QRZ! Ham Radio 8/QRZ Ham Radio Callsign Database - Volume 8.iso/mac/files/p_thenet/x1j.exe/DEVMETER.TXT

Text File | 1993-08-12 | 14KB | 307 lines

                 Deviation Meter For TheNet X-1J
1. INTRODUCTION

     The deviation meter is actually a peak audio level meter. It
     is  the first of a series of applications built into  TheNet
     X-series  firmware that makes use of an Analogue to  Digital
     Converter  ( ADC ). The ADC can be (optionally) added  quite
     simply to a TNC2.

     This paper describes the circuit, its configuration and  its
     operation.

2. CIRCUIT OVERVIEW

     The basic structure of the circuit is shown below :

     << **************************************************** >>
     << diagrams are only in MS-word version of the document >>
     << **************************************************** >>

     The  audio  input is amplified and converted to  a  DC  peak
     representation. This voltage is read by the ADC, which has a
     range  of 0 .. 3 volts, and converted into an 8  bit  binary
     value in the range 0 .. 255.

     The  software  is configured to read the ADC at the  end  of
     each  valid packet. A packet is considered valid if its  CRC
     is  correct. As soon as a valid CRC is detected, the ADC  is
     instructed  to  start  converting  on  the  deviation  meter
     channel, this conversion taking 30 microseconds. Just before
     the  formatted  frame is linked into the  chain  of  pending
     received  frames, the ADC is read and the value stored  with
     the frame.

     The reading therefore corresponds to the peak audio level at
     the  end  of a packet. If there are no non  linear  elements
     between  the receiver's discriminator and the  meter  input,
     this   will  correspond  fairly  accurately  to   the   peak
     deviation.

     Care  must  be taken over its interpretation.  It  does  not
     measure  independently the two tone levels - it  is  assumed
     that whatever local standards that relate to pre-emphasis  (
     i.e. use it or not ) have been implemented. If used for 9600
     baud FSK operation, this is not a problem.

     The  meter  will  give the wrong  answer  on  the  following
     conditions

            A badly distorted audio signal
            Badly off frequency
            Incorrect adherence to local pre-emphasis
                   standards if used for AFSK
            A noisy signal

3. SOFTWARE


     As explained in section 2, the deviation meter is  activated
     on  receipt  of  a  valid  packet.  The  software  interrupt
     routine,  upon detecting a valid CRC, whether the  deviation
     meter  is  installed or not, whether it is enabled  or  not,
     will instantly write the 'start channel conversion'  command
     to the ADC ( I/O address 0x20 ) as soon as the interrupt  is
     serviced.  It  will therefore also be signalled  when  other
     errors such as framing or invalid CRCs are detected.

     If  the  CRC is correct, then about 50 to  100  microseconds
     later,  the  Z80  will  read the  ADC  and  will  store  the
     corresponding  value  with the packet. Again,  this  happens
     whether or not the function is enabled (it is quicker to  do
     it than to test whether to do it or not ! ).

     The value received is transferred to the internal heard list
     format  without conversion. It is still therefore the 8  bit
     count ( 0 to 255 ).

     When  a  user displays the heard list, the  deviation  meter
     parameter  (  as  set with the METER command )  is  used  to
     control  the display. If it is zero, the  deviation  details
     are not displayed in the heard list. If it is non zero, then
     the  8 bit count value is multiplied by the METER  value  to
     give  a  deviation reading in hertz. This  is  displayed  as
     kilohertz in the heard list. If the METER setting is '1'  it
     will have a full scale range of 0.2 KHz ( not very useful ).
     If set to 10, it will have a full scale range of 2.5 KHz. If
     set to 255, it will display up to 64 KHz. A typical  setting
     will  therefore be in the range 15 to 30, depending  on  the
     analogue  setting. The objective is to set it such that  the
     maximum  possible audio level from the receiver,  given  its
     filters, corresponds to just under the 3 volt maximum  input
     to the ADC.

     The deviation is displayed to a resolution of 100 Hz, but it
     should not be assumed to be this accurate.

