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I

IF
Intermediate Frequency

IF INTERFERENCE
Spurious responses of the analyzer caused by the use of superheterodyning. Crosstalk between circuit modules may lead to a signal value being displayed when an input signal with a frequency equal to one of the intermediate frequencies is applied, even if the analyzer is not tuned to this frequency.

IF REJECTION
A measure of the degree to which IF interference is suppressed by the analyzer.

IMAGE FREQUENCY
A displayed interference signal caused by ambiguity in mixing. Since conversion to the IF level takes place according to the relationship:

f_IF = |f_RF - f_LO|

There will be exactly two receive frequencies which generate the same IF for a fixed setting of the local oscillator frequency:

f₁ = |f_LO - f_IF|
f₂ = |f_LO + f_IF|

f₂ is called „image frequency“.

If the intermediate frequency is above the maximum RF signal frequency, the image frequency can be rejected by first passing the signal through a low-pass filter. If the intermediate frequency is below that of the RF signal, then the image frequency can only be removed by using a preselector.

IMPEDANCE
Usually refers to the internal resistance of the analyzer at the input port. Z = 50 O is most often used as an impedance value in the RF sector. However, also Z = 75 O can be found (e.g. TV). If measurements are made on a system whose impedance differs from the analyzer’s impedance, an impedance transformer or impedance matching adapter is to be used.
Referring this value to a characteristic impedance value gives the !reflection coefficient or the standing wave ratio.

Note: If connectors and cables of different impedance values are connected with one another, severe measurement errors can result and the connectors might even be destroyed! (Example: N connectors 50 O & 75 O differ considerably in the diameter of their center conductor.)

IMPULSE BANDWIDTH
The bandwidth of a hypothetical filter with rectangular passband characteristic having the same area as the resolution filter under the characteristic curve (voltage linear) (i.e. the same voltage transfer function). For a Gaussian filter, the following relationship applies:

IBW = 1.5 RBW

(RBW = 3 dB resolution bandwidth)

Impulse bandwidth of a Gaussian filter

INPUT ATTENUATOR
A variable attenuator placed between the input and the preamplifier stage or first mixer stage of the analyzer. For automatic operation, the attenuator is normally set to give maximum interference-free dynamic range. It can be set manually to cope with other requirements (sensitivity, intermodulation). A manually set input attenuator has the advantage of allowing the operation of the unit with a low-distortion or low-noise setting. In this way the optimum load characteristics of the unit can be selected for the measurement task. At the same time it is possible to e.g. assign distortion-based spectral portions to the DUT or the analyzer by changing the attenuation value thus avoiding any erroneous measurements.

INTERCEPT POINT
Descriptive measure for intermodulation: a theoretical (extrapolated) level at which an intermodulation product is as high as the fundamental signal. Various orders of intercept point are distinguished; the most important factor in the dynamic range of a spectrum analyzer is the 3rd order intercept point (TOI). At the TOI, the 3rd order intermodulation products have the same level value as the fundamental of the test signal (this is a theoretical value, since compression already occurs at much lower values).
If the TOI point is known, it is possible to calculate the 3rd order signal to intermodulation noise ratio (a) for any mixer level.

a_d3/dBc = 2 x (IP₃ - L_in)

where:

a_d3 = 3rd order signal-to-intermodulation noise ratio
IP₃ = 3rd order intercept point (TOI)
L_in = input level to 1st mixer

Determining the intercept point

INTERMEDIATE FREQUENCY (IF)
The spectrum analyzer input signal is converted and selected into fixed frequencies, known as the intermediate frequencies, by heterodyning (mixing) with the aid of a tunable oscillator signal (local oscillator).
Converter and filter components used for converting or signal filtering are also called IF.

INTERMODULATION
If a non-linear circuit is fed simultaneously with several different signals of frequencies f₁, f₂, f₃ ... etc., the output signal will contain signals of frequencies given by the relationship n x f₁, ñ m x f_k, in addition to the original frequencies.
In the datasheet of a spectral analyzer under Intermodulation only the special case of a 3rd order intermodulation of a 2-tone signal is indicated.
The 3rd order intermodulation products of the 1st mixer stage and the IF stages are of particular importance in spectrum analysis as they govern the measurement error and dynamic range of the instrument.

• Input signal:
  f₁, f₂

• Output signal:
  f₁, f₂ original signal
  2f₁ – f₂, 2f₂ – f₁ 3rd order intermodulation products

The amplitude varies in 3rd order proportion, i.e. if the input signal is reduced by 10 dB, the intermodulation product amplitude is reduced by 30 dB (signal-to-intermodulation ratio increases by 20 dB).

Example of third order intermodulation products

INTRINSIC NOISE
Spurious responses, noise floor

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