[sdiy] Tuning indicator

[Magnus Danielson]

From: Tim Ressel <madhun2001 at yahoo.com>
Subject: [sdiy] Tuning indicator
Date: Thu, 14 Nov 2002 02:19:55 -0800 (PST)

> Hey all,

Hi Tim,

> I've got one for ya: How do you build an indicator
> that shows if one input frequency is higher or lower
> than another frequency input? Kinda like a guitar
> tuner. Would a phase-frequency comparator a la PLLs
> work I wonder?

Sure, a phase-frequency comparator with a suitable filter would indeed give
you the functionality you are looking for.

> This is for a semi-automatic VCO tuning fixture. Some
> day I hope to build several VCOs, and a gizmo that
> makes the task faster would be neat.

You could use a PLL style of comparision. In such an environment I think it
will work well. Phase-Frequency comparators have the downside of being noise
sensitive, but it should not be a problem here. You need to pump out suitable
reference frequencies, which preferably is derived from a crystal oscillator
source. You could use a 4046 or derivate thereof. The benefit is that you with
few external components get a full detector and a buffered result from the
filter.

Another method of trimming oscillators I've seen is to wire up a counter chip
like 8253 or similar to act as a frequency counter. The OB-8 uses this strategy
for the VCOs. The handtrimming is there to bring the VCOs into the land of
autotuning, and that trimming is aided anyway, so it is ultrasimple. The sad
thing is that they didn't do the same thing for the filters ;(

Cheers,
Magnus