     Small fluctuations in the reading can be expected.

     Finally,  to  guard  against  an  unnoticed  overrange,  the
     display in the heard list will precede the deviation reading
     by  a chevron ( '>' ) if the value read from the ADC is  254
     or 255.

4. Detailed Circuit Description.

     All  of the signals necessary for this circuit are found  on
     the  Z80 CPU chip located on the TNC motherboard,  with  the
     exception of a -5V power supply, and the audio signal to  be
     measured.

     We therefore decided to base the circuit around the Z80 CPU.
     We knew from experience that people are reluctant to  attack
     the  insides of their TNCs' with a hot soldering  iron,  and
     settled  on  the socket idea as the least intrusive  way  of
     gaining access to the required signals.

     The  circuit  was designed with the radio ham  junk  box  in
     mind,  with the only "specialist part"  required  being  the
     ADC itself.

     The input is first amplified (x10) by U2A a quarter part  of
     the  LM324  quad op-amp. This circuit is  also  a  precision
     half-wave  rectifier, which is used to reduce the effect  of
     D1's voltage offset. A more linear response is obtained with
     this  configuration. The output is smoothed by C2, and  then
     R3   provides   a  user  adjustable  control  for   the   DC
     representation of the received audio level into the ADC.

     The  second  part of the quad op-amp (U2B)  provides  a  low
     impedance voltage reference drive for the ADC. R6 should  be
     adjusted  such that the DC level on pin 8 (VREF) on the  ADC
     is equal to 3.0 V.

     The  address decode for the ADC is performed by 2  parts  of
     the  74HC00 (U3). The ADC is mapped to the I/O address  0x20
     and is selected by the X-1J software when required.

     The   other  three  (customisable)  ADC  channels  will   be
     available  for  use  in later releases, for  such  items  as
     temperature sensors, pressure sensors, or whatever comes  to
     mind  that will be of interest to users etc. There are  also
     two  spare  op-amps  within the LM324  for  use  with  these
     'customisable   inputs'.   Any  input   signal   should   be
     conditioned to provide a 0V...3V full scale signal into  the
     ADC .

4.1 Parts List


     QTY       REF                 PART

     2              C1, C3         1 fF @ 10V
     1              C4             0.1 fF (de coupling capacitor)
     1              C2             22 fF @ 10V
     1              R1             10K
     2              R5,R7          1K
     1              R4             1K8
     2              R3,R6          10K Miniature Potentiometers
     1              R2             100K
     2              D1,D3          IN4148
     1              D2             2V7 Zener (250mW)
     1              U1             ADC0844 (National Semi.) *
     1              U2             LM324
     1              U3             74HC00
     1              JP1            3 pin HEADER
     1              40PIN          40PIN WIRE WRAP SOCKET
     1              PCB
     1              X-1J           SOFTWARE!

     *  The  ADC0844 is a four channel ADC  available  from  most
     large  mail-order companies. At the time of the X1J  release
     (August '93), I have had some concern over the price of  the
     device.  When originally selected the device  was  available
     for  approx.  8 in the UK. In the last two  months  it  has
     risen to 18!  It may be cheaper direct from outlets  within
     the USA.

5. Construction

     PCB's,  PCB  patterns,  and  component  locations  will   be
     available  from G0JVU / G8KBB starting in  September,  1993.
     The  PCB  will be single sided and approx. 3 inches by  2  +
     inches.

     However  the  circuit  is  not  complex  and  for  the  more
     adventurous  a  small circuit can be built up  (without  the
     need for a 40 pin socket) and then wired to the solder  side
     of the TNC motherboard. In fact the original prototype built
     on veroboard measured 1 inch by 2+ inches.

     The  40 pin socket on the DEV PCB is intended to be  a  wire
     wrap  type,  soldered  to the DEV unit PCB  with  long  legs
     extending  down,  cropped so that the PCB can  be  "plugged"
     into  the  40  pin Z80 socket on the  TNC  motherboard.  The
     previously removed Z80 CPU is then re-inserted in the socket
     on the DEV PCB.

