PicoVNA™ 106 vector network analyzer

De-embedding embedded port interfaces

When it is unsafe to assume the above ideal interconnecting connectors cables or microstrip lines; for example to achieve greater accuracy or to remove known imperfections in

a test setup, we can choose instead to de-embed the interface networks on each measurement port. The PicoVNA 2 software simply requires a full Touchstone .s2p file for the

embedded interfacing network on each port. Likewise, defined networks can be embedded into the measurement to achieve a desired simulated measurement. As for a calibration,

best accuracy will be achieved when the embedding network is defined at the same frequency points as the intended measurement. Unusually for a vector network analyzer, the

PicoVNA 2 software will interpolate where necessary and possible.

Z

0 impedance reference

System measurement impedance (default 50 Ω) can be mathematically converted to any value between 10 Ω and 200 Ω. The PicoVNA 2 software also supports the use of external

matching pads and calibration in the new impedance using a calibration kit of that impedance.

P1dB utility

The 1 dB gain compression point of amplifiers and other active devices can be measured using a power

sweep, either at a test frequency or over a sweep of test frequencies. The VNA determines the small-signal

gain of the amplifier at low input power, and then increases the power and notes the point at which the gain

has fallen by 1 dB. This utility uses a second-order curve fit to determine interpolated 1 dB compression

points.

Time domain transmission step responses (top) and frequency responses (bottom) of two lowpass filters

Time domain transmission and reflectometry measurements

Time domain reflectometry is useful in the measurement of a transmission line; in

particular the distance-to-fault location of any discontinuity due to connectors, damage

or design error. To achieve this, the PicoVNA 2 software determines from its frequency

domain measurements the time domain response to a step input. Using a sweep

of harmonically related frequencies, an inverse fast Fourier transform of reflected

frequency data (S

11) gives the impulse response in the time domain. The impulse

response is then integrated to give the step response. Reflected components of the step,

occurring at measurable delays after excitation, indicate the type of discontinuity and

(assuming a known velocity of propagation) the distance from the calibration plane.

A similar technique is used to derive a TDT (time domain transmission) signal from

the transmitted signal data (S

21). This can be used to measure the pulse response or

transition time of amplifiers, filters and other networks.

The PicoVNA 2 software supports Hanning and Kaiser–Bessel lowpass filtering on its

time-domain IFFT conversions, preserving magnitude and phase, and achieving best

resolution. A DC-coupled DUT is essential to the method.