Measuring Active Components
The MegiQ VNAs provide several facilities to support active measurements.
When measuring active components we often need to provide a bias voltage or current to the device.
The VNA-0440e and VNA-0460e have a built-in bias generator with a voltage/current source. It can provide voltages from -12V to 12V with a current up to 100mA. This voltage can be switched to any, or all, of the three ports.
This makes it possible to directly provide supply to an amplifier chip so that it's pure characteristics can be measured without the influence of a supply circuit. It also makes it possible to measure the capacitance of a varactor diode or the switching characteristics of a pin diode directly. It can also provide supply to an active antenna.
The VNA ports can be configured for DC to be Open, Biased or Grounded so that it can provide a ground path for a pin diode.
Both VNAs are capable of multiple sweep levels that facilitates parametric sweeps. For example, a frequency sweep can be repeated at stepped power levels, while simultaniously adapting the input attenuators of the VNA to the changing levels. There are several applications for this.
We can measure the impedances and gain of an amplifier at varying drive levels. This will result in a set of curves representing the characteristics at the stepped power levels.
We can also measure the 1dB-compression point by sweeping the drive power level from low to high and determine where the gain starts dropping. And we can repeat this at different frequencies, in one sweep.
A varactor diode can be measured by sweeping the bias voltage at stepped frequencies. An amplifier can be characterised by stepping the bias current over frequency sweeps.
The application software provides a sweep manager to make different parametric sweep configurations. Most VNA settings can be used in a parametric sweep.
Transceiver chips need special attention. They often switch rapidly between transmit and receive mode, or between receive and idle mode. It is often necessary to use test-firmware in the device to put the chip in either TX or RX mode to measure those impedances.
When measuring a transmitter at full power, the VNA can withstand power up to 10dBm but of course there is a distortion at the carrier frequency. The VNA is well capable to measure transceiver chip impedance in TX and RX mode and match these to the desired (50 Ohm) impedance.
See also our Balanced Calibration Kit for differential chip impedances.