Hello Andy,
I cannot test into the divider as I lost a pad off the PCB and have no
more IC's left, but some are coming and I can jury rig the board for
testing even if it won't be pretty, but I added a Schmitt IC on a
breadboard with long leads, biased with two 10k resistors on the input
pin, and AC coupled with a 0.1uf ceramic. The resulting wave form is
at:
http://www.gatesgarth.com/lf-schmitt-on-collector-q2.jpg
But aside from that in messing I have found the oscillator will
sometimes not start up, but touching the board will get it going.
Also I noticed some wild HF oscillations when it played up as I was
using my better bench scope, not the USB one. So I think the crystal
Colpitts oscillator or crystal itself is suspect. I am Googling how to
derive optimum cap values for an unknown spec crystal but not getting
very far... Is there a way to derive values other than experimentally,
and what experimental methods would be correct? I have left it
oscillating away overnight to see what, if anything, it does long
term. When there were no HF additional oscillations the output even
without the Schmitt was a lot squarer and more even than my ealier
scope capture. I think Pete M0MFT is right and the oscillator itself
is also rather sub optimal in my case. N1BUG used a modern crystal of
known origin, mine is a begged bag of 4 NOS, unknown origin.
Thanks again. All child's play to most of you guys, all new to me ;)
But still much more satisfying than buying commercial black boxes!
Saturday, January 27, 2018, 4:49:55 PM, you wrote:
> Looking at the schematic, I see a 74AC74 divider is used which
> works at over 100MHz clock frequency. You are hitting its edge
> triggered clock inputs with low frequency waveform (461kHz) with a
> slow and indeterminate rise time caused by crude semi-linear
> amplification plus clipping. It really is no wonder the system is
> not driving the divider properly. That sluggish edge has to hit
> the D input on both flip-flops and cause them to toggle together.
> I'm surprised it even appears to work at HF where the design originated.
> That design with no proper logic level squarer is just asking for
> trouble and I'd be surprised if you ever get it to work properly.
> It might, perhaps, if the divider were changed to a slower 74HC74
> device, but that's a bit speculative; it may be just as bad.
> I would suggest you stop fiddling about with component values,
> trying to frig an unsuitable design. You MUST feed a proper
> shaped logic level to such high speed divider chips. There really
> is no getting round that fact. They won't work properly otherwise -
> just look at the specifications for AC series logic.
> You can keep the simple single transistor buffer, but use its
> output to feed the input of a shaping gate. A Schmitt like a 74HC14
> (a package that has 6 suitable gates) ought to do you nicely.
> Bias the input mid way between its two threshold voltage - these are
> different for HC and HCT family devices, so consult the data sheet.
> AC couple your RF to the mid-biassed input and connect output to the
> divider. Look at the gate output on a scope and you'll have a
> beautiful square wave width lovely vertical edges and perfect
> quadrature generation however low a drive signal you put in.
> Andy G4JNT
--
Best regards,
Chris mailto:[email protected]
|
