PicoVNA frequently asked questions

There are no plans right now for a 75 Ω VNA. However, the PicoVNA 106 and software do support 75 Ω measurement, either:

• mathematically using nominal Z₀ impedance conversion
• using a port impedance adaptor and a 75 Ω calibration kit

If you are working with 75 Ω systems you may already have a suitable port impedance adaptor and cal kit.

Time-domain network analysis and frequency domain network analysis are very similar measurements. The former applies a spectrum of discrete frequencies to the unknown network: a step or impulse incident waveform is applied and an oscilloscope or sampling head captures the reflected and transmitted waveforms. The latter applies a series of discrete frequencies and captures reflected and transmitted amplitude and phase using phase-sensitive (IQ) receivers.

The frequency-domain VNA approach has better dynamic range because the applied power at each frequency can be constant and relatively high, and the receivers can have restricted noise bandwidth.

TDR/TDT can theoretically be quicker because a single step or impulse could give all necessary information. However, the high sampling-time resolution required by this method tends to call for a sequential sampling oscilloscope such as the PicoScope 9311. This captures only one sample point on each cycle of a step or impulse, so only repeating signals can be tested. Even so, our TDR or TDT solution is still slightly quicker than the VNA. See below regarding multiple forward, reverse, transmission, and reflection measurements.

The PicoVNA vector network analyzer and the Pico 9311 TDR/TDT Sampling Oscilloscope solutions are comparable on primary features and performance:

1. In the context of cable or transmission line testing, time-domain measurements have the advantage of directly showing impedance continuity v time (TDR) or actual pulse response (TDT). Assuming propagation velocity is reasonably well known, a direct interpretation or readout of impedance v distance is possible. Fortunately, the PicoVNA includes a time-domain readout without extra cost, so both solutions can achieve the ideal readout.
    
2. Where it is necessary to combine measurements with other system elements (measured or simulated) then scattering parameters, Smith charts, etc. tend to be preferred.  While S-parameters and time domain measurements can be interrelated by an FFT, only the PicoVNA supports both output formats.
    
3. The VNA is limited to 6/8.5 GHz and best effective time resolution of 120/85 ps. Assuming a line propagation velocity of 2c/3, this will resolve impedance v distance along your line to around 24/17 mm [with short/open fault location around five times better). The PicoScope 9311 fast step has a system transition time of around 60 ps, giving it roughly the same time (2 x 60 ps) and distance resolution, giving it roughly the same resolution as the PicoVNA 106.
    
4. At its best time domain resolution, the PicoVNA can test at 4096 k discrete sample points. This translates to a maximum path length of around 100 m. At lower resolution, longer path lengths can be accommodated. The longest path length supported by the PicoScope 9311 is theoretically limited by the widest available pulse width of 4 µs, which translates to 400 m in a reflection measurement. In practice, however, at this length, the resolution of a TDR measurement will be lost to high-frequency cable losses and a VNA may match or even outperform it. TDR distance can be increased by instead using the lower frequency capability of PG900 fast step generators.

The larger differentiators between the two are:

1. The VNA automatically sequences reflection and transmission measurements and can do this in both directions through an unknown 2-port network. Greater speed and much-reduced manual intervention are significant benefits when more than a single port reflection or a single transmission measurement is required. Single-direction measurement is typically sufficient for transmission line measurements, but for devices with transmission losses or gain, bidirectional measurements are far more commonly needed and the automated VNA wins even more convincingly.
    
2. In differential line applications the PicoScope 9311 is likely to win. It has differential de-skewable step sources and its two channels can be used to receive either differential reflection or differential transmission. In the case of a differential line without a nearby ground, such as a twisted pair in free space, the PicoScope 9311 can determine differential impedance v distance from a single measurement setup. The differential test stimulus develops a virtual ground (or signal null) at the midpoint between the conductors. The VNA can only stimulate one conductor with respect to the other and cannot resolve differential impedance. It could, however, determine an impedance for a given core and how that changes with length, and could be used in comparison with a gold standard—for instance, both conductors separately measured, the other grounded— in this case.

If we now consider the opposite extreme example of a differential line: the individually screened twinax pair. Here a ground surrounds and isolates the two lines of the differential pair. In this case, the two differential cores can each be measured separately as individual coaxial lines. Their differential impedance will be the sum of the individual impedances. The PicoScope 9311 and the VNA can both do this in a single measurement setup. The two ports of the VNA can be used to separately measure the two cores, but only one TD readout is permitted at a time.

Of course, a great many differential lines lie somewhere between these two extremes with varying degrees of ground and pair coupling. The usefulness and accuracy of the VNA measurement vary accordingly.

Pico is uniquely positioned to support either solution and will be happy to support your decision-making for a given application.

Not at present, but we would be interested to know your requirements.

