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Saturday, January 5, 2013

Signal Generator Ideas

Hi, Bill

Howzabout a generator that goes from 1 Hz to 40 MHz with 1 Hz resolution, adjustable cursor so you can step any of the 8 digits, presettable offsets for if frequency, repeater offset, or r.i.t., directly synthesized output so almost no phase noise and 12 bit waveform accuracy? How about a price tag of less than $20 for all the major parts? Start by Googling dd_synth.asm for control software, and rather than buying the PCB recommended in the notes at the head of the program, search for AD9850 on Ebay for the synthesizer board and LCD 16x2 for the display. A 16f628a PIC is a couple bucks. Both the display pinout and the synthesizer pinout match standard perfboard. For a little more than $10 more get the AD9851 board and the upper frequency limit is 60 MHz.

A better way to transistorize the old tube-type generator is to use a dual-gate RF mosfet for the oscillator. You can trim the gain for waveform purity by adjusting the bias on gate 2. Once you find the optimum bias on each band, diode-switch the bias level with the bandswitch. Since gate 2 has an extremely high impedance you can use resistors in excess of a megohm, so it won't affect the Q of the tank coil.

Jim Daldry
Raleigh NC

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1 comment:

  1. Hi Bill,

    If you are a bit adventurous, I might recommend that you get one of the "Softrock" style kits with the synthesized receiver - such as the "Softrock RX Ensemble II" - or one of the many similar kits.

    On the face of it, it's an SDR-type receiver, but it also contains a computer-controlled synthesizer, some of the output of which may be siphoned off (and sent through a buffer amp) and be used as a stable, accurate (after calibration) signal generator.

    As noted by Jim, there are other alternatives like the AD9851 - also the Si570 (like that in the aforementioned kit) that are available as self-contained, stand-alone devices, as you have probably found out.




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