VFO Page


 
 

7 MHz Popcorn VFO

Discussion

I wanted to construct a popcorn VFO to put the Direct Conversion Main Frame receiver on the 40 meter band. The design criteria was simple; low cost and a low parts count. A tapped Hartley JFET configuration was chosen for simplicity and for builder reproducability. I have yet to build a tapped Hartley oscillator that failed to oscillate! One thing which may be trouble some for some builders is that the VFO tank was designed for a variable capacitor with a relatively low frequency swing ( 9 to 24 pF ) . It maybe wise to use a smaller L to C ratio in the tank to allow for larger air variable or varactor usage for the capacitive component. 19 turns on a T50-6 , with a tap at 4 turns from the grounded end maybe a good start if a smaller LC ratio is preferred.




Figure 1

This VFO has excellent short and long term frequency stability and the measured output voltage was ~6 dBm. While the output voltage was less than perfect for a diode ring product detector, it more than fulfilled my criteria for a popcorn 2 stage VFO/buffer.
L1 was wound and then boiled in water for ~5 minutes. It was glued to a single sided circuit board with Polystyrene Q-Dope from GC Electronics. The coil does not lay on copper as I used a motor tool to grind away the copper in the shape of a circle with a diameter slightly larger than the wound coil. The inductor was doped face down on this circuit and covered with 2 seperate coats. It is vital that your inductor is secured so that it is unmovable!

The fixed capacitors in the LC tank were NP0 ceramic types. Note that I used four capacitors for C Total to aid in stability. I disconnected and measured C Trim and it read 18 pF. Therefore with C Tune fully meshed, a total of 143 pF was required to resonate the VFO at 7.40 MHz. The total capacitance is mentioned for ballpark reference purpose only. It is unlikely to be a reproducable number. The main tuning cap was purchased from Oak Hills Research years ago when they sold parts. It is a very nice unit which features built in reduction gear and smooth action. I coupled it to the inductor with C Trim to limit the tuning range of the variable capacitor. With C Series at 10 pF as shown the tuning range was about 70 KHz. If C Series was reduced to 5 pF, the tuning range was ~ 30 KHz. Perhaps a 7 pF cap for C series would be a good compromise between the above mentioned caps as I generally like my VFO's to swing around 50 KHz maximum.

The buffer amp is not new, however the trifilar wound transformer for Q2 is a first for me. This idea was borrowed from Rick Campbells binaural receiver project.( A Binaural I-Q Receiver by Rick Campbell, KK7B for QST March 1999 ). This article is a "must have". Hotter transistors for Q1 and Q2 may result in greater output power and less popcorn factor. The Q2 source resistor may have to be increased for a hotter JFET. See Rick's article schematic.

To increase the popcorn factor of this VFO design, the air variable cap should probably be dropped and replaced with tuning diodes. Some experiments using rectifier , varactor and BJT's as tuning diodes were untaken in August 1999, however much more work is warranted before any results can be posted to this web page. The best thing about air variable caps for tuning is their uncompromising stability and linearity.


10.1 MHz VFO Project



Tunes 10.1 - 10.165 MHz , 2 volts pk-pk into 50 ohms

Discussion

It is unfortunate that some QRP operators are easy to spot on the amateur radio bands. I have often encountered transmitted signals that suffer from chirp and have poor short-term drift. Sometimes the operator is utilizing a novelty-style rig and is excuseable, however some better quality rigs and even commercial kit projects use VFO designs or building techniques which are not optimal for emitting a stable, cleanly transmitted signal. The amateur radio literature has extensively covered VFO design and construction practices making poor VFO design/construction somewhat inexcusable in better quality radios. The Clapp series-tuned has been described to have good short and long term drift so I built one for 10.1 MHz and was pleasantly rewarded with a VFO that would be suitable for a good quality 30 meter transmitter.

