Elenco Electronics MO-1251 Uživatelský manuál stáhnout pdf (Strana 24)

100

Only this time,

your digital

word

consists of the top 8 bits of

24 -bit

adder

/accumulator.

At a constant

and

high

clock rate, a fixed phase

increment is

added

the ac-

cumulator. For instance

a "1"

input

could advance the phase

count

slowly that you'll

get

a 1 -Hz sine

wave, while

a "2"

would

give you 2

Hz,

on up

to the much larger num-

bers

which give

you

much higher

frequencies.

Advantages

of the

method

are

that

you

are directly synthesizing

the final frequency, which

elimi-

nates

all

the hunting

and

the noise

bandwidth of

phase

-lock

loops.

Thus, your spectral purity can be

extremely high. There is also no

bad transient whenever you

change

frequency

-just a smooth

and unbroken

transition.

For lower

frequencies,

a person-

al computer

will work just fine,

and it should be trivial to generate

up to several kilohertz using an

Apple

Il. You

can do so

in 1 -Hz

even smaller resolution steps.

To work

at any

higher frequen-

cies, speed limitations on those

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hardware adder

-accumulators

can

get to be a problem.

One ex-

tremely expensive

source of ultra -

fast

chips

for that is Stanford Tele-

communications,

while

slower

and much

lower -cost kits

are ob-

tainable

from A

A Engineering.

One source of additional details

the

Radio Amateur's Handbook.

Minimum -order hassles

One of the biggest hacker

help -

line

complaints

concerns all of

those steep minimum orders that

many of the electronics dis-

tributors seem

insisting

upon.

The

problem

bad and

it is

getting

much worse. How can you

cope

with it?

First, note that

just

is simply

not possible in this day and

age for

anyone

profitably

offer the di-

rect -mail sales of electronic

hard-

ware if their average mail order

ends up

less than $25. Those $15 or

$25 minimums

or any

$5 to $8 be-

low- minimum service charges

from

the "new

-age"

good -guy

dis-

tributors are all

necessary for their

very survival.

On the other

hand,

several

the "old- line" distributors

have

gone as

high

as a $250

minimum

order.

Even worse

yet,

several of

them

now have

intolerable $100

per line item

minimum. Which

means

if you want a two -cent part,

you now

have

to buy 5000

identical

ones

at once, or else forget

it.

The Bell Electronics people

have

just garnered a

ZZZ rating and

moved to the

very

summit of

Synergetics black

list for

their un-

acceptably high

line minimums

and

all their outright arrogance.

(All I wanted

were

few jelly -bean

regulators.)

Unfortunately,

those

epsilon

minuses are not alone.

The sad fact is that,

if you

are

individual hacker, the deck

gets

very

much

stacked against

you.

the other

hand, that just

may

end up as the only

game in town.

So, how can

you

cope

with steep

minimum

orders?

Here are a

baker's dozen

partial solutions...

(1) Plan ahead.

If you

run in pan-

ic mode, you

will almost always

end

up wasting

money.

Find the

best dealer

with the

best source

and

the lowest

minimums. Com-

bine

what you

need with

what

you

think you

may need

for

other

up-

coming

projects.

Try to get every-

thing from

one or

two

suppliers,

rather than a dozen.

(2) Try to always

deal

with

"new -age"

distributor, such as

Mouser,

Active, DigiKey, or

Jameco, instead of using those

"old-

line"

houses such as

Schweber, Allied, Cramer, New-

ark,

Bell,

Hamilton.

(3) Fill

out

your minimum

order

with

other

goodies which you

would someday like

to play

with.

(4) Rather than

using a dis-

tributor, request free samples

di-

rectly from

Applications Engineer-

ing

of the firm actually building the

part. Use a laser- printed

or other

business letterhead. Request only

as many parts

as you

need,

and tell

them exactly what you are going to

with

them.

(5) Check into your local walk -in

surplus stores.

Often

you might

find reasonable substitutes

in-

credibly low prices,

especially on

unadvertised

odd

lots. The

savings

can even make a 100 -mile

drive

worthwhile.

(6) Build

up your own

personal

inventory of "in-

stock"

parts

that

you are

likely

to use

the

future.

(7) Network with friends in

ham or computer club,

with

en-

gineers

techs from

an aerospace

company

whatever. Be

able to

swap parts both

ways.

Become a

resource for the

other

party.

(8) Move to Silicon Valley, where

all of the 24

-hour

convenience

grocery markets

also carry all the

other known types

of chips.

minimum.

Or,

if you

are too

far

away, always be sure to try Radio

Shack.

(9) Naturally, we would

hope

you would

always check out our

Radio

-Electronics advertisers first

for

any component part.

That's

why we

put the bingo card

the

magazine. But two other great

source for

oddball components

are the unique

Nuts

and

Volts

bar-

gain shopper and all

the

distress

merchandisers found in

that clas-

sified ad section of

Electronic

News. While the latter always will

have steep minimum line charges,

the prices are often so ridiculously

low that

may

not matter.

(10) Aggressively subscribe to all

the electronics trade journals,

such

as EDN, Electronic Design,

E.E. Times, Electronic Products,

and

/or the Electronic Component

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