Unexpected Image/Spurious Signal at 3.7 GHz from a 950 MHz Source

[bluzye]

Device & API Information:
API Info lib: 2.4.1 Mar 4 2026 10:59:26 drv: libusb
found devices: 1
dev# 0: A1D610000134
hw_revision: 3.0.0
fw_version: 2.2.0 Aug 25 2025 18:22:53
manufacturer: RigExpert
product: Fobos SDR
serial: A1D610000134

Software Used:
SDR++

Steps to Reproduce:
Use a USRP in close proximity to transmit a 4 Msps GFSK signal at 950 MHz.
Configure the Fobos SDR in SDR++ with the following settings:
Sample Rate: 50 MHz
Port: RF
LNA: 1
VGA: 15

Tune the Fobos SDR to 950 MHz. The measured signal strength is approximately -55 dB.

Tune the Fobos SDR to 3730 MHz.

Observed Behavior:
An image/spurious signal appears between 3.7175 GHz and 3.72 GHz, with a maximum strength of approximately -72 dB.
When tuning the frequency in 1 MHz steps starting from 3730 MHz, the signal produces the stepping pattern shown in the attached screenshot. When the USRP stops transmitting, the image completely disappears, confirming it is generated by the 950 MHz transmission.

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The official documentation states: "IQ Image rejection 52 dB (software)". However, the difference between the primary signal (-55 dB) and this observed image/spur (-72 dB) is only about 17 dB, which seems to indicate an issue with image rejection or internal harmonic mixing at these specific frequencies.