HP 3562A, Service Manual

Page: 267 / 570

C I RC U IT D ESCRI PT IONS
MOD E L 3S62A
Baseband If fi ltering is requi red and the frequency span is 1 00 kH z, the filter
control ler passes the incom ing data to the fi lter ICs in serial format.
The fi lter ICs m u ltiply the incom i ng data by the LO data signal. This
signal is a constant when not in zoom . The fi lter ICs fi lter the data
and return it to the control ler IC Depend ing on the processing required,
this may be a m u ltipass operation. The control ler IC converts the
fi ltered data to a paral lel format.
Zoom
Zoom is used when the selected span is less than 1 00 kHz. The fi lter
controller passes the data to the fi lter ICs in a serial format. The fi lters
down-convert the i n put data by m ixing it with a digital representation
of the span center freq uency from the LO board. The resu ltant sum
and difference are fi ltered to remove the sum component and yield
a red uced frequency span. See figure 6-ASb. The fi lters retu rn the
processed data to the control ler IC, which converts the data to a
parallel format.
Input Signal
Real
From ADC Board Filter
Signals From
{
Sin
LO Board
(Freq.=Center Freq.J
Figure 6-ASb Zoom Implementation
Imag
Filter
Data Ready For
Single Channel
Complex FFT; Yields
Frequency Span Other
Than 0-1 00 KHz
W hen he processed data is ready for storage, the control ler ICs req uest the digital fi lter
local data bus. When granted use of the bus, the controller I Cs transfer the data to the
global data bus interface. The global bus DMA control ler sends a memo ry request to
G l obal RAM. When G lobal RAM retu rns a memory grant, the data flows on the global
d ata bus to G lobal RAM.
.'�
.. ... ..
6-46
f i l ter data busses. The remainder of the process is the same as described previously.
AS,
Digital Filter Block Descriptions
Refer to figure 6-ASc, AS Block Diagram, for the following ci rcuit block descriptions.
D I G ITA L FI LT E R
E ach digital fi lter cons ists of a contro l I C and two fi lter I Cs, one for real data and one
for imaginary data. When in the zoom mode, the filters mu ltiply the input data by a d igital
s ine (real) and cosine (imaginary) function from the LO assembly.