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DEVMETER.TXT
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1993-08-12
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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
+44 473 682266 ( 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
+44 394 274795 (GMT 19:00 - 22:00)
Neville Pattinson
10, High Hall Close
Trimley St Martin
Felixstowe
Suffolk
IP10 0TJ