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

[Andy Talbot <andy.g4jnt@gmail.com>]

That is EXACTLY what happened in my case.  The slow waveform hitting the linearly biassed gates generated instability at the crossover causing the divider to fail.   I think I may even have seen divide by 3 at some point.    

It was very clear on a scope what was happening.

The proper solution , for low frequencies, is to build a squarer circuit using comparators or Schmitt triggers.   It's just not worth bodging things.  Linearly biassed CMOS gates are not always the best way to go from sine to square

Andy  G4JNT

On 26 January 2018 at 16:23, DAVE PICK <daveyxm@virginmedia.com> 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

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