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Simple square wave and pulse generator
A.K.A. 1960-ies style guerilla homebrew 6 Hz - 2 MHz function generator
with 4 transistors :)
Introduction
Recently I needed a function generator for the workshop at the place
where I work. Detailed inspection showed that we had four.
One is in good shape, made in 1979, and capable of producing sine,
triangle and square waveforms. This one is retained and used as main
function generator for testing purposes at my main office location.
Other three were not working, or working sporadically. They were all
from 1950-ies or early 1960-ies, with electron tubes, of two different
types. High
voltage was ok on all of them, but output was lacking or intermittent.
I decided to scrap them, keep the working tubes, transformers and metal
for future projects, and make a new function generator using only
transistors. This new generator will be used in the machine shop
location.
Old vacuum tube based
function generators
Old Krohn-Hite function generator images. Two non-working examples were
found. They were of very solid build. Useful parts were kept for future
builds.
Old EICO function generator. Much poorer workmanship quality, thin
steel chassis plates, etc.
Transistors only? Why?
New generator was supposed to have all the bells and whistles, and use
case and if possible other parts from the scrapped vacuum-tube
generators. Transistor-only design might seem strange at this day and
age. However, I wanted to challenge myself. And there was another,
somewhat nostalgic reason: I have spent some years in a war zone, and I
can appreciate how difficult it is to extract chips from boards (and
get their datasheets) during times of war. Transistors were easily
removed from equipment, and were not critical when in different role.
BC108 will work just as well as 2N2222 in a LF amplifier for instance -
often without any change in external components.
Therefore, transistor design has a practical second and third world
use, IC based one much less so. No special chip? Tough luck. It's not
going to arrive on a mortar round. So, for all the people in the world
that are maybe now in the similar situation that I was some 20 years
ago, it is
going to be a simple and reliable, no frills build.
Use of a voltage regulator IC is not cheating. Device works perfectly
well without it, frequency is just more stable with regulator.
Frequency drift using unregulated power supply is only 2-3 %. Frequency
drift with 5 V voltage regulator, measured in a 24 hour run, is always
less than 1%. And anyway, 78L05 is a three terminal device :)
Complaint that's more to the point is the dual rotary switch with 5
positions. If you can't find it, you can use regular DPDT switch for
two bands only. Bands can be "streched" by using smaller band
shortening fixed resistors (1 Kiloohm in my schematic). This will
increase total ratio of resistance change and provide wider range (and
coarser) tuning. Other option is to use Megaohm dual-ganged base
variable resistors with Darlingtons instead of regular transistors (see
here,
Figure
14).
With
those measures, it is possible to achieve say 10 Hz - 1
KHz band A and 1 KHz - 100 KHz band B. No DPDT switch / just lazy you
say? Use one band only and you don't need a switch at all ...
You have no dual-ganged variable resistors? Ok, just find two regular
potentiometers. Your tunning will now involve adjusting 2 pots, but
that is a small price to pay for a working function generator, if
nothing else is available.
Nothing else in this schematic is critical: change transistors, supply
voltages, resistors, capacitors. It will work with a very wide range of
components.
Sinewave and
squarewave generators
For several weeks, when time permitted, I was testing different
configurations of sine and square-wave oscillators. They all had some
advantages and shortcomings. Very good overview page: http://www.sentex.ca/~mec1995/tutorial/xtor/xtor7/xtor7.html
Phase shift oscillator: narrow frequency range (1:2), meaning I would
not be able to have continuous wide frequency coverage.
Wien-bridge oscillator: I tried two different transistor based designs
and they only worked well in narrow range, and there was a number of
other problems. Gain had to be set to little above 3 initially and then
lowered to 3 for least amount of distortion. A small lightbulb was
often used in the past to provide changing resistance for gradual gain
reduction. Resistance of the bulb would increase after the power is
applied, decreasing positive feedback and gain.
LC oscillators: really hard to use at lower frequencies. And your 500
picofarad variable cap is not going to change much of a frequency when
connected to a large 100 mH coil. So a variable coil is needed. LC
oscillators
are excellent for radio range though.
Astable
multivibrator: produces square waves and pulses, easy to get to
work, stable and works with wide variety of voltage supplies and
frequencies. No sinewaves though.
After some thinking and experimentation, I found out that I will have
too many dials and controls brought out if I use both square wave and
sinewave oscillators. Therefore I decided to use square wave oscillator
(astable multivibrator) only, but provide it with all the controls I
might possibly need. Pulse generation, fine Ton/Toff regulation and
frequency adjustment are easily added. Total period of oscillation is
equal to T=Ton+Toff, where Ton and Toff can be easily manipulated with
a potentiometer. Periods are function of connected resistance and
capacitance. If sinewave is really needed, it can be had by filtering
the output to supress all harmonics. But I do hope to build a pure
sinewave oscillator when time permits, in a separate metal case, saved
from second tube generator.
