Most SDRs (and many other radio receivers) can benefit from a preamp to boost the signal from the antenna.This one is nice and simple, low in cost, easy to build and works well over most of the HF range.It can be built with variable or fixed gain. Variable gain is ideal as it allows you to avoid overload on strong signals, while still taking advantage of the improved selectivity of a tuned front end.It's a fairly compact unit when completed, and runs from a 5V power supply, which in some cases can come from the receiver itself via the Preamplifier's output lead, using 'phantom power'.The circuit of the Tunable HF Preamplifier is shown in Fig.l.The input signal is fed into chassis-mount BNC connector C0N1, then to the PCB via pin header CON2 and onto DPDT switch Si, which passes it to one of two transformers. This provides two different tuning ranges, allowing the tuning to be more selective.T1 covers arange of about 5-llMHz, while T2 covers 11-24MHz. Both are tuned by dual variable capacitor VCl, with its two gangs wired in parallel to give a 6-200pF range.The tuned signal is then fed to gate 1 of dual-gate MOSFET Ql. The signal is DC-biased from the nominally +5V rail via a 150kΩ resistor and 10nF low-pass filter capacitor, to reject supply noise.
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