[Premium-Rx] Chirpsounders

[Richard M. MC Clung]

Barry has everything pretty close to right. 

A Chirpsounder® receiver is a sweeping HF receiver that tunes through
the HF spectrum typically at a 50 kHz per second or 100 kHz per second
rate beginning at 2 MHz in a 4 minute and 40 second period.  

When synchronized with a Chirpsounder transmitter located at the other
end of an HF circuit, the receiver provides a measurement of
ionospheric propagation conditions for that path.  The Chirp
transmitter at one end of the HF circuit emits a FMCW signal which
starts at 2 MHz and sweeps upward in frequency at a constant linear
rate (50 kHz/second for 2-16 MHz sweep; 100/kHz second for a 2-30 MHz
sweep).  

An internal clock in the Chirp receiver starts its sweep synchronously
with the transmitters start time and precisely tracks the sweeping
transmitter signal.  Any radio energy that can propagate over the HF
circuit is received by the Chirp receiver.  The radio energy propagates
by various modes (e.g.  surface wave, one-hop, two-hop, etc.) which
have different propagation delays, characteristics, and signatures.  

The time required for the travel of the RF signal from the transmitter
to the ionosphere and back to the receiver causes the ionospherically
returned signal to arrive slightly behind the exact tuned frequency of
the Chirp receiver.  Assuming that the transmit and receive sweeps are
started at exactly the same time, the receiver sweep will have advanced
to a slightly higher frequency by the time the transmit signal arrives
at the receiver.  Thus, the receiver is tuned to a slightly higher
frequency than the transmitted signal arriving by skywave.  This
received frequency difference is amplified in the receiver I. F. and
converted to a “baseband audio” tone.  A tone at zero hertz indicates
no time delay.  Tones of increasing audio frequency (up to 500 Hz)
indicate increasing time delay of the ionospherically returned signal. 


In practice, multiple tones are present in the sounder’s baseband audio
output which represent the various delays caused by the different
modes, or layers of the ionosphere.  The multiple tones are processed
by the Chirp receiver's 200-line real time spectrum analyzer to
separate the tones into discrete components which identify
corresponding ionospheric modes.  The output of the spectrum analyzer
is then displayed on a CRT graphic display as a function the RF sweep
frequency of the sounder.  The CRT vertical axis indicates received
signal time delay (ionospheric mode structure) versus the CRT
horizontal axis of radio frequency; 2-16 or 2-30 MHz.  

The result is a display of propagating modes versus frequency, referred
to as a Chirpsounder Record.  As radio energy at various amplitudes is
received, the receiver gain is automatically adjusted to provide a
constant level output to the spectrum analyzer.  The digital AUTOMATIC
GAIN CONTROL (AGC) settings versus radio frequency provides a measure
of the total received signal strength versus frequency, which is
displayed as a bar graph on the CRT above the Chirpsounder Record at
the top of the screen.  In addition to the AGC bar graph two other
versions of the bar graph can be displayed.  The SIGNAL STRENGTH bar
graph subtracts the received background noise level from the total
received signal strength of the AGC display, and is therefore a more
accurate measure of the received strength of the actual transmitted
signal.    The SIGNAL QUALITY bar graph displays a communications
quality rating based on signal strength and propagation mode
characteristics.  

Thus, a Chirpsounder receiver operating at one end of the radio circuit
in association with a Chirpsounder transmitter at the other end
provides a real-time measurement of propagation conditions of the HF
circuit.  

RICH  WA6KNW