Fig. 1 shows displays with different RBW settings. In the first display, RB is 100 kHz and the sideband signals have merged to appear as a single signal. In the second display RBW is 10 kHz, and in the third display RBW is 100 Hz. Here the sidebands are clearly separated and resolved as different frequency components.
Thus for spectrum analysers, frequency resolution is to be specifiedin order to resolve different frequency components.
In the case of frequency generators, resolution is the ability to set the frequencies that are distinguishable. This is the smallest increment by which the output frequency can be changed.
Frequency resolution is an important parameter for frequency synthesisers. Frequency synthesisers are basically variable-frequency generators that are very accurately and quickly settable and highly stable. The output frequency can be varied in steps—the smallest step being the resolution.
Frequency synthesisers are used as the reference source for phase-locked oscillators. The output of phase-locked oscillators will be some integral multiple of the reference source. For example, if the multiplying factor is 50 and the phase-locked oscillator frequency is to be changed in steps of 1000 Hz, the synthesiser should have a resolution of at least 20 Hz. Thus the resolution specification depends on th requirement.
In the case of instruments that measure power levels, resolution is the ability to distinguish two power levels. If resolution is 0.1 dB, power levels separated by less than 0.1 dB cannot be distinguished. In case, the stability of power amplifers is 0.2 dB/day, to measure it, the power meter should have a resolution higher than 0.2 dB.
Rise time. Some specificationsare relevant to a particular type of measuring instruments. For example, rise time is relevant to oscilloscopes. For pulse waveform measurements, rise-time specificationis very important. It is defined as the time taken to reach fro 10 per cent to 90 per cent of the maximum value. Rise time depends on the bandwidth of the oscilloscope:
Rise time (in nanoseconds)=350/Bandwidth (in megahertz)
A 100MHz oscilloscope will have a rise time of 3.5 nanoseconds. For analysing fast-rising waveforms, oscilloscopes should have a fast rise time. The measured rise time, the oscilloscope rise time and the actual rise time are related by:
Thus for minimal measurement error, the oscilloscope rise time should be small compared to that of the waveform.
FM noise. This specification is important for frequency synthesisers. Phase-locked oscillators are used for carrier generators in communication links. Synthesisers are used as the reference source for phase-locked oscillators. If multiplication factor is ‘N,’ FM noise after multiplication will increase by 20 log N. In higher FM noise, carriers may overlap with adjacent channels. Also, in narrowband communications, this will affect signal detection.
Hence FM noise in a frequency synthesiser should be properly specified. It is specified as single-sideand phase noise and given as X dBc/Hz at Y Hz away from the carrier. This means the phase noise is X dB below the carrier frequency at Y Hz away from the carrier frequency in a measurement bandwidth of 1 Hz.
In spectrum analysers also, phase noise is important as it determines purity of the measured signal. Also, unequal signals cannot be resolved as low-level signals will merge with the noise.
In Fig. 2, because of high FM noise, noise level close to the carrier is very high, masking the low-level signal. Thus noise sidebands affect the resolution of low signals close to the carrier.
General specifications. General specificationsare common to all types of instruments. For example, GPIB/RS-232 interface is needed to connect the instrument with a PC for control and status monitoring. The instruments can be controlled remotely through the PC and all the front-panel readings monitored on the PC. Power source specifcation and environmental specificationslike operating temperature and relative humidity are also common to all instruments.
Table I shows major specificationsthat are applicable to various measuring instruments. ‘Yes’ means the particular specifiation is applicable for the instrument and ‘No’ means the specification is not applicable
In oscilloscopes, where CRT readout is available, frequency resolution may be specified. In instruments lik spectrum analysers, power meters, frequency counters and oscilloscopes, where signal is fed to the instrument, input impedance is specified Whereas in instruments like generators/frequency synthesisers, where output is taken, output impedance is specified.
Apart from the above specifcations, some specificationsfor quick checking of the instruments should be specifed. For example, in power meters and oscilloscopes, CAL out is available for checking the instrument. In signal generators, there may be requirement for amplitude modulation (AM), frequency modulation (FM), etc. For voltage standing-wave ratio (VSWR) measurements in transmission lines using slotted line, squarewave modulation is needed. These specificationsare required in synthesised frequency generators.
Thus the various specifications should be carefully selected depending upon the measurements required.
Table II shows the specificatio comparison of various instruments from different manufacturers. For example, spectrum analysers of Agilent Technology—models ESA-L series and ESA-E series—are compared. In ESA-L series, three models are shown, which cover different frequency ranges. All models have almost the same sensitivity and resolution bandwidths. Optional lower-resolution bandwidths are also available. In ESA-E series four models are shown, which cover different frequency ranges. All have the same sensitivity and resolution band-width specifications. Optional 1Hz resolution bandwidth is also available. So one has to select a model depending on the frequency of operation and resolution bandwidth requirements.
Another example shown is RF power meter. Power meter from Boonton Electronics Corporation covers different frequency ranges and different power level ranges with different models of power sensors. One has to select a particular sensor depending on frequency range and power measurement range.
Agilent Technologies has a power meter with power sensor covering up to 18 GHz and power range from 1 microwatt to 10 milliwatts. Another meter is a portable handheld model with built-in power sensor covering up to 6 GHz and power range from 1 nanowatt to 100 milliwatts. This also operates on battery. It is suitable for transportable installations where space is a constraint. All power meters have a resolution of 0.001 dB. You can select a power meter that meets frequency and power range requirements of your application.
For example, for a satellite communication earth station operating in C-band frequencies, frequency range up to 6 GHz is sufficient.For stations operating in Ku-band frequencies, frequency range up to 14 GHz is required.
To sum up
While selecting a test instrument, keep in mind present as well as near-future requirements. Accordingly, generate specifications.Compare these specificationswith the specificationsof instruments available from reputed vendors and narrow down your choices by selecting instruments that meet the required specifications. Now fro these options you can select one that costs the least.
The author has retired from ISRO as a scientist/engineer