January 2014 21
QUESTIONS and ANSWERS
the frequency. I only need to take the fifth output of a
seven-stage counter to get 49.848230 kHz.
Bomar Crystal Company has agreed to make the
VCXO in a 14-pin DIP package which I incorporated into
the design shown in Figure 3. I was going to wind my
own inductors using ferrite cores that I bought on eBay
(got hundreds of them for $20!), but found that the filter
worked fine with 5% inductors which I could buy for
47 cents each. Plus, they were much smaller than ones I
could build. The capacitors are low temperature drift
(COG) and I provided for trim caps if needed. (See the
schematic in Figure 3.) The power supply is five volts, and
must be stable and low noise to maintain a low noise
oscillator signal. The tuning voltage must also be low
noise; perhaps a battery would be needed. I added an
emitter-follower stage so the filter will not be affected by
the load impedance.
I analyzed the filter using LTSpice; see Figure 4.
The Fourier transform shows some harmonics, but there
is a 100 ohms load on it; it should be better with a
Figure 5 is the typical response.
■ FIGURE 3.
■ FIGURE 4. ■ FIGURE 5.