SpecAl <77> SpecAn

Posted on Feb 7, 2014

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

SpecAl <77> SpecAn
Click here to download the full size of the above Circuit.

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...

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    ICX424 Minimal Head (A2076) Manual
    Discrete Sliding Tone (Frequency Ramp) Doorbell
    Exterior LampCircuit Of 1998 Chevrolet Blazer
    smps modeling
    Solar robotics
    Infrared Remote Tester
    Transistor Tutorial history sample circuits