2017-04-21 Signal Generator for 27 MHz

I need a source of signals on various frequencies to test how well my bandpass filters are aligned.  I built a Vackar Oscillator with tuning over a few CB channels, mainly 27.095 to 27.255 MHz.  These are radio control frequencies.  Like a Colpitts oscillator, the Vackar’s resonant circuit has two capacitors in series with the center point grounded.  The difference is that Vackar has the transistor’s parasitic capacitance somewhat isolated from the resonant circuit.

I’ve been dealing with building a stable RF inductor and trying to get it to be stable with changes in temperature.  Last night I put a coil on it wound with 26 AWG solid enameled copper wire, wound on and secured to a 1 inch length of bamboo skewer stick, and it measured 0.8 uH.  I tuned my Kenwood R-1000 receiver to it.  By this morning it had drifted 6 kHz.

I’m taking advice and adding capacitance to the resonant tank and reducing the inductance.  I changed the coil to 9 turns 20 AWG solid enameled wire on a T50-2 iron powder toroid core, measured at 0.72 uH.  In order to bring the frequency down, I added a variable capacitor, 8 to 50 pF, N750 tempco, in series with a 47 pF silver mica to the ‘cold’ (left) side of the resonant tank.

Update Apr 24 – several days ago I ordered some powdered iron cores, T37-7, which are supposed to have very low temperature coefficient.  I got them today, so I wound one with 23 AWG solid plastic insulated wire from a Cat5e datacomm cable.  I started with 11 turns, but I removed some until the inductance measured 0.62 uH.  The dip meter showed the signal was above 30 MHz, so I added another 100 pF in parallel with the 100 pF C1, and changed the 47 pF to 68 pF.  When I adjusted the 8 to 50 pF, I got the carrier on the receiver.  I tweaked the 0.2 to 5 pF, and got it to be on 27.145 MHz.  I also added a 33 ohm resistor between the collector and C5.

I left it alone while I was doing something else, and came back later and found that it was still at about the same frequency.  I have noticed that while I’m working on the circuit the radiant heat from the soldering iron and even from my body made the circuit drift.  But with the latest modifications the long term drift has decreased, and I can go away for hours and the circuit is still at about the same frequency.  Short term drift isn’t as good, the frequency goes up and down dozens of kHz over the period of a minute.  The toroid core moves around when I pick up the circuit, and the frequency warbles as the coil moves, but I can fix that by gluing the core down.

While I was poking around I noticed that I could make the RF modulate with hum when I touched the point where the two R6 resistors, 10k and 22k joined.  So I connected a PSO to this point to modulate the RF with about 1kHz.  But when I tune the receiver across the band, I notice that I hear the tone at several places over about 10 kHz.  This points me to believe that the modulation is more FM than it is AM.

Update Apr 25 – I have been increasing the capacitance of the resonant tank.  I got to the point where the variable capacitors would not tune high enough, so I removed one turn from the core.  Then the frequency was too high so I had to add more capacitance.  According to what I’ve read the Vackar oscillator should have a 6 to 1 ratio of the divider capacitors, so I changed them to 10 and 47 pF which is 4.7 to 1 ratio, better but still not quite 6.

While I was juggling the capacitor values I used one that was 220 pF Y5P.  This worked but the frequency drifted a huge amount, varying tens of kHz in a few minutes.  So I replaced it with a silver mica capacitor, and the drifting came back down to a point that was much more stable.  The temperature coefficient of these capacitors is very important for good stability – the lowest is best.

Along the way I disconnected everything to the right of C5 and R7.  It just wasn’t effective at adjusting the frequency.

The toroid core is still a T37-7 with 10 turns, and it’s about 550 nanohenrys.  I determined the total capacitance using 27 MHz and 550 nH, and it came to 73 picofarads.  That’s better than approx. 44 pF I started out at.  The Xc = Xl = 80 ohms, which is well below the 100 ohms maximum recommended by others.

Update 2017-05-05 – I have begun another blog with further updates.

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