DUT Types

Fixed Frequency DUTs

Fixed frequency DUTs, such as low noise amplifiers, do not translate the input frequency to a different output frequency. For such DUTs, f_IF=f_RF.

Frequency Converting DUTs

Frequency converting DUTs, or mixers, translate the input frequency to a different output frequency using a local oscillator (LO). For such DUTs, f_IF≠f_RF.

Frequency converting DUTs can further be classified as upconverters or downconverters based on the frequency at the output port. The two possible frequency products at the output port for a given f_RF and f_LO are |f_RF - f_LO| and f_RF + f_LO.

For downconverters, f_IF = |f_RF - f_LO|, and for upconverters, f_IF = f_RF + f_LO

Image Mixing and Sidebands

For a given f_IF and f_LO, there exists two input frequencies that can map to f_IF. These frequencies are f_IF = |f_RF - f_LO| and f_IF = f_RF + f_LO. One of these frequencies is the signal frequency (f_RF), while the other frequency is the image frequency (f_IM). To distinguish between the signal and image frequency, sideband information for f_RF is required.

• When the sideband is Lower Side Band (LSB), f_RF = min {f_IF + f_LO, |f_IF - f_LO|} = |f_IF - f_LO|.
• When the sideband is Upper Side Band (USB), f_RF = f_IF + f_LO

The following diagram shows an example of the signal and image frequency mixing in a downconverter with signal sideband as LSB.

Image Rejection

Image rejection is the relative gain suppression of image and is defined as the ratio of the gain of the DUT at the image frequency to that at the RF frequency.

Noise Figure measurement requires knowledge of the image rejection specification of the DUT for accurate noise figure measurements. Image rejection of 0 dB implies that both the signal and image frequencies undergo the same DUT gain, while image rejection of 999.99 dB implies that the image is almost entirely suppressed. Any value between 0 dB and 999.99 dB implies partial suppression of the image, relative to the RF frequency.

The following figure describes the noise model for frequency translating DUTs, where the signal and image frequency paths are modelled independently.

where,

N_S(f_RF) is the noise power incident at the input port of the DUT, at f_RF

N_S(f_IM) is the noise power incident at the input port of the DUT, at f_IM

N_D(f_RF) is the noise power added by the DUT, at f_RF

N_D(f_IM) is the noise power added by the DUT, at f_IM

G(f_IF, f_RF) is the DUT frequency translation gain from f_IF to f_RF

G(f_IF, f_IM) is the DUT gain from f_IF to f_IM

N_out(f_IF) is the noise power at the output port of the DUT at f_IF

IEEE definition for frequency translating DUTs [1] is as mentioned in the following equation. The noise factor of the DUT, F_D(f_IF, f_RF), is defined for a pair of input and output frequencies, where f_RF is the input frequency and f_IF is the output frequency.

where,

T_D(f_IF, f_RF) is the noise temperature of the DUT at f_RF

T_D(f_IM, f_RF) is the noise temperature of the DUT at f_IM

G_REJ(f_IF, f_IM) is the image rejection

References

^[1] Otegi, N., et al. "SSB noise figure measurements of frequency translating devices." IEEE MTT-S International Microwave Symposium Digest. IEEE, 2006.