HP 8508A - Page 34

Operation
Since the circuit impedance is 50 ohms, and the measurements are at relatively low frequencies, we can neglect the
loading effects of the probe tips on the circuit response. For more sensitive applications, the HP 11576A 10:1
Divider or HP 10216A Isolator accessories can be used with the probes to minimize loading effects and measure-
ment errors. :
Measurement details
1. Connect both probes at point “J1” (the amplifier input) in the circuit. Press [PRESET] to return the HP 8508A to
a known starting state.
2. Check that the circuit under test is powered up and that the HP 8508A A UNLOCKED indicator is off. Press [B/
A MAG] and [B-A PHASE] to display vector ratio.
3. Select [DISPLAY] dB to measure the amplifier’s gain as a log ratio. Press [SHIFT] then [SAVE REF] to store
the reference and set the ratio displays to magnitude 0.0dB and phase 0 degrees.
4. Move the B probe to point “J2” (the amplifier output) and measure the gain and phase change. Change the _
frequency of the oscillator over its operating range and check the frequency response variation. In a circuit with
adjustable gain, the HP 8508A METER display is an aid to tuning. (Use of nes meter display is covered in the next
example.)
Any unmodulated RF signal in the frequency range 100kHz to 1GHz‘can be used as a source for these measure-
ments. This includes a source that is part of a larger device under test.
This diagram shows all the keys you will need to make the measurements.
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> Figure 3-4, Front Panel Keys
In the descriptions throughout this section, keys on the instrument are described by writing the key name in square
brackets, for example [PRESET].
Keys that have multiple selections are described by writing “select”, followed by the key name in square brackets,
then the required function in bold type, for example “Select REFL MEAS] SWR”. When you press this type of key,
the next valid state will be selected. Press the key the required number of times to light the LED beside the state
requested. Some selections are only available in combination with other keys, so don’t worry if all the selections are
not available.
3-10
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