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LNB Phase Lock Loop (PLL) vs DRO: What’s the Difference?
When specifying an LNB for a VSAT terminal, the product sheet lists noise figure, frequency range, and gain. What it often does not lead with is the oscillator technology — and that is the specification that determines whether your modem can lock, stay locked, and maintain the modulation order the link budget requires.
Two oscillator technologies are used in LNBs: the DRO (Dielectric Resonator Oscillator) and the PLL (Phase Lock Loop). The difference between them is not subtle. A DRO LNB and a PLL LNB can both receive a satellite signal, but only one of them is appropriate for professional VSAT operation.
This article explains how each oscillator works, what the specifications mean in practice, and how to select the right LNB type for your application.
What Is an LNB Oscillator?
An LNB contains a local oscillator that generates a reference frequency. The incoming satellite signal (in the 10.7–12.75 GHz range for Ku‑band) mixes with this local oscillator frequency, producing an output at L‑band (950–2150 MHz) that travels down the coaxial cable to the modem.
The oscillator frequency must be stable. If it drifts, the downconverted signal drifts with it. The modem’s demodulator has a carrier acquisition range — typically a few hundred kHz to a few MHz — but if the LNB oscillator wanders outside that window, the modem loses lock. In a two‑way VSAT system, a lost lock means a dropped link.
The two technologies differ fundamentally in how they generate and stabilise this reference frequency.
DRO LNBs: Ceramic Resonance, No Reference Lock
A DRO oscillator uses a small ceramic disc (the dielectric resonator) whose physical dimensions determine the oscillation frequency. It is a self‑contained, free‑running oscillator — there is no external reference, no feedback loop, and no mechanism to correct for drift.
How DRO drift occurs
The ceramic disc’s resonant frequency shifts with temperature. As ambient temperature rises, the disc expands slightly and the resonant frequency drops. As temperature falls, frequency rises. An LNB installed on a rooftop in Dubai will see a temperature swing of 50°C or more between winter night and summer midday. Over that range, a typical Ku‑band DRO oscillator will drift by 1 MHz to 3 MHz from its nominal frequency.
DRO frequency stability
- Typical frequency stability: ±1 MHz to ±3 MHz over the full operating temperature range
- Initial accuracy: similar magnitude
- Long‑term aging: gradual additional drift over years
PLL LNBs: Crystal Reference, Closed‑Loop Stability
A PLL LNB uses a crystal oscillator as a low‑frequency reference (typically 10 MHz, 25 MHz, or 40 MHz) and a phase‑locked loop circuit that multiplies and locks the high‑frequency oscillator to that reference. The crystal is stable by design; the PLL continuously corrects the output frequency to track the crystal.
PLL is not a single specification — it is a technology with multiple performance tiers determined by the quality of the crystal reference:
Standard crystal (XO)
Low‑cost quartz crystal, no temperature compensation. Stability: ±25 kHz to ±500 kHz. Used in lower‑cost VSAT LNBs where exact frequency stability is less critical.
TCXO (Temperature Compensated Crystal Oscillator)
A thermistor network compensates for the crystal’s temperature coefficient. Stability: ±1 kHz to ±25 kHz over the full operating temperature range. The standard for professional Ku‑band VSAT operation in the GCC and MENA region.
OCXO (Oven Controlled Crystal Oscillator)
The crystal is held at a constant elevated temperature in a small internal oven, eliminating thermal drift almost entirely. Stability: ±50 Hz to ±1 kHz. Required for high‑throughput SCPC links, DVB‑S2X with 16APSK or higher modulation, and teleport‑class installations.
Phase Noise: The Other Oscillator Specification
Frequency stability tells you where the oscillator sits. Phase noise tells you how clean it is. A real oscillator does not produce a perfect single‑frequency tone — it produces a carrier with random phase fluctuations that spread energy into sidebands on either side.
In an LNB, oscillator phase noise adds directly to the received signal. High phase noise raises the noise floor, degrades EVM (Error Vector Magnitude), and limits the maximum modulation order achievable on the link.
Phase noise is specified in dBc/Hz at a given offset from the carrier. Lower (more negative) numbers are better.
