A lot can be learned when using strict design criteria to build a project. I set out to build an entire receiver using only 2N3904 transistors and at the end settled upon the design shown above. This design resembles that of the Ugly Direct receiver on this web site in a lot of ways and is also a low-cost popcorn project. A great deal of time was spent building and testing various VFO designs and investigating an interesting single-balanced mixer using two 2N3904 BJT's. The design process and reasons for abandoning my original criteria in the case of the mixer and VFO will be discussed.
If you do not have access to test equipment, tune the resonators at the center frequency while listening to the receiver in the headphones to obtain the greatest possible band noise. Confirm your adjustments by tweaking the trim caps while listening to a QSO as well.
The diode ring mixer ultimately used has 50 ohm ports and can be a homebrew or commercial unit such as the popular SBL-1 from MiniCircuits.
An (LC) VFO for 30 Meters
This design was by far the most stable design for both short and long term drift and is the most stable VFO that I have ever built.
The wound inductor should be cemented face down onto the PC board after removing a small portion of copper big enough to fit the inductor on so that it is not touching any of the PCB copper surface. I used a hobby tool and sanded off the copper in a circular shape about 3/4 inch in diameter. The inductor was glued on with epoxy. The Qu of these home spun audio inductors is very low and consequently have very low loss. The 0.56uF cap I used was a miniaturized metallized polyester film (DigiKey EF2564-ND) which is an expensive part at 95 cents Canadian currency.
AF Preamp Chain
Following the diplexer is the familiar grounded base amplifier popularized by Roy Lewellyn, W7EL. This stage presents a low noise, wideband ~50 ohm input impedance to the diode ring detector and diplexer. An active decoupler is used to help prevent any hum getting into this stage. The 22uF capacitor in the decoupler circuit is capacitively multiplied by the beta of Q1 and has an effective filtering value of 22000 uF. The second stage is an amp designed by Wes Hayward, W7ZOI. The DC negative feedback provides bias stabalization for this stage. It is interesting to note that W7ZOI made a break in the DC feedback loop with a 10uF cap to ground so that there is no negative AC feedback around the amplifier and it operates at maximum gain.
The source follower and two low pass stages were pulled from Solid State Design for The Radio Amateur published by the American Radio Relay League. The original article had the a ~1KHz cutoff frequency using 3K3 ohm resistors. The above schematic uses two 3K9 ohm resistors in each low pass stage for a cutoff frequency of 870 Hz. Other cutoff frequencies can be set by adjusting these resistor values as desired. The lowpass filter stages serve to improve QRM copy ability and attenuate a lot of the wideband noise generated and/or boosted in the preceeding stages.
AF Amp and Driver
Driving the final amp is a high gain common-emitter amp with its output connected to a 10K pot for volume control. The 0.0022 uF bypass cap is used as a highpass filter to help remove hiss.The final AF amp is a simple common-collector amp set for approximately 37 mA of emitter current. The 180 ohm resistor could be dropped to 150 ohm (~45 mA Ie) providing a heat sink is used on the BJT. A piece of PC board glued to the flat part of the transistor could be used to fashion a heat sink if you decide to stand more current than the original design.The 10 ohm resistor and the 22uF capacitor on the collector of Q8 form an RC filter to decouple the AF stage from the positive voltage supply. I have found this amp sufficient to drive a pair of Walkman style headphones with reasonable volume. Do not expect ear-shattering volumes levels however. Three sets of cheap headphones were tried and one pair gave very low volume when compared to the other sets. Keep this in mind if your not getting reasonable volume to your ears. The headphone jack used for this rig is a 1/8 inch (3.5 mm) stereo jack with both channels connected together for monoaural output.
Like all electronic projects, this receiver should be built and tested one section at a time. Ugly construction easily allows this to be done. I started with the final amp and then worked backwards through the schematic until the antenna input was reached. Build the 2 low pass filters and the source follower as one section as the source follower is needed to bias the lowpass filter stages. The AF amp stages can be tested with a home brew AF oscillator such as a free-running multi vibrator.