Re: Frequency adjustment on a simple cw transmitter


Jim Kortge
 

On 12/23/2013 12:38 PM, jdcompton@mindspring.com wrote:


I have recently built several QRP cw transmitters, all based on Lew
Smith's Simple 17 meter QRP Transmitter design, but in different
bands.
Where is that circuit published?

Without exception, the transmitting frequency of all of them
has been between 1 - 2 kHz below the crystal frequency. Adding a
capacitor to the circuit, either in series or parallel with the
crystal will supposedly change the frequency, as will a
trimmer/variable capacitor (as in Simple Simon VXO) but I don't know
if it be changed either up or down.
A capacitor in series with the crystal will always move it up in frequency.

My latest build for 15 meters uses a 21.060 crystal and the output
frequency of the transmitter is 21.048. I can and will experiment
with capacitors, but it would help knowing where to start.
That starting point should be to put the specified load capacitor in series with the crystal. In many cases, that would be on the order of 18-27 pF and is often written right on the crystal case or specified by the manufacturer or seller.

My question: does anyone know where in the circuit to add
capacitance to raise the final frequency?
Directly in series with the crystal, either in the hot lead or the one going to ground. It doesn't matter which lead. The grounded lead is the more common location though.

.....or lower it for that
matter.
To lower the frequency, you add inductance in series with the crystal. You will have to play with the value. Above a certain value, the crystal will stop oscillating. 10-68 uH would be a good range to play with for starters.


Or is there an alternative method, such as changing the two
filter coils?
Not if it is crystal controlled. The crystal frequency has to be changed to move the transmit frequency.

Adding a capacitor would be simpler, if that would work.

It will.... Experimentation is required though. It can't be easily calculated unless the crystal has been fully characterized, which takes a bunch of measurements that most folks can't do.


Jim, K4JDC
72 and GL Jim,

Jim, K8IQY

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