Antennas/Antennas_CD-ROM_Walnut_Creek_September_1996.iso/net94/docs/netlegen.asc

Text File | 1996-06-30 | 3KB | 71 lines

CADA
COMPUTER AIDED DESIGN FOR AMATEURS

R. P. HAVILAND, W4MB

LEGENDS

Fig. 1. Some of the equivalent circuits usable in design. a)
Replacing a transformer with a T-network. b) Tee to Pi transform:
see the ARRL Handbook for equations. c) Low frequency and high
frequency equivalent circuits for a FET. The Low frequency model
is used by the program. The three capacitors and the resistor may
be added as external elements, and are typically required above
about 10 MHz. d) As c), but for NPN transistors. In addition, the
decrease in beta with frequency can be approximately modeled by
placing a capacitor across the load resistor. The magnitude is
equal to 1/(2xPIxFxR) where F is the beta cutoff frequency and R
is the load resistance.

Fig. 2. Simple circuits for practice in analysis. a) A voltage
divider. b) Series tuned circuit, Input at 1) and output at 2) or
3). c) Parallel tuned circuit. d) A pi low pass filter,
approximately for the low end of 80 meters. e) An opamp. For
correct modelling in the inverting mode, the 10K resistor between
the minus input and common should be in series with the input.
Note the difference in performance for the +/- inputs with this
connection.

Fig. 3. An example of the component connection list sent to the
printer. This list should be a part of the record for the model
run. This is for Fig. 2b.

Fig. 4. The saved file of matrices for the model of Fig. 2b. The
first two numbers are the specified nodes, and the number
actually used. The line of seven zeros are output values, and
show if the model was run before saving values. A line of 6
values are for the A, B, P, Q, R, S, and T matrices starting from
elements 1,1, 1,2 and progressing to N.N.  The model connections
can be recovered from this file, but not easily. See the program
list for exact file structure.

Fig. 5. The main output of a model run of Fig. 2b, always sent to
the screen and optionally to the printer. For most work, the
first three columns are the important ones. The presence of
resonance is shown by both amplitude and phase change.

Fig. 6. The graphical version of Fig. 5. This graph form is used
to allow good detail, since the graph can extend over several
pages. Note that the frequency scale can be linear or
logarithmic. If several decades of frequency are covered, set the
number of points to number_decades*points_per_decade+1. Except
around resonances, 2 or 5 points per decade are usually
sufficient.

Fig. 7. A run result for drive impedance, printed for real and
imaginary components. The data may be presented in the format or
graph of Figs. 2 and 3.

Fig. 8. A more complex practice example, a simplified diagram of
a G-G linear. A good exercise is to try adjusting LC values for
maximum output. Check also the impedances at nodes 2 through 6.
See text for use of node 1.

Fig. 9. A simple example of use of the JOINNET program to create
a more complex net. a) shows Fig. 2b, used as is for the first
net. b) shows how it is also use for the second net, by using a
paralleling resistor. c) shows the combined net, as recorded by
the matrix file. d) shows how this is prepared for analysis using
NET94, by adding the paralleling resistor, and two jumpers,
represented by one-milliohm resistors.