What is the pulling figure of a VCO and how does load impedance variation affect frequency stability?
Load Pulling Effects
Every oscillator is sensitive to its load impedance because the load affects the resonator's effective Q factor and resonant frequency. When the load impedance changes (due to cable movement, temperature, or downstream component impedance variation), the oscillation frequency shifts. This frequency shift is the load pulling effect.
| Parameter | Passive Diode | Active FET | Subharmonic |
|---|---|---|---|
| Conversion Loss/Gain | 5-9 dB loss | 0-10 dB gain | 8-12 dB loss |
| LO Drive Level | +7 to +17 dBm | -5 to +5 dBm | +5 to +13 dBm |
| IP3 (typical) | +15 to +30 dBm | +5 to +20 dBm | +10 to +20 dBm |
| Noise Figure | 5-9 dB (= conv. loss) | 8-15 dB | 9-14 dB |
| LO-RF Isolation | 25-45 dB | 15-35 dB | 20-40 dB |
Frequently Asked Questions
How do I measure pulling?
Connect the VCO to a variable-length transmission line terminated in a short circuit (or use a precision load tuner). Vary the line length to present all phase angles at a fixed VSWR. Record the maximum and minimum frequencies. Pulling = fmax - fmin.
Does an isolator eliminate pulling?
An isolator provides 20-25 dB of reverse isolation, reducing the effective load VSWR seen by the VCO by the same amount. This reduces pulling by approximately 20-25 dB but does not eliminate it completely. For the most critical applications, cascade an isolator with a buffer amplifier.
What about temperature effects on pulling?
Temperature changes the load impedance (cable phase shift, component impedance variation), which modulates the VCO frequency through the pulling mechanism. This creates a temperature-dependent frequency instability that adds to the VCO's own temperature coefficient. Isolators mitigate this effect.