Re: Receiver bandpass filter questions.

From: Robert C Monsen (rcsurname_at_comcast.net)
Date: 08/17/04 

Date: Tue, 17 Aug 2004 18:46:09 GMT

<Rubicon> wrote in message
news:4122edcc.13580699@news.netaccess.co.nz...
> On Mon, 16 Aug 2004 20:15:19 GMT, "Robert C Monsen"
> <rcsurname@comcast.net> wrote:
>
> >
> ><Rubicon> wrote in message
> >news:41219528.5520302@news.netaccess.co.nz...
> >>
> >> Hello,
> >>
> >> I have some questions about my first basic bandpass filter for an
> >> ultrasonic receiver circuit.
> >
> >I don't know much about filter design, but I've had really good
luck
> >using LM567 chips in 40kHz ultrasonic applications such as this.
Its a
> >'tone decoder', basically a PLL with a logic output when the thing
> >locks. It can be a bit persnickety to get going, but does a very
good
> >job once its tuned up. It is VERY sensitive.
> >
> >LM567s are out of production, at least at national, but you can
> >occasionally get them surplus. Also, I think NTE still sells them.
> >
> >However, I'm guessing you could use any PLL, such as a 4046, using
the
> >phase comparator output as an indication of lock. Here is a lock
> >indication circuit I lifted from a Forest Mims book:
> >
> >
> > Vcc ------------------------------------+----------------+
> > | |
> > V RED V
GRN
> > - LED -
LED
> > | Unlocked |
LCKD
> > .-. .-.
> > |1| |1|
> > 4001 4001 |k| 4001 |k|
> > __ 100k __ '-' __ '-'
> > PC1-----|>=| ___ +-|>=| | +-|>=| |
> > |1 |o--+--|___|--+-----+ |1 |o----+----+ |1 |o-----+
> > PC2-----|__| | | +-|__| +-|__|
> > '--->|----+
> > 1N4148 |
> > |
> > --- 0.05 uF
> > ---
> > |
> > GND
> >
> >created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
> >
> >Its meant to indicate lock of a 4046; PC1 goes to pin 1, and PC2
goes
> >to pin 2. Thus, it works no matter what phase comparator you are
> >using. The PC1 or PC2 input goes high, and the little circuit
between
> >the first two NOR gates acts like a filter, filtering out the
> >transistions caused by the phase comparators. I'm guessing that the
> >4046 + digital output is similar to an LM567. The 4046 + 4001
probably
> >uses far less power, if thats an issue.
> >
> >Regards,
> > Bob Monsen
> >
> >
>
>
> Bob,
>
> Thankyou for all the info there. Phase comparators are components I
> haven't yet had to deal with but I'll look into them now.
>
> Regards,
>
> Andrew.
>

Andrew, A phase comparator is part of a phase-locked loop (PLL), which
is what a 4046 is called.

A PLL consists of a voltage controlled oscillator (VCO), and a phase
comparator circuit. The phase comparator takes as input the input
frequency and the output of the VCO, and outputs a voltage. That
voltage is used to control the VCO. Thus, you have a feedback system
where the input frequency is tracked by the VCO. The circuit above is
used to indicate whether the VCO is tracking the input freqency. The
first LED is on if its not, and the second LED is on if it is...

Thus, the way you set it up is to configure the 4046 to lock near
40kHz (using information in the datasheet), and to feed the amplified
output of your receiver transducer into the PLL input. When the input
is near 40kHz, it'll lock, and the green LED will turn on.

You can use this to turn on your switch.

The advantage of the LM567 mentioned before is that its designed with
this application in mind, so you don't need the output filter circuit
given above. Here is a link where you can get 5 of these for 99 cents
+ shipping.

http://sales.goldmine-elec.com/prodinfo.asp?prodid=8036

Regards,
 Bob Monsen

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