Re: LF: Softrock RX divider IC problem, unexpected frequency out

[N1BUG <paul@n1bug.com>]

I replied to Chris elsewhere on this and I agree about the 
oscillator being fishy.
When it was working his numbers seemed right... 115.364 LO * 4 = 
oscillator at 461.456, seems right for a 461.5 kHz crystal. All of 
mine have oscillated just a bit below the crystal frequency. (In the 
LF version my 455 kHz crystal oscillates at 454.916.
He now says he's measuring 464.444 at the input to the dividers, 
that seems wrong!
I suspect the oscillator has gone free-running or has spurious 
output(s). It could be his crystal isn't happy with the capacitor 
values I used. They worked for me but I used a different crystal. It 
may also be that resistor which I changed from 22k to 10k. At 22k my 
oscillator was not running correctly. 10k fixed it here but it may 
still be too close to the edge, he might try going a bit lower with 
that resistor.
Paul


On 01/26/2018 11:23 AM, DAVE PICK wrote:
> Chris
>
> You seem to have made a divide by three rather than four...
>
> I'm not sure what's causing that but make sure any unused inputs are 
> tied up or down as appropriate.
> You have measured the frequency of the oscillator but is it a good 
> square-wave on 464?
> Something isn't quite right with the oscillator if its 3kHz off..
>
> If it's a bit wobbly it may be mis-triggering. A capacitor change 
> may be needed around the oscillator part in order to keep the 
> low-frequency crystal happy.
> Dave
>
> > On 26 January 2018 at 13:45 Chris Wilson <dead.fets@gmail.com> wrote:
> >
> >
> >
> >
> > Hello LF and VLF folk!
> >
> > I have built a couple of Softrock Lite II IF frequency receivers,
> > normally they would be used to view another receiver's IF on a
> > bandscope, but popularly they are also used on low and medium
> > frequencies as stand alone SDR receivers. The MF one works fine. Th LF
> > one was intermittent, then failed with high current draw. I isolated
> > it to a faulty or shorted divider chip. The original one was marked
> > 78C49NM. I couldn't find one of those, so plumped for one from RS
> > Components here in the UK, a CD74AC74M96G4, RS part number 662-6939
> > The oscillator worked on the bench and from an xtal frequency of 461.5
> > kHz gave the expected divide by 4 output of 115.364 or very close.
> > Then suddenly it started receiving broadcast stations and the divided
> > outputs Q and /Q were on 153.814 KHz! There they remain. Fiddling I
> > briefly saw a return to 115.36 Khz, but it was not for long. All
> > voltages to it's pins are correct at 5V. I am not sure if it's a
> > faulty divider, or a bad connection or short.
> >
> > Xtal case is grounded and seems stable. R16 in the oscillator changed
> > on advice from 22.1k to 10k as they apparently tend to stop running
> > with 21.1k there. I attach the schematic. Wrong IC? Details of the
> > divider's operation are at 
> > http://www.wb5rvz.org/softrock_lite_ii/03_div
> > The new IC details are at 
> > https://uk.rs-online.com/web/p/flip-flop-ics/6626939/
> > I see these frequencies on these pins:
> >
> > Pin 3 : 464.444 kHz
> >
> > Pin 11 : 464.444
> >
> > Pin 12 : 153.814
> >
> > Pin 8 : 153.814
> >
> > Pin 2 : 307.625
> >
> > Pin 2 : 307.625
> >
> > Is this a random IC fault or does it have some mathematical
> > correlation? My maths is pitiful!
> >
> > Not sure what to do now, I have another IC but before risking pad
> > damage thought it prudent to ask, many thanks.
> >
> > Pins 14 and 2 on the mixer IC show good, 90 degree out of phase square
> > waveforms of normal amplitude on a scope, but at 153.814 KHz....
> >
> > Schematic is at http://www.gatesgarth.com/schematic.jpg
> >
> >
> >
> > --
> > Best regards,
> > Chris mailto:dead.fets@gmail.com