On Sun, 25 May 2014, gareth wrote:

I wonder what others are building into their projects for a VFO
these days?

The PA0KLT from www.sdr-kits.net looks intriguing, and might
settle all arguments in favour of Vackar-Seiler or Franklin for some
years to come?

It depends on what you want to do. Since most people want multiple bands,
it starts making sense to shift to DDS or a synthesizer, since since the
output will be clean.

But there was a period when it was a sort of hybrid, an analog VFO feeding
a PLL, so you could change bands by a new coil for the VCO and changing
the crystal that mixed the VCO down to the frequency of the VFO. There
the output wsa on the injection frequency, unlike some rigs at the time
that mixed the VFO with crystal oscillators for the injection frequency.

So about 1974 I was playing with that sort of PLL and found some car radio
PTOs at the local surplus tore. The mechanics taken care of, I just had to
add an oscillator to the coil. At that frequency, around 1MHz, it was nice
and stable.

Then a year later, I found a bunch of Collins PTOs really cheap at a
hamfest, and bought some (though then I was talked into parting with all
but one). Those were of course tube PTOs and complete oscillators. Those
were easy to change to an FET, and since they too ran at 1 to 1.5MHz, they
were nice and stable, and it required more turns of the knob to travel the
band.

Since that sort of thing needs a dial or frequency counter, that adds to
the complication, another reason to shift to a synthesizer.

On the other hand, if one was going to use a frequency counter, might as
well add the minimal circuitry for "huff and puff" to help stabilize the
VFO, and then actually run the VFO on the injection frequency. I've never
tried one, but I remember receivers being described in ham magazines where
the VFO ran above the signal frequency, and thus was running quite high in
frequency, but apparently with the "huff and puff" was stable enough for
SSB. Except that such things are fading, it's easier to build a frequency
counter now than it was in 1974, since you can get higher densticy
components even at the logic level. A 74LS390 (I think it wsa) is a dual
decade counter, like two 7490s in the old days. And a latch intended for
computers are 8bits, better than the 7475 of the old days that were 4bits.

Michael