet capacitance at the bias points your circuit actually uses (datasheets usually quote it at two voltages, such as 1 V and 4 V, or 1 V and 28 V), not just a single nominal figure. Varactor capacitance depends strongly on bias, so a part rated at a different voltage can land your oscillator or filter far off frequency.
- Capacitance ratio over your tuning voltage
- Compare the ratio between the low-voltage and high-voltage capacitance within the control-voltage span you actually have, such as a 3.3 V or 5 V PLL output versus a higher tuning supply. The ratio sets how wide a frequency range you can tune. A part that needs a high tuning voltage gives little range from a low-voltage control line.
- Junction profile: abrupt vs hyperabrupt
- Choose hyperabrupt for wide tuning range at low voltage, and abrupt for higher Q and a more predictable capacitance-voltage curve. Hyperabrupt parts give more tuning per volt but cost some Q and linearity. That tradeoff decides VCO phase noise and filter loss.
- Q factor / series resistance
- Check the Q, or series resistance, at your operating frequency. Lower series resistance is better for VHF, UHF and microwave work. Series resistance adds loss and phase noise in tank circuits. This is what separates RF-grade varactors from general-purpose ones.
- Maximum reverse voltage
- The tuning voltage plus any RF swing on top of it must stay below the rated reverse breakdown voltage. Large RF swings in power oscillators can forward-bias the diode or exceed breakdown, which causes distortion or failure.
- Package and matching
- Pick a package your board can handle (SOD-323, SOD-523, SOT-23 or through-hole). Use dual common-cathode parts or matched sets when two tuned circuits must track together. Package parasitics matter at RF, and tuned stages that track each other need diodes with closely matched curves.
Brands that matter
- Skyworks: SMV-series hyperabrupt and abrupt varactors, widely used in RF VCO designs.
- Infineon: BB-series silicon tuning diodes for TV, radio and RF tuners, with good datasheets.
- Nexperia: Former NXP discrete line, including BB-series varicaps for tuner applications.
- Toshiba: 1SV-series variable-capacitance diodes, common in AM/FM and tuner circuits.
- onsemi: Supplied the classic MV-series hobbyist varactors. Check whether a given part is still active.
- MACOM: Microwave-grade varactors for higher-frequency and tuning-network work.
- Microchip (Microsemi): High-reliability and microwave varactors for specialised RF use.
What to avoid
- Unbranded bulk lots with no datasheet or capacitance-voltage curve, because you cannot design a tuned circuit around unknown capacitance.
- Using a rectifier diode, LED or zener as a varactor in anything but casual experiments: the capacitance range is small and Q is poor.
- Parts whose datasheet quotes capacitance only at a tuning voltage your circuit cannot reach.
- Assuming a cross-referenced substitute will tune the same; compare the capacitance-voltage curves, not just the headline capacitance.
- Leaving out a bias resistor or RF choke on the tuning line, which loads the tank and spoils Q.
Questions
Hyperabrupt or abrupt varactor: which do I need?
Hyperabrupt if you need wide tuning from a low control voltage, as in most PLL VCOs. Abrupt if Q, low distortion and a predictable curve matter more than range.
Can I use an ordinary diode reverse-biased as a varactor?
It will vary capacitance, but only slightly and with high loss. A purpose-made varactor gives a specified curve, far wider range and much better Q.