Typical phase noise values (at 1 kHz offset)
- DRO LNB: approximately −40 to −55 dBc/Hz
- PLL standard crystal: approximately −65 to −75 dBc/Hz
- PLL TCXO: approximately −80 to −90 dBc/Hz
- PLL OCXO: approximately −95 to −105 dBc/Hz
For QPSK and 8PSK operation, PLL TCXO is more than sufficient. For 16APSK and 32APSK, oscillator phase noise contributes measurably to EVM — TCXO or OCXO is the appropriate specification at these modulation orders.
LNB PLL vs DRO: Specifications at a Glance
| Parameter | DRO | PLL Standard | PLL TCXO | PLL OCXO |
|---|---|---|---|---|
| Oscillator type | Free‑running ceramic | Crystal + PLL | TCXO + PLL | OCXO + PLL |
| Frequency stability | ±1–3 MHz | ±25–500 kHz | ±1–25 kHz | ±50 Hz–1 kHz |
| Phase noise (1 kHz) | Approx. −50 dBc/Hz | Approx. −70 dBc/Hz | Approx. −85 dBc/Hz | Approx. −100 dBc/Hz |
| Temp. compensation | None | None or minimal | Thermistor network | Oven‑controlled |
| Typical cost | Lowest | Low–medium | Medium | High |
| VSAT suitability | Receive‑only only | Entry‑level VSAT | Professional VSAT | High‑throughput, teleport |
| DVB‑S2X (16APSK+) | Not suitable | Not suitable | Yes | Yes |
| iDirect / Comtech | Not compatible | Not recommended | Required standard | Yes |
When to Choose Each LNB Type
Appropriate only for consumer DTH reception. Not suitable for any two‑way VSAT terminal, modem‑connected system, or network where modems must maintain carrier lock. Do not specify a DRO LNB for any professional VSAT application.
Acceptable for entry‑level VSAT with low‑order modulation (QPSK) and wide‑carrier‑acquisition modem configurations. Use when cost is a significant constraint and link conditions are benign. Not recommended for GCC deployments where temperature swings are large, or for iDirect / Comtech modem platforms.
The standard specification for professional Ku‑band and C‑band VSAT in the GCC and MENA region. Required for iDirect, UHP, and Comtech modem platforms. NJRC NJS‑series, Norsat 1000H‑series, and Swedish Microwave C‑band LNBs are TCXO‑based. If you are specifying a VSAT terminal for enterprise, oil and gas, maritime, or managed service in the Gulf — this is the LNB to specify.
For high‑throughput SCPC links using 16APSK, 32APSK, or DVB‑S2X. Teleport and broadcast uplinks. Any link where phase noise contributes measurably to EVM or where very high spectral efficiency is required. Norsat 3000‑series and selected NJRC models cover this tier.
GCC VSAT Context
In the GCC enterprise VSAT market, PLL TCXO is the de facto standard. Every major VSAT modem manufacturer — iDirect, Comtech EF Data, UHP Networks — specifies a minimum oscillator stability of ±25 kHz or better for their platforms. This rules out DRO LNBs entirely and favours TCXO over basic PLL crystal references.
Arabsat, Es’hailSat, Yahsat, and SES satellites serving the MENA region carry transponders where PLL TCXO LNBs lock and maintain lock without difficulty. For maritime VSAT in the Arabian Gulf — where vessel motion, humidity, and temperature variation add stress to the outdoor unit — TCXO stability provides the additional margin that ensures the modem does not drop lock in rough conditions or during summer temperature spikes.
Frequently Asked Questions
Conclusion
The choice between DRO and PLL LNBs is straightforward for VSAT applications: DRO is for receive‑only broadcast only, and PLL is required for any two‑way modem‑connected terminal. Within PLL, TCXO is the professional standard for GCC enterprise VSAT, and OCXO is appropriate where very high spectral efficiency or high‑order modulation is required.
For most VSAT deployments in the UAE, Saudi Arabia, Qatar, and the wider MENA region — oil field camps, maritime terminals, enterprise offices, and managed service sites — a PLL TCXO LNB is the correct specification. It provides the frequency stability and phase noise performance that iDirect, Comtech, and UHP modems require, at a cost‑to‑performance ratio that makes it the default choice for professional integrators.
