Software Vault: The Diamond Collection/The Diamond Collection (Software Vault)(Digital Impact).ISO/cdr40/x1j4pk96.zip/DEVMETER.TXT

Text File | 1995-01-28 | 14KB | 344 lines

Deviation Meter Details                               Page 1

             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, but there is a separate set of
     files that describes the deviation meter in more
     detail, containing the artwork, netlist etc. If you
     can't find the archive of this set of files (
     originally named X1JDEV.ZIP ), contact G0JVU.

2. CIRCUIT OVERVIEW

     The basic structure of the circuit is shown below :


     << circuit is in windows file version >>


     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  disabled,  the
     deviation  details are not displayed  in  the  heard
     list.  If it is enabled, then the 8 bit count  value
     is  multiplied  by the multiplier value  to  give  a
     deviation  reading in hertz. This  is  displayed  as
     kilohertz  in  the  heard list.  If  the  multiplier
     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 ╡F @ 10V
      1    C4      0.1 ╡F (de coupling
                   capacitor)
      1    C2      22 ╡F @ 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
                   Semiconductor) *
      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.  ú8  in
    the  UK. In the last two months it has risen to  ú18!
    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  Cable shield for audio
         1     signal (Earth)
      JP1 Pin  Audio input from rig (5pin
         2     DIN inside TNC)
      JP1 Pin  -5V (from TNC motherboard)
         3


     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