Astable multivibrator
is selected
After consulting "Transistor manual" (General Electric 1964) and the
internet, I finalized my circuit testing with this design (http://www.sentex.ca/~mec1995/tutorial/xtor/xtor7/xtor7.html
Figure 16), but with changed circuit values. Collector resistors were
changed to 2.2 Kiloohms, base resistors to dual-ganged 100 Kiloohm
potentiometer (frequency adjustment) in series with current limiting
(and frequency range shortening) 1
Kiloohm resistors. One additional (on/off switched) 500 Kiloohm
potentiometer, in parallel with one 100 K dual-pot section is used to
regulate pulse width, if pulses are desired instead of square waves.
Very fine tuning is achieved with 2.5 Kiloohm pots. There are 5
frequency ranges, two capacitors are switched for each
range. Values are 100 pF, 1 nF, 10 nF, 100 nF and 1 microF ceramic.
This provides 6 Hz - 2 MHz range of possible frequencies in 5 ranges.
Transistors as originally used: 2N4401 - dirt cheap general purpose NPN
silicon transistors.
Initial protoboard testing. Also visible: dual potentiometer (frequency
tuning) and rotary capacitor switch (frequency range selection).
Output can be picked from either multivibrator transistor collector
(normal/inverted) and is then fed to a TTL stage which outputs TTL
level signal (TTL OUT). Variable signal level is available on the VAR.
OUT connectors.
Schematics
Top part of the schematic is a power supply. Transformer provides 12
Vac on the secondary. Half-wave rectifier is used because of small
current drain and voltage regulator. If voltage regulator is not used
it would be better to have a full-wave rectifier or, failing that,
large electrolytic capacitor in filter stage.
Transistors are labeled as follows:
T1 - multivibrator transistor 1
T2 - multivibrator transistor 2
T3 - buffer and TTL level output driver
T4 - variable output driver
Transistors T1 and T2 are forming standard astable multivibrator.
Frequency band is selected with rotary switch that connects 2
capacitors at the same time. Frequency tuning is done with dual-ganged
100 Kiloohm potentiometer. Fine Ton/Toff and (at the same time)
frequency tuning is done with two 2.5 Kiloohm potentiometers. 500
Kiloohm potentiometer is used to adjust the pulse width if square
wave/pulse switch on the front panel is closed.
1N914 diodes serve to improve the sharpness of the square wave. See http://www.sentex.ca/~mec1995/tutorial/xtor/xtor7/xtor7.html
for details. T1 and T2 collector resistors are as small as possible,
but have to be larger than Rbase/Gain, in order to ensure transistor
saturation. For example, if base resistor is 100 Kiloohms, and
transistor gain is 50, collector resistor of minimum 2 Kiloohms must be
used (and that's pushing it). T3 is buffer/TTL driver stage, with small
collector resistance for fast switching. T4 is added driver stage for
variable voltage output. It is connected after the TTL stage to reduce
loading on multivibrator transistors. This provides somewhat worse
rise/fall times for T4 versus T3 however.
Build
There were a number of problems with the build. Initially, no
oscilloscope was available except for the venerable Tektronix 502A from
1963. Newer Tektronix TDS 2002 from 2005 died in 2010. Display unit
failed - Tek asked $550 for replacement display unit. No way I was
going to pay for that. Rigol DS1052E ($399) was ordered late into the
build. Old 502A has a bandwidth of only 1 MHz (and it was heavily out
of
calibration), so it was used only as a partly-trusted temporary help.
Parts soldered onto printed circuit board. Wires were eventually
shortened considerably.
Venerable Tektronix 502A provided useful service during the first part
of testing. However it was severely limited by its 1 MHz bandwidth.
This would cut-off square-wave harmonics at high frequencies and
provide incorrect waveform display. With all this in mind, it was able
to show to some extent what improved square wave shape - and what did
not. This was especially true at lower frequencies, bellow 100 KHz.
Initial testing with 502A scope showed that waveform was distorted at
fairly low frequencies. I knew that scope was flaky but this was too
much (I compared the waveform with known good function generator).
Literature recommended replacing collector resistors to smaller values
to improve parasitic capacitance charge/discharge time. This was done
on T3 and T4 and reduced response time considerably. T1 and T2
collector resistors could not be made much smaller than 2.2 K because
transistors would not saturate and multivibrator would cease to work.
It was also discovered (datasheet) that 2N4401 transistors have poor
rise/fall times when compared to 2N3904 transistors. Also they required
larger collector currents than 2N3904 to even approach similar
rise/fall times. 2N4401 transistors were therefore swapped for 2N3904s,
with marked further increase in performance.
Some examples of gradual improvements to the waveform through the
change in components. Signal rise is shown much magnified.
Waveform with 2N4401 transistors.
Waveform with 2N3904 transistors.