The SMA and PC3.5 connectors on the PicoVNA make ideal relatively low-uncertainty ports from which to port-adapt both to the larger format and legacy interfaces and to the emerging smaller format and point-contact interfaces. In many cases, port adaptors can achieve sufficiently low measurement uncertainty for systems with connector types of poorer repeatability, and in other applications where performance is not limited by the connector.

Where measurement demands are high, you may be able to obtain port adaptors with either de-embed data or reference plane offset values; both these correction mechanisms being supported by the PicoVNA. In the absence of data, you can purchase a mating pair of port adapters such as PC3.5(f/m) to N(m) and PC3.5(f/m) to N(f). Mate the pair at the N-ports and measure them, then attribute half of the transmission parameters and an average of the reflection parameters to each. This is often how the manufacturer measures adaptors that are provided with data.

Ideally, the adapted ports would be calibrated with port-matching cal kits. These are available from a range of suppliers.

Likewise, within-series and adapting test leads are even more widely available than cal kits. However, it seems likely that emerging small-format connectors will demand a lighter-gauge, more flexible test lead or a flexible port adaptor. Please let us know if you have special requirements in this area.

Using the example of the PicoVNA 106, we have:

• 140 kHz bandwidth
• 10 MHz to 6 GHz sweep
• 201 points (12-term cal): 37 ms for full s2p result set. = 184 µs per point for forward AND reverse sweep.  > 5400 points/sec
• 201 points (S21 cal): 25 ms for single S21 result set. = 124 µs per point for a forward sweep.  > 8000 points/sec

Be careful when comparing with competitors, as they tend only to quote figures for the single-point single sweep. That compares with our faster figure above and not with the slightly longer time for a forward and reverse sweep necessary for a full set of S-parameters. This product is amongst the faster units available. See the competitive comparison on the web site under the Reviews tab.

The sweep speed reduces according to bandwidth settings as the measurement at the output of bandwidth filter has to settle to its full accuracy.

If we first set a 10,001 pt sweep, the forward/reverse switch is proportionally less of an interruption and we can estimate its duration from the two trace lengths that we have.

At 140 kHz 170 us / pt for full s2p (two sweeps):

• Forward / reverse switch implied at around 14 µs.

Testing at other bandwidths for 10k point sweeps:

• At 10 kHz: 430 µs/pt for full s2p (two sweeps)
• At 1 kHz: 3 ms/pt
• At 100 Hz: 28.5 ms/pt
• At 10 Hz: 285 ms/pt

How can I do TRL (through-reflect-line) calibration and de-embedding with the PicoVNA?

TRL (through-reflect-line) calibration is a complex subject. TRL calibration and de-embedding are two separate mechanisms for the removal of fixture errors within a measurement. In fact there are at least five mechanisms that may be appropriate, depending upon the fixture, its errors, the measurement and the accuracy required:

1. Reference plane shift or rotation – Removes the effect of delay in the fixture feed lines, but not their loss, mismatch loss or flatness errors. Applies for transmission (S21, S12) and reflection (S11, S22) measurements.
2. Normalization – Removes the effects of loss and flatness of the feedlines for transmission measurements.
3. Time-domain gating – Assuming that time-domain translation is available for the accurate time location of fixture errors, an additional time-domain gating function can be used to gate them out and then convert back to the frequency or s-parameter domain. This is effective for reflection measurements and removes fixture mismatch and delay, but not fixture loss.
4. De-embedding – Uses S-parameter data files for each of the test port feedlines. These are mathematically backed out of (de-embedded from) all transmission and reflection measurements, wholly removing their errors (at least to the level of accuracy in those de-embed files).
5. TRL calibration – While we can readily and accurately define and apply SOL (short-open-load) calibration standards to a connectorized test port, we can't so readily define SOL standards to apply at the end face of any given fixture feedline. If we could, we would be able to define a calibration reference plane right at the DUT terminals and remove all errors to the feed side of that reference plane. TRL calibration instead uses lengths of transmission line that have identical structure to the feed lines, often several lengths (to achieve a broadband calibration) and with various terminations; a practical, if inconveniently time-consuming, process and one that cannot fully address fixture mismatch errors.

The PicoVNA supports mechanisms (1), (2), (1)+(2) and (4).

Despite having the time-domain function available, we do not have the gating function that would support (3).  Nor do we support (5) in its many complex forms that in practice become necessary.

Our User's Guide has some good background and of course instruction in the use of reference plane shift, normalization and de-embedding. This will help you to decide which method is most appropriate to your application.

If you have to apply de-embedding to your measurement, there are two options for the derivation of the de-embed s-parameter files. Most commonly a simulation model of the feedlines would be used. You might also have a measurement option. If the feedline and launch points are geometrically identical, for instance, you might measure Port 1 and 2 feeds as a through pair and attribute half of the transmission parameters and an average of the mismatch parameters to each. The choice depends on the details of your fixture, your available fixture test pieces and your application.