Theory of Operation
3-34 2715 Spectrum Analyzer Service Manual
RFOPTIONS
1dBStep
0
2
4
6
8
10
5MHz
300kHz
30kHz
3kHz
10MHz/−10 dBm
toLog Amp
PINDiode
Attenuator
10MHz/−20 dBm from RF Mother Board
1MHz, 100 kHz, 10 kHz, 1 kHz,
and300 Hz F ilters
10
10
10
1010
Figure 3- 15: VariableResolution Module Diagram
The PIN attenuator design is basedupon a pair of matched pin diodes used in a
tee configuration. This co nfiguration maintains the impedance mat ch at each end,
providing the product of the PIN diode resistances is equal to the square of the
end terminations. This can be approximated if the sum of the currents in the PIN
diodes is equal to a constant. The DAC has complementarycurrent outputs
whose total is set by an adjustment. The attenuationrange is approximately
12 dB with a 1 dB minimum loss. The worst case return loss at theend points is
15 dB. The step size for an 8 bit DAC is 0.04 dB steps in the centerof its range.
PIN Attenuator