Posted on Feb 6, 2014

An 5 pole elliptic low pass filter (impedance: 50Ohm, -6dB cut off frequency 33MHz) connects the antenna signal to the diode ring mixer, a traditional SBL-1, which offers good performance even below 500kHz. A high pass / low pass diplexing filter leads the 45MHz mixing products to a first IF amplifier, while the lower frequency mixing products are

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wasted in a 50Ohm load. I am not too sure the diplexing filter is really needed as the first IF amplifier offers already a fairly broad band and low input impedance, but I don`t think it will hurt a lot. The first IF amplifier is derived from the well known Hayward & Lawson design, it uses a BF494 transistor featuring a moderate ft of 250MHz. The collector current is set to a fairly high 15mA, improving the dynamic range. The output transformer converts the relatively low load impedance by a factor of 9, increasing collector load and hence the gain. A 6dB attenuator sets the output impedance of the first IF amplifier at 2500Ohm, which is the impedance of the 45MHz IF filter. The total gain was measured at 15dB, including the attenuator. The 45MHz IF filter is a paired set of two pole monolithic crystal filters, sold by Golledge as GMCF-45 - 45G15B1 . It offers a bandwidth of 15kHz (3dB ripple), a stop band rejection of 70dB, and an impedance of 650Ohm, see Golledge`s Web site. A second IF amplifier is similar to the first, having a higher input impedance though, and closes the receiver front end part. The overall gain was measured to be 13dB for the entire front end combination of filters, mixers and amplifiers. Cascaded with the ELEKTOR DRM-RX module, a total gain (HF to 12kHz) of 36dB is achieved, which is adequate for an MDS of less than 1 µV. A single voltage controlled VHF oscillator tuneable between of 45 75MHz, a...

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