I decided to try my hand at building a spectrum analyser after seeing the design by Roger Blackwell, G4PMK, in the Radio Communication Handbook5. I had to adapt the design because I couldn`t get the Motorola MC 3356 IC he used. This is an account of what I came up with, how it performed and how it could be improved. Figure 1 is a block diagram of
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the analyser. It`s a dual conversion superhet. Input signals in the range DC to 50 MHz are up converted to a first IF of 170 MHz where they are amplified and filtered before being down converted to a second IF of 10. 7 MHz. The first IF was chosen to suit a helical filter I had in stock. There`s no reason why another frequency (e. g. 145 MHz) couldn`t be used instead. The crystal filter determines the resolution bandwidth. The helical filter has a bandwidth of 2MHz and serves only to remove spurious responses. Without it there are spurs at 26. 45, 31. 8, 38. 833, 45. 967 and 63. 6 MHz where VCO harmonics fall exactly 10. 7 MHz above or below harmonics of the second local oscillator. The 31. 8 MHz response is generated via an amazing intermediate frequency of 4 * (170 + 31. 8) = 10. 7 + 5 * 159. 3 = 807. 2 MHz showing what the NE602 mixer is capable of. Why so many The design was experimental. I didn`t know how much gain would be needed to raise the VCO output to the required level until I`d built and tested it; and I didn`t want to put too much on one board in case something went wrong! Each module was tested separately. I built two versions of the second IF to try different crystal filters. The VCO is a varicap-controlled FET LC oscillator with an emitter-follower buffer. The amplifier is a pair of cascaded MSA-0404 MMICs. I`m not going to describe these two modules in greater detail because I recommend using a commercial...
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