Re: LF: RE: frequency doubler and divider

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

To prolong an already long thread :-)

 

A differential line receiver of the 26xx32 and 26xx33 type gives you a good performance.   It has guaranteed switching thresholds; bias both input at half way and AC couple to one of them.  But also has a small amount of symmetrical hysteresis - .   The '32 devices has 200mV, the '33 has 500mV,  

 

You'll find hysteresis very advisable when you actually start radiating a signal using the interface.   A normal comparator has so much gain that if you were to remove the RF source, there is more than likely to be enough pick up from the antenna to feed back round and cause all sorts of spurious oscillations.  (In my case 700 watts of 137kHz at a frequency set by the resonance of the loding coil / antenna).  Having the hysteresis means a minimum signal has to arrive before there can be any square wave generated.  

 

Precede the comparator input with a two stage bandpass filter , Ql = 10 or so, , and you should be able to feed it with any waveform in the knowledge a nice 50% square wave out will result.

 

Andy

www.g4jnt.com

 

On 2 August 2011 15:55, <victor13@online.nl> wrote:

Hi Stefan,Rik,

i didn't want to make the story too long,but i agree with you.

73 gl

Victor

-----Oorspronkelijk bericht-----
Van: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org]Namens Stefan Schäfer
Verzonden: maandag 1 augustus 2011 19:04
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: Re: LF: RE: frequency doubler and divider


Hi Victor,

Yes, that should work too. But as Rik says, there must be a 50% duty
cycle. And if it's a sine wave, using CMOS gates is not a good idea i
think, because there is a wide range where the gate can switch from L to
H or H to L. So a comparator must be choosen. And so the sine wave
should look clear (no modulation).
Anyway thanks, these are all useful ideas that makes us a bit more
creative :-)

73, Stefan/DK7FC

PS: Hmmmm, indeed, not bad that idea! My PA driver has a rectangular
output. I will feed that to the optocoupler stage (6N136) to prevent EMC
problems in the shack (already works sufficient). Amplified by a BC548
and a transformer between + and collector to a core that comes in
saturation, one could form a curve at 2x f input that has a sharp rising
slope and a lower falling slope. This 2x f signal should be suitable to
drive the SG3525. Ah, i'll try all that tomorrow. Hope the PA will be
active next weekend :-)


Am 01.08.2011 16:40, schrieb victor13@online.nl:
> Hi Stefan,
>
> wat about a bridge rectifier with a divider? Also broadbanded?
>
> 73
>
> Victor
>
> -----Oorspronkelijk bericht-----
> Van: owner-rsgb_lf_group@blacksheep.org
> [mailto:owner-rsgb_lf_group@blacksheep.org]Namens Stefan Schäfer
> Verzonden: zondag 31 juli 2011 15:06
> Aan: rsgb_lf_group@blacksheep.org
> Onderwerp: LF: frequency doubler and divider
>
>
> LF, VLF,
>
> I just re-invented the wheel ;-) Just by thinking i found one possible
> way how to double and divide a rectangular signal, to achieve an
> accurate 50 % duty cycle :-)
>
> One could take a simple XOR gatter like a CD4070 and a RC circuit. The
> time constant has to be about the half of the period of the wanted
> frequency, e.g. 3.6 us for 137 kHz. The output of the RC circuit
> (voltage across C) and the original signal is fed to one single input of
> the gatter. Then the output frequency will be 2x f input.
>
> All this can be found in the internet as well, of course. But if you
> don't know about that possibility you don't know what to seach in the
> web too. At least i haven't found that method by searching and it also
> wasn't discussed here in the recent time. Furthermore i never saw it on
> a PA design by amateurs i found in the web.
>
> The output signal of the 4070 is then fed to the well known frequency
> divider consisting out of e.g. a CD4013. Then output frequency will be
> the same as on the input but the duty cycle will be accurate 50%!
>
> This is one method to (re-) obtain a suitable drive signal for a H
> bridge PA or class D PA after passing the driver signal through an
> optocoupler or so. I'm sure i will use that method now in several
> circuits, like in the new LF PA that still causes some problems,
> excactly on that matter!
>
> Later i found on the German wiki site:
> http://de.wikipedia.org/wiki/XOR-Gatter#Frequenzverdopplung
>
> 73, Stefan/DK7FC
>
>
>