Elmer 001 - The Pixie 2 Project

"The Oscillator" 
By Brice D. Hornback, KA8MAV
Date 04/14/2001

Overview

The Pixie 2's oscillator is a single frequency "Colpitts Oscillator" with it's frequency set by a single installed crystal.  It's broadband design and components allow any crystal from 3 MHz to about 22 MHz to be used to set the 2N3904 (or 2N2222A / PN2222A) transistor (Q1) to the crystal frequency.  Any crystal above or below this range will require a change in the two 100pF feedback capacitors (C1 & C2) attached to transistor (Q1).  For example, to get this circuit to oscillate on the 10-meter band, try replacing the two 100pF capacitors with 47pF capacitors.  Also, a 2N3904 as supplied in the kit may not oscillate on 10-meters.  If not, replace it with a 2N2222A or a PN2222A or equivalent.

The output of the oscillator is coupled via a capacitor (C3) to the second transistor (Q2), which serves to MIX, AMPLIFY, and DETECT (convert to audio), signals entering the Base and Collector.

Assembly

This may be the first time some of our builders have used a soldering iron, so we'll briefly go over the basics of soldering.  Soldering is not complicated.  Things to remember are don't use too much solder, use enough heat (but not too much), and try to get it right the first time.  Typically, any mistakes that are made can be fixed.  But, if the pads and traces are overheated they may come loose from the circuit board. If a solder bridge or short forms, simply use a piece of solder wick to clean it up.  Double-check all solder joints before proceeding to the next step.  Make certain all the components are soldered and the excess leads are trimmed.  Check for solder bridges and cold solder joints (dull / not shiny).

We'll start by installing the parts for the oscillator section into the printed circuit board (PCB).  If a socket is going to be used for the crystal (XTAL) install it now.  Machine pin sockets such as those made for IC's work perfectly although the holes on the Pixie 2 board may need to be enlarged a little bit so the larger pins will fit.  

Most of the parts on the Pixie can be inserted either way.  However, the two transistors, three electrolytic capacitors (look like metal cans), the diode, and the LM386 IC will be damaged if inserted incorrectly.  If you have any doubts as to which direction to install the parts, please ask.  Typically, the electrolytic caps will be marked with a minus sign on the negative lead, the diode is marked with a stripe at one end indicating the cathode (or negative) side, and the IC is marked with an indentation on one end and possibly a dot nearest pin 1.  The transistors in the Pixie 2 will be installed with the FLAT side facing towards the right side of the board away from the crystal.  

Refer to the completed Pixie 2 photo for a close-up view of the completed board showing the correct parts orientation.

Start at the upper left side of the board and install the parts in the following order:

Note: Leave the output side lead of the capacitor C3 about 1/2" long for now.  This will come in handy later during testing as it gives you something to attach a probe to.  Don't forget later to cut this lead flush with the board like the other leads.

Pixie 2 - Oscillator
(Note: I left the output side lead of the capacitor C3 long and
brought it up through the hole to the right of it for testing.) 

Next, connect a 9-volt battery connector to the board.  A hole has been provided for the + but not for GND.  I simply tacked the black "-" battery lead to the GND side of the crystal.  Make 100% certain that you connect the battery the correct way.  If not, your Pixie 2 will not work and the board may be damaged.  We'll be using a 9-volt battery for now.  If you use a 12-volt supply your test results and measurements will be different.

Testing / Troubleshooting

Apply power to the circuit.  If it smokes, remove power immediately.  If it doesn't, it's passed it's first "smoke test" and you can continue with the testing process.  Next, using the RF Probe you've previously built, measure the voltage at the output of C3.  Don't worry about what the measurement is... just that it is measurable.  (Using a fresh 9-volt battery the "Elmer 001 Pixie 2" measured 2.541 volts DC using the KA8MAV RF Probe and a digital volt meter.)  If you have an oscilloscope, look at the signal at the output of C3. You should see a sine wave with a peak voltage of roughly three or four volts and no DC offset. It is unlikely the oscillator will put out a perfect sine wave.  Don't worry about that as long as the frequency is correct. There are ways to clean up the signal by adjusting the bias of the transistor Q1.  

Note: Without adding additional inductance and/or capacitance in series/parallel with the crystal, a 7.040 MHz crystal is not going to oscillate exactly on 7.040 MHz.  The "stock" Pixie 2 will probably be operating around 7.039 MHz.  (The "Elmer 001 Pixie 2" oscillates at 7.038.850 MHz.)

This completes the Oscillator section of the Pixie 2.

73/72/71! de Brice KA8MAV
ka8mav@arrl.net 

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Additional Pixie 2 information can be found within the pages of the "Pixie Information Project".

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