[GPS_Standard] VE2ZAZ

[Roberto Barrios]

> Date: Tue, 8 Jun 2010 10:12:06 -0700
> From: dplatt at radagast.org
> To: rbarrioss at msn.com
> CC: gps_standard at mailman.qth.net
> Subject: Re: [GPS_Standard] VE2ZAZ
> 
> 
> > I’m not using that circuit on this application, but it could be tuned for
> > it. Yes, 200nS is quite a long time.
> > 
> > The PPS output can deliver up to 5mA, so we could lower the 10K resistor
> > down to drive the gate capacitance harder, and bring the rise time to
> > shorter times (note we actually need to sink current in this application)
> > 
> > Note that the rise time from the Resolution-T directly is “typically less
> > than 20 nanoseconds”, so it may not be very important to have a very fast
> > pulse as long as the trigger level of the Schmitt Trigger in the PIC remains
> > constant over time (which should if we keep temperature and supply voltage “
> > constant”)
> 
> I had a somewhat similar issue in my board, as I wanted to
> drive both the controller's PPS input, and a Maxim MAX238
> TTL-to-RS-232 level shifter (for DCD output to the PC) from
> an Oncore's PPS output. The Oncore PPS driver isn't terribly
> strong, and I was afraid that it couldn't drive both devices
> without slowing down too much.
> 
> I chose to use a classic two-stage common-emitter driver,
> which is (I think) what Bert was alluding to. Two
> transistors (I had a whole bag of 2N2369 fast NPN switches
> in my junk box) and three resistors. This really isn't
> all that much more than the one-MOSFET, two-resistor level
> shifter you alluded to, and I think it's a lot faster (the
> 2N2369 has turn-on and turn-off times of under 20 ns).
> 
> With fast-switch bipolar transistors being such "jellybean"
> parts these days, why risk degrading the PPS rise-time?
> You can even get "digital transistors" which have the
> necessary base resistors build right into the package
> (NEC sells 'em)... two of these daisy-chained together,
> and maybe one external resistor would do the trick.
> 

 

Hi Dave,

 

I agree with you, this particular application may not be the ideal place to use the MOSFET circuit because of the gate capacitance. It's more interesting to use it in an RS232<>TTL converter where the timing is not so critical.

 

I just believed it could be done and it was an interesting conversation anyway :-)

 

Regards,

Roberto Eb4EQA


 
 		 	   		  
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