📡 Probe Tuning Simulator NMR

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🔌 Tuning & Matching Circuit ▼
RF 50Ω C_M C_T L (coil) R sample

CT shifts resonance · CM sets 50 Ω match · they couple, so tune iteratively.

🔬 About the Probe ▼

The NMR probe contains a solenoid RF coil around the sample tube. Tuning means lining up the spectrometer frequency with the probe's resonant circuit so RF power enters the probe efficiently instead of reflecting back.

Tuning capacitor (CT) moves the dip left or right by shifting the probe resonance toward the target frequency.

Matching capacitor (CM) deepens the dip by matching the probe to the 50 Ω transmission line.

💡 CT and CM affect each other, so tuning is usually a short back-and-forth process.
Reflection Curve (S₁₁)
Target: 400.224 MHz
Signal Quality 0%
Probe Freq
400.22 MHz
Target Freq
400.22 MHz
✅ Tune Success Metrics ▼
Return Loss
0.0 dB
VSWR
∞
Probe Quality Factor
—
(Return Loss shows reflected power and more negative is better. VSWR shows mismatch and values near 1 are best. Probe Quality Factor shows how sharp the resonance is and a healthy tuned probe usually keeps it reasonably high.)
Controls
Start Here
Click Auto-Tune to tune automatically.
If you want to tune by hand, open Manual Tune just below for the short step-by-step guide.
🛠️ Manual Tune ▼
  • Move CT to bring the dip toward the target frequency.
  • If the dip is too far left, lower CT. If it is too far right, raise CT.
  • Move CM to make the dip deeper once it is near the target.
  • Use small steps and go back and forth between CT and CM.
  • Stop when the dip is centered on the target and is as deep as possible.

⚙️ Tuning Capacitor (CT)

Capacitance 15.0 pF
Adjustment Step
Tap for a single nudge or hold for continuous motion.

⚙️ Matching Capacitor (CM)

Capacitance 10.0 pF
Adjustment Step
Use coarse first to find the dip, then fine to center and deepen it.

🧪 Sample Dielectric

Dielectric Factor 1.00

🎯 Target Frequency

MHz
Enter your Larmor frequency (¹H, ¹³C…), then press Set followed by Auto-Tune or Manual Tune.
📚 Facts About Tuning ▼
📐 Resonance Frequency ▼

The probe's LC circuit resonates at:

f = 1 / (2π√(LC))

where L is the coil inductance and C is the total circuit capacitance (tune + match in network). Adjusting CT shifts this frequency.

📐 Reflection Coefficient ▼

The reflection coefficient Γ measures impedance mismatch:

Γ = (ZL − 50) / (ZL + 50)

Return Loss in dB:

RL = −20 log₁₀|Γ|

Deeper dip (more negative dB) means better matching. Less RF power is reflected.

💡 Goal: RL < −20 dB (less than 1% reflected power)
📐 VSWR ▼

Voltage Standing Wave Ratio:

VSWR = (1 + |Γ|) / (1 − |Γ|)

VSWR = 1.0 is perfect. VSWR < 1.5 is excellent for NMR. High VSWR means standing waves on the cable that waste power and can damage amplifiers.

📐 Larmor Frequency ▼

The nuclear spin precession frequency in a magnetic field B₀:

f₀ = γ · B₀ / (2π)

¹H NMR: γ/2π = 42.577 MHz/T → at 9.4 T → 400 MHz

¹³C NMR: γ/2π = 10.708 MHz/T → at 9.4 T → 100.6 MHz

This is the frequency your probe must be tuned and matched to.