Low-cost RF Detector Circuit

Hi,

I'm looking to design an RF circuit that detects the signal strength of
a sinusoidal pilot signal between approximately 90-120MHz and just
outputs a digital signal (so it can be read by a microprocess)
indicating the frequency (within 100KHz) with the greatest amplitude.
Cost is the key factor. Right now, the best design I've been able to
come up with is using a Motorola MC145170 PLL, a mixer and filter or FM
discriminator chip. I was wondering if there is an ASIC out there that
performs this function or can be adapted to perform this function, or
if there's a lower cost alternative.

Thanks.

Jeff

 

[Tim Shoppa]

I'm looking to design an RF circuit that detects the signal strength
of
a sinusoidal pilot signal between approximately 90-120MHz and just
outputs a digital signal (so it can be read by a microprocess)
indicating the frequency (within 100KHz) with the greatest amplitude.

As long as there's only a single dominant signal, it sounds like you
want a 3 or 4-digit frequency counter that goes up to 120MHz.

If you take your signal and just feed it into a bench frequency
counter, does it read what you want?

Tim.

 

Hi Ken,

Thanks, that's a pretty interesting idea. One question is, how stable
would the frequency oscillator be be without the PLL and how accurately
would the correlation between the PIC PWM signal and the actual
frequency be?

Also, could you elaborate a bit on the second part of your reply about
the PIC being run from the VCO and the sampling from the ADC? Thanks
in advance!

Jeff

Ken Smith wrote:

In article <1106683891.271200.123140@z14g2000cwz.googlegroups.com>,
jt.tseng@gmail.com> wrote:
Hi,

I'm looking to design an RF circuit that detects the signal strength
of
a sinusoidal pilot signal between approximately 90-120MHz and just
outputs a digital signal (so it can be read by a microprocess)
indicating the frequency (within 100KHz) with the greatest
amplitude.
Cost is the key factor. Right now, the best design I've been able
to
come up with is using a Motorola MC145170 PLL, a mixer and filter or
FM
discriminator chip. I was wondering if there is an ASIC out there
that
performs this function or can be adapted to perform this function,
or
if there's a lower cost alternative.

Let me be the first to say "Use a PIC".

A PIC has an ADC built in. The ADC can't sample at a high enoungh
frequency to actually digitize the signal you have but, you may not
have
to.

The PWM output of the PIC can control a VCO. The output of the VCO
can be
mixed with your signal and the result low pass filtered. The PIC can
then
sweep the VCO up and down while watching the low pass filter. As you
go
past the signal, the low pass filter will have a beat frequency on
it. If
the PIC watches for the beat being large an low frequencied, it can
find
the carrier.

You may even be able to skip the mixer and low pass. If the PIC is
run
from the VCO, the sampling of the ADC would do the mixing. If you
can
figure some way to reject the various aliases, you could have a very
simple circuit.

--
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kensmith@rahul.net forging knowledge