Background

I have read about but never tried making a series-tuned Clapp Oscillator until this project. The late Doug DeMaw, W1FB wrote about one he built for 1.9 MHz in Solid State Design for the Radio Amateur. The Clapp circuit allows a greater amount of inductance to be used than would be possible in a parallel-tuned oscillator. This in turn tends to minimize the effect of stray inductance from the surrounding circuitry. In addition, the high capacitance of the paralleled 270 pF caps in the feedback circuitry helps to minimize the effect of changes in Q1's junctional capacitance when Q1 is switched on. The XL of L1 is ~393 ohms and this was chosen pragmatically. I basically wound as many turns of # 22 AWG wire that would fit easily on a T68-6 toroidal core. In Solid State Design, it is recommended that L1 has an XL of ~ 260 ohms for the Clapp circuit. Looking at the work of other authors using the Clapp circuit, it seems to me that 260 ohms would be the minimal value to use as wide variation above this value was observed.
For the feedback capacitors, I used four of the largest value NP0 ceramic that DigiKey sells; which are 270 pF. Reviewing the amateur literature again revealed a huge variation in capacitance value versus frequency for the feedback capacitors. I tried increasing the feedback capacitance above the 270 pF paralleled values and could not measure any improvement in short term frequency stability.

Design and Construction Information

L1 was wound and then boiled in water for 6 minutes. It was glued to a single sided circuit board with Polystyrene Q-Dope from GC Electronics. The coil does not lay on copper as I used a motor tool to grind away the copper in the shape of a circle with a diameter slightly larger than the wound coil. The inductor was doped face down on this circuit and covered with 2 seperate coats. Good quality NP0 ceramic caps were used for all the fixed picofarad value capacitors in this project, however the 68 pF cap in the output tank of Q3 need not be NP0. I have found that not all NP0 caps are optimal. Surplus caps sold by some venders allowed drift which was reduced when swapped out for better quality new caps. The units sold by DigiKey have proven excellent, all though I am sure other reputable suppliers also have good quality product.

Q2 and Q3 are lightly coupled to to minimize loading of the previous stage. All leads on all components were cut short as practical. Note that Q1 and Q2 are run at a lowered voltage to minimize component heating. I hate duplicating the VFO tank capacitance values in other people's VFO designs. You never seem to have the variable capacitors and some odd fixed-value caps mentioned in their design. This VFO is no exception. For interest sake the total capacitance of the various fixed value plus the trimmer capacitor to tune 10.1 MHz was just over 40 pF. This is with the main tuning capacitor fully meshed. You will have to redesign the capacitor values to suit the components that you have on hand. Do not use a cheap low-Q trimmer cap. I used a Johnson air-variable unit in my design.

The main tuning cap was purchased from Oak Hills Research years ago when they sold parts. It is a very nice unit which features built in reduction gear and smooth action. I coupled it to the inductor by a 5 pF NP0 capacitor to limit the tuning range of the variable capacitor. As you can see, the VFO tunes up to 10.165 MHz, which is too much for 30 Meters. If this VFO is used for exciting a transmitter on the air in the future, I will further limit the tuning range to prevent inadvertant transmitting out of the 30 meter band limits.
This VFO was built for my test bench and has a built in 50 ohm -3dB attentuator for use with diode ring mixers as the output voltage is 10 dBm into 50 ohms. (not shown) The biggest drawback with the Clapp VFO in my view, is that shifting the VFO for a frequency offset can be difficult at some frequencies. Shifting this VFO 600 hertz off frequency for transmitting would require a very very low capacitance change and would require some major engineering. For this reason, I prefer not to use an offset on Clapp designs.
The VFO was enclosed in a Hammond die-cast box to minimize temperature changes and RF leakage. These Hammond cases. although expensive seem to be great VFO enclosures from my experience. The VFO should be powered with a well filtered DC supply.
The output of this VFO is a clean looking sine wave. This is the most frequency stable VFO that I have ever built. I placed this VFO near a commercial superhet receiver that has a 500 hertz wide crystal IF filter and tuned the receiver so that the VFO signal (evaluated as an audio tone) was centered perfectly in the passband. I heard a very stable 650 hertz audio tone. One hour later, the audio tone was unchanged and the VFO was still centered in the xtal filter passband. Listening to this VFO as an audio tone reminds one of listening to a crystal oscillator. I evaluate all my oscillators this way as variations in frequency are easily detected by variations in pitch by the ultra-sensitive human ear.
The output tank on Q3 will require ~84 pF to resonate and the 68 pF fixed value cap plus a 5 -20 pF trimmer tuned it well. The turns ratio is 27:5 and I used AWG # 26 enamel covered wire. The output impedance is close to 50 ohms. A low pass filter can be added for further signal washing. The 470 microHenry RF chokes used on Q1 and Q2 were epoxy conformal coated inductors that are color coded and resemble resistors somewhat. (DigiKey M7845-ND)

Conclusion

It is hoped that this page will be helpful to those QRP ops who pride themselves with frequency stable, chirp free transmissions on the amateur radio bands.


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