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From: Andy Talbot <andy.g4jnt@gmail.com>
Date: Fri, 26 Jan 2018 16:39:45 +0000
Message-ID: <CAA8k23RYAbGQSBVNMyxdViOKpmpUnOg4a_Rk08Ej_nOqSU9BAw@mail.gmail.com>
To: LineOne <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: Softrock RX divider IC problem, unexpected frequency out
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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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<div dir=3D"ltr">That is EXACTLY what happened in my case.=C2=A0 The slow w=
aveform hitting the linearly biassed gates generated instability at the cro=
ssover causing the divider to fail.=C2=A0 =C2=A0I think I may even have see=
n divide by 3 at some point.=C2=A0 =C2=A0=C2=A0<div>It was very clear on a =
scope what was happening.</div><div><br></div><div>The proper solution , fo=
r low frequencies, is to build a squarer circuit using comparators or Schmi=
tt triggers.=C2=A0 =C2=A0It&#39;s just not worth bodging things.=C2=A0 Line=
arly biassed CMOS gates are not always the best way to go from sine to squa=
re</div><div><br></div><div>Andy=C2=A0 G4JNT</div></div><div class=3D"gmail=
_extra"><br><div class=3D"gmail_quote">On 26 January 2018 at 16:23, DAVE PI=
CK <span dir=3D"ltr">&lt;<a href=3D"mailto:daveyxm@virginmedia.com" target=
=3D"_blank">daveyxm@virginmedia.com</a>&gt;</span> wrote:<br><blockquote cl=
ass=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;p=
adding-left:1ex"><u></u>


   =20
 <div><p>Chris<br></p><p>You seem to have made a divide by three rather tha=
n four...<br></p><p>I&#39;m not sure what&#39;s causing that but make sure =
any unused inputs are tied up or down as appropriate.<br></p><p>You have me=
asured the frequency of the oscillator but is it a good square-wave on 464?=
<br></p><p>Something isn&#39;t quite right with the oscillator if its 3kHz =
off..<br></p><p>If it&#39;s a bit wobbly it may be mis-triggering. A capaci=
tor change may be needed around the oscillator part in order to keep the lo=
w-frequency crystal happy.<br></p><p>Dave<br></p><blockquote type=3D"cite">=
<p>On 26 January 2018 at 13:45 Chris Wilson &lt;<a href=3D"mailto:dead.fets=
@gmail.com" target=3D"_blank">dead.fets@gmail.com</a>&gt; wrote:<br><br><br=
><br><br>Hello LF and VLF folk!<br><br>I have built a couple of Softrock Li=
te II IF frequency receivers,<br>normally they would be used to view anothe=
r receiver&#39;s IF on a<br>bandscope, but popularly they are also used on =
low and medium<br>frequencies as stand alone SDR receivers. The MF one work=
s fine. Th LF<br>one was intermittent, then failed with high current draw. =
I isolated<br>it to a faulty or shorted divider chip. The original one was =
marked<br>78C49NM. I couldn&#39;t find one of those, so plumped for one fro=
m RS<br>Components here in the UK, a CD74AC74M96G4, RS part number 662-6939=
<br>The oscillator worked on the bench and from an xtal frequency of 461.5<=
br>kHz gave the expected divide by 4 output of 115.364 or very close.<br>Th=
en suddenly it started receiving broadcast stations and the divided<br>outp=
uts Q and /Q were on 153.814 KHz! There they remain. Fiddling I<br>briefly =
saw a return to 115.36 Khz, but it was not for long. All<br>voltages to it&=
#39;s pins are correct at 5V. I am not sure if it&#39;s a<br>faulty divider=
, or a bad connection or short.<br><br>Xtal case is grounded and seems stab=
le. R16 in the oscillator changed<br>on advice from 22.1k to 10k as they ap=
parently tend to stop running<br>with 21.1k there. I attach the schematic. =
Wrong IC? Details of the<br>divider&#39;s operation are at <a href=3D"http:=
//www.wb5rvz.org/softrock_lite_ii/03_div" target=3D"_blank">http://www.wb5r=
vz.org/<wbr>softrock_lite_ii/03_div</a><br><br>The new IC details are at <a=
 href=3D"https://uk.rs-online.com/web/p/flip-flop-ics/6626939/" target=3D"_=
blank">https://uk.rs-online.com/web/<wbr>p/flip-flop-ics/6626939/</a><br><b=
r>I see these frequencies on these pins:<br><br>Pin 3 : 464.444 kHz<br><br>=
Pin 11 : 464.444<br><br>Pin 12 : 153.814<br><br>Pin 8 : 153.814<br><br>Pin =
2 : 307.625<br><br>Pin 2 : 307.625<br><br>Is this a random IC fault or does=
 it have some mathematical<br>correlation? My maths is pitiful!<br><br>Not =
sure what to do now, I have another IC but before risking pad<br>damage tho=
ught it prudent to ask, many thanks.<br><br>Pins 14 and 2 on the mixer IC s=
how good, 90 degree out of phase square<br>waveforms of normal amplitude on=
 a scope, but at 153.814 KHz....<br><br>Schematic is at <a href=3D"http://w=
ww.gatesgarth.com/schematic.jpg" target=3D"_blank">http://www.gatesgarth.co=
m/<wbr>schematic.jpg</a><br><br> <br><span class=3D"HOEnZb"><font color=3D"=
#888888"><br>-- <br>Best regards,<br> Chris mailto:<a href=3D"mailto:dead.f=
ets@gmail.com" target=3D"_blank">dead.fets@gmail.com</a><br><br></font></sp=
an></p></blockquote><span class=3D"HOEnZb"><font color=3D"#888888"><p>&gt;<=
/p></font></span></div></blockquote></div><br></div>

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