     If your TNC's Z80 is not socketed, you may like to  consider
     adding  one  now! Otherwise you will need to work  out  some
     other way of getting the necessary signals to the DEV board.
     I  suggest fitting a  "tin-plate" folded pin socket, as  the
     wire wrap sockets will locate into them. Turned pin  sockets
     present extreme difficulty in mating with the square section
     wire wrap sockets.

     If   there  is not enough clearance in the TNC box  for  the
     extra PCB, you could consider using a header plug, with  the
     necessary  signals  being  transferred to the  DEV  board  ,
     mounted elsewhere, via a short cable.

     There are three other connections that need to be made  from
     the DEV PCB to the TNC motherboard:

     JP1 Pin 1      Cable shield for audio signal (Earth)
     JP1 Pin 2      Audio input from rig (5pin DIN inside TNC)
     JP1 Pin 3      -5V (from TNC motherboard)


     Solder  in the components in the usual way, taking care  not
     to make any solder bridges to the tracks running  in-between
     the IC pads. Ensure the chips, and polarised components  are
     the  right way around, and take the usual  ESD  precautions.
     There are four wire links needed on the PCB to complete  the
     single-sided circuit.

6. Alignment

     There are four adjustable controls that will need to be set-
     up for correct operation.

     1) Radio's   audio  drive  signal  into  the  TNC.   (Volume
        control).
     2) The deviation signal level control (R3).
     3) The reference voltage for the ADC. (R6)
     4) The METER parameter within X-1J.

     The best set-up sequence (todate)  is:-

     a) Adjust R6 on the DEV board to give 3.0V input into pin  8
        of the ADC.
     b) Attach  an  oscilloscope (if available) to pin 1  of  the
        LM324.  Turn  the  rig's squelch control  such  that  the
        squelch  noise is driving the input of the  DEV  circuit.
        Adjust the rig's audio drive signal into the DEV  circuit
        so  that  the output of the op-amp is just  clipping  the
        power  rails.  This will be seen as a 10V  peak  to  peak
        signal.
     c) Adjust R3 to give the maximum DC signal into pin 3 of the
        ADC. This should be 3V maximum.

     Note: The function of R3 is to scale the input voltage  into
     the  ADC. The maximum signal available from the  circuit  is
     approx. 3V which corresponds to the full range of the ADC. A
     3V input into the ADC will enable the circuit to present the
     widest  possible  range  of values  to  the  X-1J  software.
     However significant "unexplained" variations in the reported
     deviation (MHEARD list) can be reduced by lowering the input
     voltage  to the ADC. If you do need to adjust this  voltage,
     then you will also have to adjust the METER parameter within
     the X-1J software again.

     d) The  "METER"  parameter  within  X-1J  gives  the   final
        adjustment to the circuit. Converting the received binary
        number into a meaningful deviation in KHz.
        In practice with the unit operating as described above, a
        value of 47 was found to provide a true representation of
        the  displayed  deviation to the measured  deviation.  To
        ensure an accurate measurement is given to the user,  you
        will need to calibrate the function using a signal with a
        known deviation!

     As  you  can  see from the above set up  procedure.  If  you
     change the setting of the volume on the rig afterwards,  all
     will  be  lost!   It is preferable to take  the  audio  from
     before  the  audio  amplifier sections  of  the  receiver  (
     ideally  direct from the discriminator ) and to  adjust  the
     parameters or op-amp gain accordingly.

7. Contacts.

     G8KBB can be contacted via...

     G8KBB @ GB7MXM
     [phone] ( GMT 19:00 - 2200)

     Dave Roberts
     7, Rowanhayes Close
     Ipswich
     IP2 9SX
     ENGLAND

     G0JVU can be contacted via....

     G0JVU @ GB7MXM
     G0JVU @ G0JVU.ampr.org
     NEVILLE @ SWFMC1.SINET.SLB.COM
     Compuserve: 100021,3307
     [phone] (GMT 19:00 - 22:00)

     Neville Pattinson
     10, High Hall Close
     Trimley St Martin
     Felixstowe
     Suffolk
     IP10 0TJ