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Software Vault: The Diamond Collection
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The Diamond Collection (Software Vault)(Digital Impact).ISO
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SMETER.TXT
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1995-01-28
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S Meter details Page 1
Signal Strength Meter for TheNet X-1J Release 4
This file contains a description of the S-meter extensions
necessary for TheNet X- 1J to display received signal
strength.
The software assumes that there is a signal strength meter
available that produces a voltage proportional to the
logarithm of the input signal strength. If there is no such
output available from the receiver, it is often possible to
add such a function to it.
If there is such a meter output, the ADC expects an input
voltage in the range 0 to 3V. It is not necessary for the
voltage to be referenced to zero for no signal, as the
software can compensate for this. It must not exceed the ADC
reference voltage ( 3V ).
If there is no such meter output, then one may be created by
adding a second IF to the receiver. If a device such as the
MC3356 or MC3362 is used, it has a logarithmic Received Signal
Strength Indicator ( RSSI ) output of surprising accuracy. The
first prototype I built had a deviation from linearity of less
than 1dB over the main part of its range, with a kink at low
signal levels and compression at the high end. If you can
print out the Word for Windows version of this file, a graph
of the calibration data is appended to the file. If not, the
raw data is contained in the file 'smeter.csv' in comma
separated spread sheet format. The next one built had 2 dB
variation in its linearity over the operating range.
The prototype circuit is contained in the Word for Windows
version of this file. It consists of an FET input buffer ( so
that the receiver is not unduly loaded ) followed by a low
pass filter. The filter has a cut-off of 1 MHz. This is
connected to the IF input of the receiver chip, and the output
of the RSSI taken from pin 14.
The circuit is also shown in the file 'smeter.ljt'. This is an
HP PCL printout file. Copy it ( a binary file with the '/B'
switch if using DOS COPY ) to an HP Laserjet or compatible
printer.
You must consider the circuit as a design idea that will need
to be modified for your radio. My prototype was fitted to the
455 KHz IF signal from the second conversion mixer, and the
low pass was needed as there was a significant component of
the 10.245 MHz second conversion oscillator in the signal. The
IF strip of the MC3356 will operate from 200 KHz to 50 MHz, so
without the low pass it can be driven by a 10.7 or 21.4 MHz
IF. What is important is that the signal is taken after the
main receiver selectivity, usually its crystal filter, and
before any limiting IF amplifier stages. It is also important
that the signal levels are correct, so that a signal that is
just detectable on the receiver just starts to increase the DC
output of the RSSI. It may be necessary to adjust the signal
level, for example by adding an amplifier stage before the
MC3356 input.
Note that there are many devices with RSSI outputs - use any
of them that are handy but remember you need one with an
accurate and large range. The operational range of the MC3356
is between 50 and 60 dB, and I am told that more modern
cellular radio IFs have up to 90 dB range !.
To calibrate the meter, you need a known signal, for example a
signal generator of known output, and a switched attenuator
with at least 5dB steps and preferably 2 dB steps. Connect a
DC voltmeter to the output of the MC3356, and connect the
signal generator to the receiver input operating frequency (
144.625 for the prototype ) via the attenuator. The signal
should be increased in 2 dB steps and the voltage noted for
each step. The results need to be plotted as a graph. In
calibrating the prototype, slight errors were noted in the
calibration of the switched attenuator. These need to be
subtracted out from the data.
On the graph, draw a straight line through the curve as a
'best fit' ignoring the end of range effects of noise floor,
hysteresis or overload. Where the line crosses the noise
floor, note the DC voltage and dBm level at this point.
Calculate the slope of the curve in units of dB per volt. You
should then have the following data items :
The noise floor DC reading
The slope of the best fit calibration curve
The dBm point that corresponds to the crossover of
the noise floor and the best fit calibration line.
The dB multiplier is calculated as :
dB_multiplier = X . Vref / V
where X dB change in input caused V volts DC change ( i.e. the
slope of the best fit line from the graph ), and Vref is the
ADC reference voltage.
The data are input as follows :
The signal strength meter noise floor is entered as an integer
in the range 0 to 255 ( hopefully a small number about 50 ish
) calculated from the DC noise floor reading from the graph (
V ) and the ADC reference voltage ( Vref ) as
256 * V / Vref
The dBm meter display format multiplier is entered as
calculated above from the graph. In my prototype, 54 dB change
caused 2V DC change in output with a 3V reference voltage, so
the multiplier was 81.
The dBm noise floor is entered at a positive integer
corresponding to the complement of the dBm zero point from the
graph. For example, 0.65 V DC was the noise floor reading for
my prototype and the calibration line crossed this noise floor
level at a dBm reading of -113 dBm. The dBm noise floor is
entered at 113 ( i.e. drop the '-' ).
The S meter multiplier is set by trial and error depending on
your perception of what constitutes an S9 signal !.
Alternatively, it is set to the dB_multiplier divided by the
number of dB per S point, so in the previous example, if you
want 4 dB per S point, set it to 20. Note that there are
several 'standards' for the number of dB per S point, all
vociferously defended and justified. It is better to use the
dBm scale.
The output of the RSSI needs to be connected to the ADC in the
TNC. The easiest way to do this is to use the squelch line in
the standard TNC2 5 pin DIN connector ( pin 5 ). This signal
is frequently unused in nodes. The RSSI output is connected to
pin 5 in the radio, and in the TNC the signal is disconnected
from the squelch circuits and connected instead to channel 2
of the ADC ( one of the unused pads on the ADC ). In TNCs such
as the BSX2, the squelch signal is connected into the TNC
circuits via a diode that forms a logical AND gate with the
modem DCD. The easiest way to disconnect pin 5 from these
circuits is to lift one end of that diode.
The lead from radio to TNC must be reasonably short as the
output impedance of the RSSI is not low. If problems are
found, an op-amp buffer may need to be added to give a low
impedance drive.
When exploring the innards of radios looking for suitable tap
point, a degree of care and ingenuity will be needed. Finding
one with about the right signal level, prior to a limiter,
after the main bandpass filter and without undue loading on
the radio circuits is not always easy.
<< circuit diagram not present in ASCII version of file >>
Example Node heard list showing dBm format
IPNET:G8KBB-5}
Callsign Pkts Port Time Dev. dBm Type
G8KBB-2 1129 1 0:0:0 Node TCP/IP
FELIX 869 0 0:0:6 5.7 -79
G0JVU-2 4285 0 0:0:40 5.9 -78 Node TCP/IP
G7MNS 368 0 0:1:17 4.1 -89
G8STW-5 6227 0 0:4:54 5.0 -102 TCP/IP
G1YRE 61 0 0:5:27 6.2 -82
GB7MXM 326 0 0:7:6 5.8 -78
FB1ICL 1 0 0:13:40 6.9 -104
G0TMH-5 1 0 0:13:57 6.1 -107 TCP/IP
G0OEY-5 2288 0 0:14:10 6.1 -93 Node TCP/IP
G1DVU-5 1 0 0:18:39 7.6 -107 TCP/IP
G8HUE 90 0 0:21:50 5.5 -92
G7BKO 1 0 2:0:14 7.0 -96
G4ZEK-14 13 0 3:39:22 5.7 -79
G0NJA 29 0 4:8:54 6.6 -91
G7JVE-5 259 0 5:23:33 4.3 -105 TCP/IP
G8INE 5 0 8:11:28 6.3 -112
G4IZC-5 69 0 8:26:29 6.8 -112 TCP/IP