Why does LO phase noise matter?

The LO's phase noise directly limits receiver performance through reciprocal mixing. When a strong interfering signal (blocker) at frequency offset f_m from the desired signal mixes with the LO phase noise at offset f_m, it produces noise at the IF that adds to the receiver noise floor. The noise power created is: P_noise = P_blocker + L(f_m) + 10 log(BW). For a GSM receiver with a minus 23 dBm blocker at 3 MHz offset and channel BW of 200 kHz: if L(3M) = minus 140 dBc/Hz, P_noise = minus 23 + (minus 140) + 53 = minus 110 dBm. This must be below the receiver sensitivity (approximately minus 104 dBm for GSM), so the LO needs L(3M) better than minus 134 dBc/Hz. For LTE with closer channel spacing and wider bandwidth, the requirements are even more stringent. In transmitters, LO phase noise causes spectral regrowth that violates the adjacent channel leakage ratio (ACLR) specification. The integrated phase noise over the adjacent channel bandwidth determines the TX noise floor in that channel.

What LO source technologies exist?

PLL frequency synthesizer: the most common LO source. A VCO is locked to a crystal reference via a phase-locked loop. The output frequency is N times f_ref (integer-N) or (N + F/M) times f_ref (fractional-N). Inside the loop bandwidth, the phase noise follows the reference (scaled by 20 log N). Outside, it follows the VCO. Modern fractional-N PLLs achieve minus 100 to minus 120 dBc/Hz at 100 kHz offset at 2 GHz. DDS (Direct Digital Synthesis): generates an arbitrary waveform by stepping through a sine lookup table with a phase accumulator clocked by a high-speed DAC. Sub-Hz frequency resolution, fast switching (nanoseconds), phase-continuous tuning. Limited to about f_clock/2.5. Spurious from quantization and DAC nonlinearity. YIG-tuned oscillator: uses an yttrium iron garnet sphere as the resonator, tuned by a magnetic field. Extremely wide tuning range (2 to 20 GHz continuous with one device). Excellent phase noise (minus 120 dBc/Hz at 100 kHz offset at 10 GHz). Slow tuning (milliseconds due to magnet inductance). High power consumption (magnet current). Used in spectrum analyzers and signal generators. DRO (Dielectric Resonator Oscillator): uses a high-Q dielectric puck (Q of 5000 to 20000) to stabilize a transistor oscillator. Fixed or narrow-band. Excellent phase noise (minus 110 to minus 140 dBc/Hz at 100 kHz). Used for radar LO and point-to-point microwave links.

What is high-side vs low-side injection?

High-side injection: f_LO is above f_RF. f_IF = f_LO minus f_RF. The frequency conversion inverts the signal spectrum (upper sideband becomes lower and vice versa). The image frequency is at f_image = f_LO + f_IF = f_RF + 2 times f_IF (above the LO). Low-side injection: f_LO is below f_RF. f_IF = f_RF minus f_LO. The spectrum is not inverted. The image frequency is at f_image = f_LO minus f_IF = f_RF minus 2 times f_IF (below the RF). Choice factors: image frequency location (which is easier to filter), LO frequency range (wider tuning may favor one side), spurious products, and synthesizer performance. In a receiver covering 800 to 900 MHz with IF = 70 MHz: high-side LO = 870 to 970 MHz, image at 940 to 1040 MHz. Low-side LO = 730 to 830 MHz, image at 660 to 760 MHz. If the antenna band is 800 to 900 MHz, high-side injection places the image closer, making it harder to filter. Low-side may be preferred. For multiple conversion receivers, the first LO is typically high-side (places image far from RF for easy filtering) and the second LO is chosen based on the IF plan.

Frequency Source

Local Oscillator

/loh-kul os-ih-lay-ter/ — LO

Reference for mixer frequency conversion. f_IF=|f_RF−f_LO|. High-side: f_LO>f_RF (spectrum inverts). Low-side: f_LO<f_RF. Image: f_im=f_RF±2f_IF. Phase noise: limits selectivity via reciprocal mixing. Sources: PLL (−100-120 @100k), DDS (fast switch), YIG (2-20G, −120 @100k), DRO (fixed, −140 @100k). LO drive: +7-17 dBm.

PLL: −110 @100k

YIG: 2-20 GHz

DRO: −140 @100k

Understanding Local Oscillators

The local oscillator is the heartbeat of every superheterodyne radio. Its quality determines the receiver's ability to reject nearby interferers (selectivity), its frequency accuracy, and its spurious-free dynamic range. A noisy LO degrades every aspect of receiver performance, no matter how good the rest of the design is.

Choosing the right LO technology involves balancing phase noise, tuning range, switching speed, spurious levels, power consumption, and cost. No single technology excels in all parameters; each application demands a different tradeoff.

LO Equations

Frequency conversion:
f_IF = |f_RF − f_LO|
Image: f_im = 2f_LO − f_RF (high-side)
Image: f_im = 2f_LO + f_RF - 2f_RF (low-side)

Reciprocal mixing:
P_noise = P_blocker + L(f_m) + 10log(BW)

PLL phase noise (in-band):
L_out = L_ref + 20log(N)
N = f_out/f_ref

LO Source Comparison

Source	Tuning	PN @100kHz	Switch	Use
PLL int-N	Wideband	−100-115	μs	Cellular, WiFi
PLL frac-N	Wideband	−110-125	μs	5G, SDR
DDS	f_clk/2.5	−130-140	ns	Agile radar
YIG	2-20 GHz	−115-125	ms	Test equip
DRO	Fixed/narrow	−130-145	N/A	Radar, P2P

Common Questions

Frequently Asked Questions

Phase noise impact?

Reciprocal mixing: blocker + LO PN = noise at IF. P_noise=P_blocker+L(fm)+10log(BW). GSM: −23 dBm blocker @3MHz, BW=200kHz: need L(3M)<−134 dBc/Hz. TX: PN causes spectral regrowth = ACLR violation. LO PN = system dynamic range ceiling.

Sources?

PLL: VCO + phase detector + reference. In-band PN follows ref+20logN, out-of-band follows VCO. Fractional-N: fine resolution, lower spurs. DDS: phase accumulator + DAC, ns switching, quantization spurs. YIG: 2-20G continuous, excellent PN, slow, high power. DRO: Q=5k-20k, lowest PN, fixed freq.

High vs low side?

High-side: f_LO>f_RF, spectrum inverts, image at f_RF+2f_IF. Low-side: f_LO<f_RF, no inversion, image at f_RF−2f_IF. Choose based on: image filterability, LO tuning range, spur locations. Dual-conversion: first LO typically high-side (large image spacing). Second chosen per IF plan.

Related Terms

← LNA Loop Ant →

Frequency Sources

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