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



