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Created: Oct. 2025
Introduction
Introduction
The cloned LZ1AQ preamp has a gain peak at ~90 MHz (fig. 1, black trace) because it omits the original's low pass filter (LPF). Unfortunately, this gain peak in the FM broadcast (FMBC) band may cause blocking & intermodulation in cheap SDRs.
Fig. 1: The cloned LZ1AQ preamp has an aberrant gain peak at ~90 MHz (black trace) that can detrimentally allow FM broadcast signals to block or intermodulate cheap SDRs. Retrofitting a low pass filter (LPF) beneficially reduces FM band gain by ~24 dB (red trace).
Material & method
Material & method
To resolve the aberrant peak frequency, we retrofitted the said LPF to the clone. The LPF's component values that are similar to the original (red dashed area in fig. 2), except for two minor differences. Firstly, the inductors L1-2 are 330 nH instead of 300 nH because the former is a standard value. Secondly, the damping resistors R21-22 are omitted because the chosen Toko L2012 inductors have low Q (~13 at 25 MHz) that can reduce the peak gain. All the added components are 0805-size and are mounted outside the shield, next to the SMA connectors (fig. 3).
Fig. 2: The LPF circuit (red dashed box) were retrofitted to the preamp to reduce FMBC blocking & intermodulation risks
Fig. 3: Placement of the retrofitted LPF components on the preamp PCB
Results
Results
Retrofitting the LPF beneficially downshifts the peak gain to ~27 MHz (red trace in fig. 4). Consequently, the gain increases in the upper HF range over 17~30 MHz compared to without the LPF. However, there is some gain reduction in the lower HF range below 16 MHz. In the FMBC band, the gain is advantageously reduced by ~24 dB; hence, lessening the potential for blocking / intermodulation (fig. 1).
Fig. 4: The retrofitted LPF beneficially downshifts the peak gain to ~27 MHz (red trace)
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