Elenco Electronics MO-1251 Instrukcja Użytkownika Pobierz pdf (Strona 14)

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LETTERS

THOSE

DARN

DIODES

We wish we had a

dollar

for

every diode we put

in

backward -

because

we'd

be.$4

richer right

now! In the article "High -Power

Hi

-Fi Audio Amp for Home

or Car,"

(Radio

-Electronics, March 1989)

Fig. 5, the 12 -volt

power supply, on

page

54

shows diodes D1 through

D4 reversed. Figure

6, the photo

on

page

55, shows the correct

ori-

entation for those

diodes. Sorry

'bout that.- Editor

CALIBRATION

CLARIFICATION

The "TV- Derived Frequency

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(Radio -Electronics,

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LETTERS

RAD/0- ELECTRON/CS

500

-0 e/-

COUNTY

BOULEVARD

FARM

/NGO.4LE,

NY 1/7.35

1988) introduces three

uncertain-

ties that can be eliminated if the

counter being calibrated

has

a

2.5

-, 5 -, or

10 -MHz reference

os-

cillator.

The

uncertainties are

those in the transmitted 3.58

-MHz

color sub -carrier, the calibration of

the standard itself,

and the

±1

count

error

in the counter

display.

Those

uncertainties are additive,

and

may amount to 3 ppm or so.

(They

could be

much less

than 3

ppm, but you can never really be

certain

of

that; nor is it easy to

ascertain

what

the

value might

be

at

any point in time.)

It's a waste of time to

keep

a

frequency counter calibrated to

an

unnecessarily

high degree of ac-

curacy.

If

2%

is

good enough for

you

in

audio-

frequency work, the

line- voltage

frequency is

a

handy

calibration

reference. Some work

may require measurements to

within 1 ppm

or better.

A

shortwave

receiver is

about

all

that's

needed

to calibrate a

fre-

quency counter

with

a

2.5

-,

5 -,

or

10

-MHz oscillator.

Let the counter

warm up and stabilize, and adjust

its oscillator for zero beat with

WWV. If the counter's

radiated

os-

cillator signal and the receiver's

sensitivity

are great enough,

not

even a dummy antenna

will

be

needed to

couple

the signals and

hear the beat

frequency.

With no

propagation

path cor-

rections, WWV carrier frequencies

are accurate

to

±0.03

ppm. The

only other uncertainty

introduced

using

this

method is your ability to

hear the beat, or difference

fre-

quency, and to adjust

it

to as close

to

zero

as posssible.

Difference

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