[DSP-10] Problems at 19.68 MHz

[Roger Hayward]

Courtney:

It sounds like the oscillator is running properly, but it is out of the
range that the VCO can tune it to.

The VCO, by itself, will tune to 19.68 MHz, plus or minus a little bit.
That plus or minus is derived from the PLL chip, which determines whether
the frequency is a little too high, or a little too low, compared to the 10
MHz oscillator.

If the VCO is running at too high a frequency, chances are that the
inductance is too small on the coil, or you have a capacitor value that
needs slight adjustment.  If I recall, even squeezing together the windings
on the toroid will help a little bit.

Remember that the PLL chip U7 won't be dividing the 10 MHz crystal
oscillator down until it has been properly programmed.  Bob W7PUA has a
couple of smaller assembly programs you can download in to the DSP-10 that
will configure the PLL's for tuneup.

The thing that I found to be the very-best with this procedure was breaking
the loop between the PLL and the VCO, and making certain that the VCO, by
itself, will actually oscillate about the target frequency.  I have some
notes on my website I'd like to point you to:

	http://www.ka7exm.net/dsp10/pll_tuneup.html

On the situation near U104, this is hopefully obvious, but make sure you
have pin 1 and pin 2 of P106 connected to one-another.  This is required to
use Y101 on-board, back to U104.  I'm guessing you have done this.  Make
sure that U104 is properly powered, etc.  I can't remember if O_OUT (pin-2)
requires programming before it is enabled....sorry, I don't have a datasheet
handy for that part.

Good luck with the progress on the rig.  Make sure to report back to the
group on your progress!

73,
	Roger
	ARS KA7EXM





-----Original Message-----
From: dsp-10-bounces at mailman.qth.net [mailto:dsp-10-bounces at mailman.qth.net]
On Behalf Of Courtney Duncan
Sent: Monday, February 28, 2005 10:39 PM
To: dsp-10 at mailman.qth.net
Subject: [DSP-10] Problems at 19.68 MHz

I'm to the point of trying to get my DSP-10 working.

After fooling with L21, I have the 19.68 MHz oscillator tuning that 
range, but the output of the PLL amplifier, U6 pin 6, swings from rail 
to rail whilc tuning C69 very delicately as if the negative feedback 
wasn't working.  All the parts are there and check out as being 
connected and not shorted to anything, however.

Looking at waveforms on the scope, 10X probe, the waveform at the 
junction of C69 and L21 is a nice sine wave, about +/- 5 v.  The output 
of U8 is distorted and the output of U9 is very distorted.  Also, the 
outputs of U8 and U9 are about 1 MHz higher than the oscillator 
frequency.  I don't know how this is possible unless distortion peaks 
are getting counted extra, consistently.  Is this a scope probe artifact 
or a real problem?

In a related, but I think, different problem, I have 10 MHz in (from an 
external source) at U104 pin 1 but nothing is coming out of pin 2. 
Again, there are no obvious shorts in the circuit.  I connected pins 1 
and 2 together to get 10 MHz down to U7 pin 1 input for working on the 
above problem for now.

Anybody have any ideas as to what's wrong here?  Do I have a bunch of 
blown parts?  If so, where's a good place to get replacements?

Courtney, n5bf/6
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