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    # Measurements
    When designing, implementing, and verifying real-world communication systems,
    there are numerous helpful measurements that can be taken. This page explains
    some of these measurements and details how they can be taken using the Keysight
    FieldFox RF Analyzer.
    ## Contents
    1. [Keysight FieldFox](#keysight-fieldfox)
    2. [Spectrum Analyzer](#spectrum-analyzer)
    3. [Oscilloscope](#oscilloscope)
    4. [Frequency Offset](#frequency-offset)
    5. [Transmit Power](#transmit-power)
    6. [Occupied Bandwidth](#occupied-bandwidth)
    7. [DC Offset](#dc-offset)
    8. [IQ Imbalance](#iq-imbalance)
    ## Keysight FieldFox
    The Keysight FieldFox RF Analyzer is a convenient tool for making a number of
    usesful measurements on digital communication systems. Its small size,
    ruggedized form, and battery make it ideal for portable applications, although
    the FieldFoxes will be limited to desktop usage for ELEN90089. The FieldFox can
    be used in one of three modes:
    - Cable and Antenna Analyzer (CAT) - typically used to to test an entire
    transmission system, from transmitter to the antenna. Similar to VNA mode.
    - Vector Network Analyzer (VNA) - used to characterize two-port RF devices by
    measuring both amplitude and phase responses to an applied stimulus.
    - Spectrum Analyzer (SA) - used to observe and measure the characteristics of
    a signal generated by some device or occurring over-the-air.
    In ELEN90089 we will use the FieldFox almost exclusively in spectrum analyzer
    mode. The following are detailed references on the performance characteristics
    and measurement setup for the FieldFox:
    - [Keysight FieldFox N9914A Datasheet](datasheets/5990-9783.pdf)
    - [Keysight FieldFox N9914A User's Guide (Abridged)](datasheets/N9927-90001.pdf)
    > **Note:** It is important to note the accuracy of a given measurement made
    > with the FieldFox will be limited by its relevant performance specifications.
    > See the [datasheet](datasheets/5990-9783.pdf) for full performance
    > specifications.
    ## Spectrum Analyzer
    Using the FieldFox as a basic spectrum analyzer is straightforward. Either
    attach an antenna or cable your transmitted signal into the *RF IN* port of
    the analyzer. An SMA to N-type adaptor should be present on all FieldFoxes in
    the lab. The FieldFox can be put into spectrum analyzer mode be selecting:
    `Mode > SA`.
    At a minimum, to observe the frequency spectrum of a signal at RF you should set
    the following parameters:
    - Center Frequency: `Freq/Dist > Center`
    - Bandwith: `Freq/Dist > Freq Span`
    The FieldFox will likely set the amplitude (Y-axis) scale automatically, but you
    may wish to adjust this to better investigate the signal of interest:
    - To use autoscale: `Scale/Amptd > More > Autoscale`
    - To set the units per division (gridlines): `Scale/Amptd > Scale`
    - To set the reference level (top of graph): `Scale/Amptd > Ref Level`
    Markers can be used to investigate specific points on the output spectrum. For
    example, to find the frequency and power of the highest peak displayed you can
    choose: `Marker > Peak`.
    Refer to the [user's guide](datasheets/N9927-90001.pdf) for full details on
    configuring the FieldFox.
    ## Oscilloscope
    The FieldFox can be used as an oscilloscope by setting the frequency span to
    zero: `Freq > Freq Span`. You can then:
    - Set the center frequency as desired: `Freq/Dist > Center`
    - Set the sweep time to change the length of the time plot:
    `Sweep 3 > Sweep Time`
    - Set the resolution bandwidth (ResBW) to change the captured bandwidth:
    `BW 2 > ResBW > Man`
    ## Frequency Offset
    The frequency offset of a device (e.g., the bladeRF) can be measured by
    configuring the device to generate a tone at a known frequency and then using
    the marker function to observe the location on the FieldFox display. Frequency
    offset is then the difference between these two values.