After reading GE Transistor manual, it transpired that further increase
in switching speed can be gained by putting small capacitor in parallel
with base resistor for T3. This converts RTL
logic into RCTL logic :) Some improvement was seen.
Further tests revealed that 60 Hz noise was somehow modulated onto the
output signal, causing unacceptable behaviour especially around 25 Hz
area, and distortion elsewhere. After inserting a large 2200 microfarad
capacitor between +5 V and GND, problems disappeared. Original 10
microfarad capacitor was retained as well.
Case
Metal steel case was taken from old vacuum tube generator. It was
cleaned and painted. New PCB was mounted onto a piece of plywood.
Plywood piece also supports new front aluminum panel on the lower end.
Three wood squares were glued into the metal case to serve as
additional supports for the aluminum front panel. Front aluminum panel
shaft and screw holes have been drilled, after which the panel was
painted.
Template for front panel hole drilling.
After case and front panel have been painted, potentiometers, switches
and other components were mounted for trials. They were removed few
times during the build, when needed. Potentiometer shafts had to be cut
down to size.
Dials, scales,
calibration
Dials (knobs) were taken from scrapped tube generators. Scales were a
big
issue: I wanted them to look nice, but did not want to spend
extraordinary amount of time on them. But there was no easy solution
and I went to a complicated solution eventually.
I did calibration with temporary circular piece of paper affixed to
front panel. Frequency was checked with frequency counter and counter
equipped multimeter. Each point was marked with a pencil. Then a scale
was drawn on a PC according to a temporary paper scale pencil marks. I
made each permanent scale in Inkscape, printed them, and cut them with
scissors to fit. Each scale also received a plexiglass (acrylic)
plastic cover 3 mm thick to prevent damage to the scale. It took
literally weeks to get all this right. Definitely not recommended for
mass production.
Old nixie-tube display counter and multimeter frequency meter were used
for calibration.
Finished product
Few pictures of the finished function generator are shown, with and
without the case. Case was salvaged from scrapped vacuum tube based
Krohn-Hite generator and repainted.
Front panel without case.
Top view without case.
Back view without case.
Back view with case.
Front view with case.
Back view with metal cover on.
Angled front look.
Controls
Remark: T=Ton+Toff, T=1/f. T=period, f=frequency. Ton=pulse high,
Toff=pulse low.
Controls are used to set:
1. Frequency of the square waves and pulses (scale calibrated for
square waves only)
2. Five frequency bands. 6-222 Hz, 59-2480 Hz, 596-25900 Hz, 6.74-289
KHz, 60.4-2220 KHz.
3. Fine frequency adjustment 1 (in fact Ton duration)
4. Fine frequency adjustment 2 (in fact Toff duration)
5. Pulse duration (if pulse selected). Variable from 1-49% (normal) and
51-99% (inverted) duty cycle (maximum, when frequency dial is in
leftmost position).
6. Output amplitude (VAR OUT connector). TTL OUT levels are unaffected
by this.
7. Normal/inverted (selects either T1 or T2 collector for output to T3)
8. Square wave / pulse selection.
9. Power on/off
Output: 5 V TTL levels and adjustable 0-3.38 V levels.
Use
Square wave generator is useful for many applications. One of them is
to quickly estimate a bandwidth of an amplifier (square wave has
infinite number of harmonics). Second is distortion check. Third is
keying of a test radio/infrared transmitter for example. Particular use
for variable-width pulse generation is severalfold: pulse width
modulation (PWM) for motor control, infrared/laser/radar pulsing (time
domain reflectometry), response time measurement etc.
Performance and
operation
6 Hz - 2 Mhz frequency range in five frequency bands.
Square wave and pulse, 1-49%, 51-99% (maximum, when frequency dial is
in leftmost position).
TTL
OUT voltage level, and variable voltage 0-3.38 V level out.
Around 40 nanosecond
waveform rise time (from 10 to 90% of amplitude).
Some images of the unit in operation are shown.
1 MHz operation is demonstrated.
Rise time is around 40 nanoseconds.
Low frequency operation.
Demonstration of pulse generation. Pulse width can be varied
approximately from 1-49%, and with signal inversion (normal/invert
switch) from 51-99% (maximum, when frequency dial is in leftmost
position).
Other builds
Oliver from Berlin, Germany, built his own modified version.
Carl from Florida, USA, sent this picture of his almost finished generator.
Literature
1. GE Transistor Manual, 1964 edition.
2. Osnovi elektronike: Radio-predajnici i radio-prijemnici, Državni
Sekretarijat za Narodnu Odbranu, 1967. Translated into Serbocroatian
from Soviet original (in Russian): Основы радиотехники и радиолокации -
радиопередающие и радиоприемные устройства, Военное издательство
министерства оборони СССР, Москва 1965. В. Г. Левичев, Я. В. Степук, Б.
И. Фогельсон.
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