June
2005, Issue 179
Precision
Frequency Meter
Cypress
PSoC High Integration Challenge 2004 Contest Winner
AUDIO
& INPUT PROCESSING
One
of the continuous time blocks provides the analog reference
voltage (AGND) at half VCC. All analog signals then
can be biased to this level.
The
frequency meter powers the radio (3 V). The radio audio
taps out of the radio after the detector but before
the audio amplifier. This ensures that the audio level
remains the same regardless of the radio’s volume setting,
thus allowing the radio volume to be adjusted as necessary.
My
radio is tuned to Radio New Zealand, which is the national
public broadcast service. The FM transmission ensures
that the audio signal remains at a constant level and
minimizes interference (especially from nearby PCs).
The
audio input from the radio is AC-coupled, biased to
AGND, and fed to a programmable gain amplifier configured
to 16 times gain. The amplified signal is output to
a pin and fed through a simple external band-pass filter
consisting of RC low-pass and high-pass filters with
a 1-kHz center frequency (the time pips’ frequency).
The filter output is biased to the AGND reference (VCC/2)
and fed back to the PSoC to two analog comparators.
The
two comparators are set to either side of the AGND reference
(0.5 × VCC):
the high comparator to 0.562 × VCC and the low comparator
to 0.437 × VCC. The two comparator signals are combined
via the LUT to produce a signal-present signal that’s
high for either a positive or negative level. However,
it’s low during periods of silence or zero crossing.
This signal is the counter clock’s capture signal. It
allows the precise clock count to be determined at the
edge of an audio pulse.
The
input signal to measure is AC-coupled by a capacitor
and then biased to AGND. It’s then fed to two inputs.
One is an analog input to an internal PSoC comparator.
The other is a digital input. The comparator output
is fed to another digital input. This permits two input
operation modes: Analog and Digital.
