EMRFD Message Archive 5580

Message Date From Subject
5580 2010-12-26 19:37:18 Chris Howard Trouble with Fig 4.11

I'm trying to build a VFO for my "First Transmitter" from EMRFD.

I have followed the Fig 4.11 diagram and I get ~ 5 Mhz output
but the output is very low. My waveform at the collector looks
nice but is just a few tens of millivolts not the 2 volts the text says.
At the junction of the colpitts caps I'm even lower.

I have gone over the circuit. One discrepancy is that the resistor
immediately at the point of application of 8 volts is unmarked in the
I have been applying the 8 volts to the top of the 3.3k resistor.

Also, I am using a 2n2222 transistor.

I have not yet tried changing the emitter resistor.
5585 2010-12-27 11:56:56 Tim Re: Trouble with Fig 4.11
With the Seiler circuit as shown and with 8V at the top of the 3.3K resistor, the collector waveform is a 1.6V bump per cycle. I think that "bump per cycle" is indicative of current limiting. If the capacitive divider (430pF and 180pF caps) is out of whack then the oscillator won't run in current limiting.

The tank voltage actually is running 14 V pk-pk but the amplitudes at the transistor pins are much lower because it's isolated from the tank by that 33 pF capacitor.

Above voltages measured with a 10x scope probe and a 2n3904 although I can't imagine it being much different with a 2n2222.

If you make the base capacitor (0.01uF in the schematic) be too large then the circuit starts up very very slowly. IMHO the startup behavior of this oscillator may be inappropriate for keying in a transmitter. It might look like that capacitor is just a bypass capacitor but really the base R's and that C set (I think! Correct me if I'm wrong) the amplitude limiting timescales of the circuit.

You might just try the J310 in a Hartley (fig 4.4 for basic circuit).

Tim N3QE.

5600 2010-12-28 13:32:10 Wes Re: Trouble with Fig 4.11
Hi Chris,

The resistor from the 8 volt supply is just a decoupling R. 100 Ohms will work fine, as would 47 or 220, etc. Look at the rest of the book and see what was used in other such slots. But don't omit it. Take a look at the info on decoupling.

How are you measuring your signal? The 2.5 volt pk-pk signal mentioned was measured with a 10X probe into the oscilloscope. If you tried to connect it directly to the scope through a piece of wire or a piece of coax, you will be down at the levels you mention.

The Seiler is a sensitive circuit. Any parts that are not not of reasonable Q will cause the levels to be low. "Reasonable" means that the inductor has a Q of 150 to 200 or more, which is easy with a T44-6 or T50-6 toroid. Reasonable C for the caps would be a couple of thousand, again a reasonable thing with ceramic leaded capacitors. See the recent thing I put on my web site regarding capacitor Q. I think the title was "The Two Faces of Q," or something like that.

The collector is a good place to extract the signal. A reasonable buffer would be a comm
5601 2010-12-28 14:18:02 Chris Howard Re: Trouble with Fig 4.11
Thanks Wes and Tim for your comments on my situation.

Also, I will check out that paper on component Q.