3.16 Quick Tips

3.16 Quick Tips

  • It is recommended to let the equipment warm up for at least 1/2 hour before each use.
  • Using an attenuator (3 dB or 6 dB) will reduce reflection between the source and the DUT and hence improve the return loss of the instrument.
  • Amplifier connection sequence: Apply Load/Apply Input/Apply Bias (gradually). Do the reverse to disassemble.
  • Ensure that all supplies are tied to the same ground.
  • Great care should be taken to be sure that there is a DC block (a capacitor) at the output and/or input of the device. Any unconfined DC voltage at either point could cause erroneous measurements not to mention that the device or instrument after the DUT could load the DUT. A typical bias T can be used here since it has a capacitor.
  • For spectrum analyzer measurements, the displayed bandwidth should be set wide to give you an indication to where the signal is. Once you have determined where the signal is, you can narrow the bandwidth to that signal.

Dynamic Range of a System

Dynamic range is defined as the difference between a receiver's maximum input level and its noise floor during a single measurement. How much dynamic range is enough depends on the application and the DUT specifications. A general rule of thumb is: System dynamic range > Device specification. How much greater is dependent on the accuracy you want to achieve. There are several techniques for increasing the system's dynamic range. These include:

  • Minimizing receiver bandwidth reduces receiver sensitivity to harmonics, spurious and broadband noise and increases system dynamic range.
For a fixed receiver bandwidth, increasing Source output power will increase the dynamic range dB per dB up to the point that the receiver is driven into compression.
  • Averaging (a sweep-by-sweep process) removes random noise.
Reducing system/resolution bandwidth (point-by-point averaging) reduces random noise. For each factor of ten decrease in system bandwidth, an approximate 10 dB of increased dynamic range is achieved. [See Hewlett-Packard Company, RF and Microwave Device Test for the '90s Seminar Papers (HP publication number 5963-5191E) for more information]

Poor probe contact in on-wafer measurements can drastically affect a measurement. For example, a shift in the resonant frequency, a change in bias current, or simply no reading at all can result. One way to verify the integrity of the contact is to gently tap the floor near the probe station with your foot while looking for changes in the displayed measurement. Erratic measurements can also be an indication of poor probe contact.

"Compensation" is different from "Calibration". Calibration is used to define a reference plane for measurement. Compensation reduces the measurement error induced by the components between the DUT and the calibration plane (e.g., test fixture, cables, etc.).

Cable Handling and Connector Care

Do maintain as large a bend radius as possible in the cable, especially when using the cable in a confined area. A large radius reduces wear on the cable and it minimizes the electrical phase shift changes introduced by the bends, as well as minimizing loss. Two identical cables with different bends will exhibit different loss. By flexible cables do not assume that they can be flexed in any direction an infinite number of times. Any metal (in this case the conductor) eventually breaks or distorts if it is bent (or flexed) enough times.

Use a loop or an s-curve in the interconnecting cables to provide connection without force. A cable can be damaged by exceeding the manufacturer's specified minimum bend radius. Avoid pinching or crushing cable assemblies. Arrange cabling and other components so that no rotational force is applied about the probe's long axis. Torque applied to the probe head is often the cause of misalignment.

When connecting connectors, align center lines before mating. Gently turn the coupling nut, checking for resistance caused by pins not mated, cross-threading, or other damage. Never screw a female connector into a male connector.

Use adapters on connectors that go through many connect-disconnect cycles to prevent damage to the connector. The adapter takes the wear and accidental damage and can readily be replaced. Damaged connectors can damage other connectors. Inspect connector wear at frequent intervals.

There are two basic DC tests that can be performed to verify the integrity of a cable: a continuity check and "ground check". It does not take much resistance to cause an intolerable insertion loss at microwave frequencies. Cable continuity can be checked with a multimeter. First you must "zero" the multimeter by touching the test leads together and adjust for a full-scale reading. Then place each test lead on the center conductor at either end of the cable and verify that you obtain a zero reading on the meter. A "ground check" can be performed using the same procedure but placing one test lead on the outer conductor. There should be no reading at all on the meter. Check that this is truly an open circuit by setting the meter to the highest resistance scale, zero it again, and repeat the procedure.

Cleaning Connectors

Clean connectors are essential for ensuring the integrity of RF and microwave coaxial connections. Consult your network analyzer manual for more information on proper cleaning procedures.

Gain Slope Equalization

In wideband microwave transistor amplifiers, the gain rolls off by approximately 6 dB/Octave per stage, unless there is compensation. Sometimes the roll-off comes unexpectedly sooner than desired (due to larger parasitics or some unaccounted for phenomenon in the design) yet we need the device for proof of concept or to be used with other devices and cannot afford to wait for a re-spin of the design.

A simple low-cost way to compensate for early gain roll-off in broadband low noise and high power amplifiers is to use high-speed gain slope equalization [1] [2]. They can also be used in broadband signal transmission to compensate for transmission medium characteristics and wherever you need slope attenuation characteristics with good impedance match. Both positive and negative slopes are possible but negative slope is used more often. In negative slope equalizers, the goal is to have a specified loss at the low end and zero loss at the high end of the band of interest. This can be achieved by a circuit that presents a short circuit at the higher end and produces some desired attenuation at the low end of the frequency band.

The 3 dB bandwidth of the amplifier/equalizer combination is extended to the equalizer pole at the expense of gain reduction (a 6 GHz extension is possible). The equalizer can be fabricated on duroid using chip resistors and capacitors, 50 ohm microstrip, and SMA connectors. For proper equalization, the zero of the equalizer should be matched with the pole of the amplifier.

There is a variety of equalizer topologies with varying complexity that can be designed for different source and load impedances, and cascaded for steeper slopes (especially those with low SWR) depending on performance required.

References:

[1] Z.M. Markovic, "Designing frequency dependent attenuators for broadband microwave circuits," Microwave Journal, pp. 242-260, May 1991.

[2] Y. Jamani, "MMIC Active Tranversal Filter Using Distributed Amplifier Technique for High-Speed Optical Communications," Master's Thesis, Queen's University, Kingston, Ontario, Canada, 1997

 

Back to Test Guide Table of Contents