Section III
Paragraphs 3-8 to 3-14
50 volts (dc or rms): for the Model 350D (input imped- 3-12. USE OF IMPEDANCE-MATCHING NETWORK.
ance, 600 ohms), 5 watts corresponds to approximately
3-13. NEED FOR INPUT MATCH. An impedance- 55 volts (dc or rms).
matching network is necessary between source and
Model 350C/D attenuator under the following CAUTION
conditions:
The Model 350C/D may be damaged by
applying power to the OUTPUT terminals or a. Source frequency is 100 kc or above.
by applying more than 5 watts to the INPUT
terminals. b. Model 350C/D is set for less than 20-db
attenuation.
3-8. OPERATING PROCEDURES.
c. Source output frequency response is affected by
3-9. MATCHED IMPEDANCE. mismatched impedance.
3-10. When the Model 350C/D INPUT and OUTPUT
terminals are terminated properly, attenuation is the d. Source output is monitored by meter which is
sum of the 10 DB and the 100 DB control settings. accurate only when source operates into matched load.
The voltage at the output of the Model 350C/D may be
determined if input voltage (or input db level) and the 3-14. MATCHING AT THE INPUT.
amount of attenuation inserted by the Model 350C/D
are known. Table 3-1 shows the attenuation factor a. When the source is not affected by mismatchand
(Af) over the attenuation range of the Model 350C/D. source impedance is lower than that of Model 350C/D,
The method for finding the input level in dbm, is a series resistor may be used between source and
explained in section IV. To find the voltage at the attenuator. The resistor value should be the difference
output terminals proceed as follows: between Model 350C/D impedance and source imped-
ance. For example, to match the Model 350C to a 200-
a. Determine the input voltage to the Model 350C/D ohm source requires a series resistance of 300 ohms.
and the amount of attenuation set on the Model 350C/D.
b. Except for the condition stated in a., a resistive
b. Locate the amount of attenuation in the db column impedance-matching network should be used.
of table 3-1 and read the corresponding attenuation Resistors should be deposited film or carbon type.
factor. Also, better accuracy is obtained if the network is
enclosed in a shielded container and connecting leads
c. To calculate the output voltage, multiply the in- are kept short. Data on impedance-matching networks
put voltage by the attenuation factor. See paragraph for the Model 350C are given in figure 3-2 and table
3-11 for an example. 3-2; data for the Model 350D are given in figure 3-3
and table 3-3. The amount of insertion loss is included
3-11. In figure 3-1 A and B the Model 350D is shown in tables 3-2 and 3-3.
connected to a matching source and load. In both cases
the Model 350D is set to attenuate the signal by 24 db.
The attenuation factor for 24 db from table 3-1 is
0.0631 and the output voltage, for the conditions shown
in figure 3-1A, is then:
IMPEDANCE OF = 1. IMPEYNCE OF
Vout = (55~) (0.0631) = 3.47 volts MODEL 350C = SOURCE OR LOAD
For figure 3-1B the attenuation factor is the same as ID-3-163
for figure 3-1A and the output voltage is:
Vout = (50~) (0.0631) = 3.155 volts Figure 3-2. Model 350C Matching Network
Table 3-2. Model 350C Matching Network Values
I
Z W Insertion
(ohms) (ohms) lu (ohms) (ohms) I Loss
50 474.3 1.166 51.40 16 db
200 387.3 .8843 256.7 9 db
600 13.22 245.2 1148.0 4 db
2000 31.3 1733.0 536.2 12 db
500 5000 11.66 4743.0 514.1 16 db
01307-1 3-3 |
ACEC
