Thursday, July 30, 2015

Constructing a simple browser controlled RF synthesizer (Part1)

Now that I had a set of modules lying around that have served their initial purpose I had to figure out another boondoggle (or just send them to the recycling center - if you are the kind of person reading this type of material you need no explanation regarding the value of boondoggles versus putting something on the shelf...)

Existing modules with a BeagleBone Black were assembled to produce a frequency synthesizer. The frequency range is 37 – 2000 MHz (CW, square wave) with at least a few kHz of resolution. The level is controllable 0dBm to -30dBm with an accuracy of +/-2 dBm.  For simplicity it uses an external 5V power supply with 100Mb/s Ethernet and web browser user interface.  You can purchase instruments like this on eBay for $200 - $600 from a few sources.  They generally have wider output level ranges and support the full frequency range of the ADF4351, come in smaller packages, and are USB only.  You won’t save yourself any money by building your own (but you’ll have more fun and learn a lot more…).

The idea is to have an  “instrument like” device – something you can plug in, turn on, use, and then turn off.  I did not want to have to spend lots of time drudging up utilities on the PC or worry about shutting the device down properly.  This is part of the reason for focusing on a web browser based control approach (and I wanted to get some hands on experience with embedded web techniques and technologies).

The basic aspects of this project are:
a) Digital and RF Hardware
b) BBB Linux Configuration
c) User Interface Software
d) Packaging

The notes are broken into multiple posts focusing on each of the steps above. The hardware involved was developed in pieces over time and is described in this entry.  In short, an ADF4351 is used as the synthesizer source and the level is controlled using an amplifier with digital step attenuator and a LTC5587 power meter. The basic control software adjusts the attenuator and synthesizer output until the final output is at the desired level.

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