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Shareware Overload Trio 2
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SMETER.TXT
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1994-03-12
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Signal Strength Meter for TheNet X-1J Release 2
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.
<< The plot of the calibration data is only in the word for
wondows file. See the file smeter.csv for the raw data >>
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