    > **Note:** The accuracy of frequency measurements are limited by the accuracy
    > of the LO used in the FieldFox, which is +/- 0.7 ppm + aging. See the
    > [datasheet](datasheets/5990-9783.pdf) for full performance specifications.
    ## Transmit Power
    Any external attenuator used (e.g., the 30 dB SMA attenuator we use for hardware
    loopback) should be accounted for in power measurements. This can be done
    automatically in the value displayed by the FieldFox by setting
    `Scale/Ampltd > More > External Gain`.
    The power level of a single frequency tone is best measured using a marker:
    `Marker > Peak`.
    For signals with non-zero bandwidth, e.g., a modulated signal, the channel
    power (CHP) within a frequency range should be measured using:
    `Measure 1 > Channel Measurements > Channel Power`.
    - To set the integrated bandwidth used in the CHP measurement:
    `Meas Setup 4 > Integrating BW`
    > **Note:** Power measurements with the FieldFox generally have an accuracy of
    > +/- 1 dBm. See the [datasheet](datasheets/5990-9783.pdf) for full performance
    > specifications.
    ## Occupied Bandwidth
    The occupied bandwidth measurement looks over a frequency span and determines
    the start and stop frequencies between which is contained a specific percentage
    of the observed power, e.g., 95% of the observed power. These frequencies are
    marked with vertical bars and both occupied bandwidth and occupied power
    measurements are displayed. To measure occupied bandwidth:
    - `Measure 1 > Channel Measurements > Occupied Bandwidth`
    ## DC Offset
    The RF front-ends of the SDRs we employ in ELEN90089 all employ direct
    conversion architectures (zero IF or homodyne) for both their transmit and
    receive chains. Direct conversion greatly reduces the number of components in
    the RF front-end and simplifies the design, but results in a number of specific
    impairments that need to be addressed. DC offset, partially as a result of LO
    leakage, is one such impairment.
    DC offset of a transmitter can be observed by configuring the device to
    generate a signal with no energy component at dc and then observing the output
    on a spectrum analyzer. As an example, we can generate a complex exponential
    rotating at a frequency f_0 at baseband, which after upconversion by our RF
    front-end should be present f_0 Hz above our carrier frequency (f_c + f_0).
    The ratio between the power at our desired frequency (f_c + f_0) and that at
    the carrier (f_c) is a quantification of the severity of the DC offset
    present. This is typically expressed in units of dBs below carrier (dBc).
    ```
    P_dc (dBc) = P_{f_c + f_0} (dBm) - P_{f_c} (dBm)
    ```
    Larger values of `P_dc` are obviously better. DC offset of a receiver can be
    measured in a similar way by feeding a passband signal with no energy at the
    carrier frequency (f_c) into the RF front-end downconverter and observing the
    resulting baseband spectrum. Care must be taken that the DC offset of the device
    generating the waveform is negligible so as not to distort the measurement.
    > **Important Note:** DC offset is both gain and frequency dependent.
    ## IQ Imbalance
    IQ imbalance is the other main impairment of direct conversion receivers that we
    discussed. It is the result of a mismatch in the gains and phases of the
    in-phase and quadrature LOs generated for use in up- or downconversion. It is
    present in all architectures that use quadrature modulation or demodulation,
    but is more pronounced in direct conversion architectures.
    IQ imbalance of a transmitter can also be measured by generating a tone at a
    known frequency offset (f_0) from the carrier (f_c) and observing the output
    spectrum. When present, in addition to the desired tone at f_c + f_0 (Hz), IQ
    imbalance results in a complementary tone at the same offset below the carrier,
    f_c - f_0 (Hz). The ratio between the power of these two tones is a measure of
    the severity of the IQ imbalance.
    ```
    P_{iq} (dBc) = P_{f_c + f_0} (dBm) - P_{f_c - f_0} (dBm)
    ```
    Receiver IQ imbalance can also be measured using the a single-tone test but,
    again, care should be taken to ensure the measurement is not capturing any IQ
    imbalance in the device generating the waveform.
    > **Important Note:** IQ imbalance is both gain and frequency dependent.