NATIONAL RADIO ASTRONOMY OBSERVATORY

TITLE: CLIPPER BANDWIDTH REQUIREMENTS

Distribution: GB LibraryCV LibraryVLA LibraryTU Libraries: Downtown File

Mountain FileJ. PayneH. HvatumR. LacasseS. WeinrebP. NapierM. BalisterC. Burgess

Under the p

clipper bandwidth has been uncertain for many years.The requi ement fo

essure of getting equipment built and in service it has been easier

CLIPPER BANDWIDTH REQUIREMENTS

R. E. Mauzy,

to ignore the question and build the clippers with what seemed a generous margin,

about 10 times the signal bandwidth. With ever increasing bandwidth this brute

force approach may not be so easily accomplished the next time around. A calcu-

lated solution was suggested but dealing with the nonlinearity was considered

oach seemed reasonable if a clipperfar more work than by measurement.

could be modified for narrow bandwidth and remain stable.

A clipper from the Model II correlator having about 200 MHz bandwidth per

stage was modified with four 3-pole low pass filters for 12 MHz, 3 dB cutoff

per stage. See Figure 1. A second clipper with full bandwidth was used for

comparison. The two units were measured in the Model III cor elator in the same

IF channel. The first tests showed excess rolloff for the modified unit begin-

ning about 2 MHz and increasing to 1.9 dB at 8 MHz. The modified clipper was

measured for flatness at a non-clipping level and the curve corrected. The re-

sult was a bandpass that dipped to -0.7 dB at 5 MHz and returned to _0.2 dB at

8 MHz. With errors this small it seemed prudent to improve the flatness of the

modified unit, check the reference unit flatness and retest. Small changes in

capacitor values in the filters eliminated the drooping output in the upper half

of the band and provided a flatness within 0.1 dB out to about 9 MHz. Late mea-

surements with a somewhat different test setup showed about 0.2 dB dip in the

center of the band. See Figure 2C. The test clipper flatness was also checked

at five input levels from -20 to -56 dBm. This data showed a very flat response,

0.05 dB at all levels.

The units were aga n checked in the co relator with a common input. The

new data showed a dip through the center of the band of about -0.7 dB and a re-

cove y to dB at 8.5 MHz with respect to the 1.5 MHz level. The results of

0 tests are shown in Figure 2A.. Because of the correlator filter shape it

is not possible to make accurate measurements above 8.5 MHz or below 1.5 MHz on

the 10 MHz band. See Figure 3. The error up to 1.5 MHz was measured by running

bandpasses at 5, 2.5 and 1.25 MHz. The average of several measurements showed a

flat response to 0.75 MHz, down 0.02 dB at 1 MHz and 0.06 dB down at 1.5 MHz.

These results are obtained by aking printouts of the bandpasses hrough both

clippers and determining the difference by scaling. The scaling is referenced

to a baseline that is predicted from the shape of the skirts not from the zero

line of the graph. An error in this line could contribute to a slope across the

band.

The curves of Figure 2A did not have the shape anticipated so were accepted

with skepticism. The next effort was to try a different test method. The band-

pass was checked at a normal input level with one, two and three CW signals,

none of which duplicated the correlator results. The following attempt was to

introduce a flat (t 0.15 dB) 10 MHz band of noise at -20 dBm with a CW signal

added in at -35 dBm. With this combination the results were similar as shown in

Figure 2B, the primary difference being a 0.25 dB rise rather than a 0. 3 dB drop

across the band. The band of the reference clipper used for corre atormeasure-

ments was checked for flatness. As a linear amplifier it had about +0.1 dB rise

across the band, as a limiting amplifier the rise averaged 0.18 dB from 1

MHz and 0.08 dB from 5 to 9 MHz. This reference slope may account for most

the negative slope in the correlator tests.

The next step could be another round of improving measurement accuracy and

clipper flatness. But the effort required to improve the results would increase

3

dramatically. Because of the relatively small effect that bandwidth has on the

spectrum, the extra effort is not considered justified. In conclusion, it is

sufficient to say that a clipper bandwidth somewhat wider than the video band

will contribute no more than 0.5 dB ripple to the spectrum.

REM/cjd

Attachments:Figure 1: SchematicFigure 2: Narrow Band Clipper ResponseFigure 3: Correlator Bandpasses

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