G8MNY TECH 25.08.19 04:46l 164 Lines 8209 Bytes #365 (0) @ WW
BID : 18857_GB7CIP
Subj: Simple 1kHz AF Test Osc
Path: OK0NBR IR2UBX DB0RES DB0OVN DB0GOS ON0AR OZ5BBS CX2SA GB7CIP
Sent: 190825/0737Z @:GB7CIP.#32.GBR.EURO #:18857 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Revised Nov 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
At my local club for a construction contest we made up several Velleman Signal Generator Mk105 kits. They all worked OK, but the output shapes were very poor, so I have had a go at re-designing mine....

THE ORIGINAL
This is a single frequency osc design using a 555 astable osc & then used various cascaded CR filters (1 Cap combination actually shorting the 555 square wave edges to ground!). A separately biased NPN was used as an active filter to help the sine wave filtering & the options then linked into an output NPN emitter follower. There were 4 shapes available all at different levels.

NEW DESIGN
I kept the 555 & set the frequency more to 1kHz (slightly dependent on battery volts) by changing the main timing R to 22k. [N.B. square wave ratio of 1:1 for no even harmonics needs charge R (1k+22k) to equal the discharge R (22k), which of course it cant be!]
The large output square wave on pin 3 is attenuated 6:1 & biased up to work the NPN at the same DC level as the other waveforms. The other 2 waveforms are self biased from the square wave average.

┌─o\o─┬───┬───────┬─┬────────────────┬────────────────────┐
│ │ 1k ┌──┴─┴─┐ 5k6 SQUARE │
│+9v │+ │ │ 4 8 │ ┌─────────┬┴───────────────o │
_│_ === ├────┤7 555 │ 18k 4k7 TRIANGLE │/
▄ 25u│ 22k │ 3├──┬┴──18k─┬──)────────────────o ─┤BC547
│ │ ├────┤6 │ 2k2 │ │ SINE │\e
│ │ ├──┬─┤2 │ ├──4k7──)──)──┬─10k─┬─22k─┬─o │ 10u
│ │ === │ │ 5 1 │ │ ┌──┤ │ │ === │ ├─┤├─┐
│ │47n│ │ └──┬──┬┘ │ │ === │ │ 22n└─────)──────┤+ │
│ │ │ │ 10n│ │ │u22 │ │u1│ │ │ │ 47k ──
│ │ │ │ === │ === │ │ │ ===47n 10n=== 4k7 Pot O/P
└─────┴───┴──)────┴──┴───┴────)──┴──┴──┴───────────┴──────┴────┴────
└──────47k───────┘

To get the triangle wave I used an 18k & u1, & to cancel any slight curve charge discharge shape at 1/6 height, I added in some of the oscillator curvy ramp that was out of phase via the 47k from pin 2. This is an unusual G8MNY BODGE!

Without correction Inverted correction Result
.-'\ slight curve more curvy waveform /\
/' `\ / \ _.-~\ /' `\ /
/' `\_ /' `\_ /' /' `\ /'
~-./' ~-._/' `\/'

For the sine wave feed, I did the same, but with a 2k2 & u22 as an initial filter CR to set the higher level needed for the sine wave feed to an active butterworth 18dB/Octave filter. That used a doubling of values to give a good filter shape. This gives a total of 24dB/Octave filtering for the sine wave on the initial square wave harmonic levels. That is about -50dB @ 3kHz, or better than 0.3% distortion all in, as the 3rd harmonic should be the only significant one. Due to the hefty filtering the level is also a function of frequency!

Only 1 NPN transistor is used in this circuit now. I changed it's emitter resistor to reduce current & the 1uF supply cap to 25uF & also the output feed cap to 10uF for a "no droop" square wave.

Power consumption is now a lower 11mA @ 9V so a PP3 battery should last 10hrs. The circuit will operate OK on 12V, but the frequency & levels will change!

