HEWLETT PACKARD (HP) 350D
Service Manual

Section III Paragraphs 3-8 to 3-14 50 volts (dc o r rms): for the Model 350D (input impedance, 600 ohms), 5 watts corresponds to approximately 55 volts (dc o r rms). CAUTION The Model 350C/D may be damaged by applying power to the OUTPUT terminals or by applying more than 5 watts to the INPUT terminals. 3-12. USE OF IMPEDANCE-MATCHING NETWORK. 3-13. NEED FOR INPUT MATCH. An impedancematching network is necessary between source and Model 350C/D attenuator under the following conditions: a. Source frequency is 100 kc o r above. b. Model 350C/D is set for l e s s than 20-db attenuation. c. Source output frequency response is affected by mismatched impedance. d. Source output is monitored by meter which is accurate only when source operates into matched load. 3-14. MATCHING AT THE INPUT.

3-8. O P E R A T I N G PROCEDURES.
3-9. MATCHED IMPEDANCE. 3-10. When the Model 350C/D INPUT and OUTPUT terminals are terminated properly, attenuation is the sum of the 10 DB and the 100 DB control settings. The voltage a t the output of the Model 350C/D may be determined if input voltage (or input db level) and the amount of attenuation inserted by the Model 350C/D a r e known. Table 3-1 shows the attenuation factor (Af) over the attenuation range of the Model 350C/D. The method for finding the input level in dbm, is explained in section IV. To find the voltage a t the output terminals proceed as follows: a. Determine the input voltage to the Model 350C/D and the amount of attenuation s e t on the Model 350C/D. b. Locate the amount of attenuation in the db column of table 3-1 and read the corresponding attenuation factor. c. To calculate the output voltage, multiply the input voltage by the attenuation factor. See paragraph 3-11 for an example.
3-11. In figure 3-1 A and B the Model 350D is shown connected to a matching source and load. In both cases the Model 350D is s e t 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:
Vout = ( 5 5 ~ (0.0631) = 3.47 volts )

a. When the source is not affected by mismatchand source impedance is lower than that of Model 350C/D, a s e r i e s resistor may be used between source and attenuator. The resistor value should be the difference between Model 350C/D impedance and source impedance. For example, to match the Model 350C to a 200ohm source requires a s e r i e s resistance of 300 ohms.
b. Except for the condition stated in a., a resistive impedance-matching network should be used. Resistors should be deposited film o r carbon type. Also, better accuracy is obtained if the network is enclosed in a shielded container and connecting leads a r e kept short. Data on impedance-matching networks for the Model 350C a r e given in figure 3-2 and table 3-2; data for the Model 350D a r e given in figure 3-3 and table 3-3. The amount of insertion loss is included in tables 3-2 and 3-3.

1.
IMPEDANCE OF = MODEL 350C IMPEYNCE OF = SOURCE OR LOAD
ID-3-163

For figure 3-1B the attenuation factor is the same a s for figure 3-1A and the output voltage is:
Vout = ( 5 0 ~ (0.0631) = 3.155 volts )

Figure 3-2.

Model 350C Matching Network

Table 3-2. Model 350C Matching Network Values
Z

l

u
(ohms) 474.3 387.3 13.22 31.3 11.66 1.166 .8843 245.2 1733.0 4743.0

W

I Insertion

I

(ohms)

(ohms) 50 200 600 2000

(ohms) 51.40 256.7 1148.0 536.2 514.1

Loss
16 db

9 db 4 db
1 2 db

500 01307-1

5000

16 db 3-3