How do I select a frequency synthesizer for a test and measurement application?
T&M Synthesizer Selection
Selecting a synthesizer for test and measurement requires matching the instrument's specifications to the measurement requirements. The most critical specification varies by application: phase noise for receiver sensitivity testing, spur level for spurious response measurements, settling time for production testing, and output level range and accuracy for gain and compression measurements.
| 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
What phase noise do I need for receiver testing?
The synthesizer's phase noise must be at least 10 dB better than the receiver's LO phase noise at all offset frequencies. For testing base station receivers: synthesizer phase noise < -140 dBc/Hz at 10 kHz offset. For testing radar receivers: < -150 dBc/Hz.
How important is switching speed?
For production testing (characterizing hundreds of units per hour): switching speed of 1-100 μs is needed. For R&D and validation: 1-10 ms is usually acceptable. For signal simulation (radar, EW): sub-microsecond switching may be required, which demands DDS-based architectures.
What about output power flatness?
Output power should be flat (±0.5 dB) across the operating frequency range. Automatic level control (ALC) maintains power accuracy as frequency and temperature change. For precision power measurements: use an external power meter with feedback to the synthesizer's ALC.