Output level is dependent on supply, but all about 1V P-P on 9V.

IN USE
The main use is for AF work, where tones can be injected into AF stages. The 3 waveforms have different uses..

SQUARE WAVE used with a scope to quickly determine the AF bandwidth..

┌._ ________ _.┐
│ ~-._ │ │ Very │ │ _.-~ │
│ Poor~│ │ │ flat │ │ │ Too │ │
│ LF │ _│ │Responce│ │ much │_ │
│ _.-~ └────────┘ LF ~-._ │
└~ ______ , ~┘
/~ Too │ │\.______
│ little │ │ │ Too │ │
HF \_ │ │ much │ _-----┘
_ Level ~~~~~~ HF │/
0dB │ │100%
│ │
│ │
│ │ S Q U A R E W A V E S P E C T R U M
-10dB_│ │ 33%
│ │ | 20%
-20dB-│ │ │ │ 14% 11% 9% 8% 7% 6% 5% 5% 4% 4% 3%
└─┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴─ Hz
1k 3k 5k 7k 9k 11k 13k 15k 17k 19k 21k 23k 25k 27k

As the bandwidth is very large, so any restriction will result in the above
tell tail shape changes. Sharp frequency cut off always produce overshoots &
ringing immediately after the vertical edges. An amplifier dummy load of 8Ω
in parallel with a 1uF fed with a square wave is often used to show amp
stability into nasty loads as it brings on overshoots due to NFB delay etc.

∩ n Ringing _______
│ U ~────┐ Slew OK Small / \
│ │ ┌─────────┐ / Slew \ /
│ ∩ │ └─────────┘ Bad \ /
U u~────┘ Large \_______/

While poor slew rates produce different slopes (bandwidth) depending on
levels. This is normally due to too much NFB masking a poor amp design!

Note that the square wave spectrum is very annoying to the ear as it
contains only non musical odd harmonics that only slightly reduce by their
harmonic number fraction.

TRIANGLE WAVE used with a scope to look at Amplifier crossover & peak &
other non linearities.
/\
/\ crossover /' `\ ideal _/\_ peak
/' `\__ __/ /' `\ straight /~ ~\compression
\ / /' `\ sides /' `\_ _/
`\/' `\ / ~\/~
`\/'

/~~\ symmetrical /~~\ asymmetrical _/\_ asymmetrical
/' `\ clipping /' `\ clipping /~ ~\compression
`\ / `\ / /' `\ /
`\__/' `\ /' `\ /'
`\/' `\/'
_Level
0dB │ │100%
│ │
│ │ T R I A G L E W A V E S P E C T R U M
│ │
│ │
-20dB_│ │ 10%
│ │ │ 4% 2% 1% 0.8% 0.6% 0.4%
└──┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴── Hz
1k 3k 5k 7k 9k 11k 13k 15k

The spectrum is like a square wave, but lower odd harmonic levels at their
harmonic number fraction squared.

SINE WAVE used with a scope or AF spectrum analyser (sound card) or just
ears, to judge level of unwanted harmonic generation in a piece of AF kit.

_ Level│ │
/~ ~\ Note the smooth dB │ │ SINE WAVE SPECTRUM
| | curves of the │ │
│ │ │ sine wave, with a │ │ Little/no
| | perfect inverted │ │ harmonics
\_ _/ image & no harmonics. └──┼──┬──┬──┬──┬──┬──┬──┬ Hz
~ 1k 2k 3k 4k 5k 6k 7k 9k

OTHER OSCILLATORS
Accurate constant level variable frequency sine wave oscillators are used to determine the true frequency response. Also undistorted sine waves are used for true RMS power measurements on PA stages.


Also see my bul on "AF Waveform Generator", & " Locking a Frequency with 555".


Why don't U send an interesting bul?

--- Mystic BBS v1.12 A43 (Linux/32)
* Origin: HAMRADIO telnet slacko.kozow.com (21:5/101)