You can check out Part 1 here: diy-audio-function-generator-part-1
I started to pull together the Schematic and PCB Layout for the board in DesignSpark. I've kind of cheated a bit and just thrown parts in the schematic and passed them through to the PCB Layout side of DesignSpark without connecting anything up yet. I did this just to get a jump start on the "look" of the layout and match it up to my concept drawing.
Next I'll go back to the schematic and start connecting things up.
I have appreciated all the supportive comments and feedback I have received about this project so far. James Grenert was kind enough to email me an old Elektor article from Nov. 1994 that has a very similar Lamp Based Wien-Bridge Oscillator Circuit as the Jim Williams one I am using as a basis for this Function Generator.
Below is the article (Note this was Google-translated to English from the original German article, so there are some miss-translated words... but it is still very readable)
SPOT-SINUS
GENERATOR – ELEKTOR 11/1994
SIMPLE AND LOW
DISTORTION
Oscillators
come in many shapes and varieties, each with its own specific characteristics.
A type known for its beautiful sinusoidal output signal, the Wien-bridge
oscillator. Here we describe a repackaged in a modern implementation, as a
special old-fashioned method of keeping the amplitude constant. Perhaps nowhere
as electronic engineers have been as creative in the design of oscillators. For
each specific application there is a suitable type oscillator. There are RC, LC
and crystal oscillators, high-and low-frequency oscillators, low-power
vermogensoscillatoren. Do you want a rectangle, saw-tooth or sinusoidal signal?
You name it. Or variable frequency oscillator, the modular-be? It's all or is
in any case to make. For the testing of audio equipment is often a source who
need a nice clean sine supplies. The best is of course an adjustable version,
but also a fixed l-kHz generator is a very useful tool, but as long as the
signal produced is low distortion. An oscillator type here eminently suited
for, is the classic Wien-bridge oscillator.
The Wien
bridge consists of a combination of an RC network and a series-parallel RC
network. The two resistors and two capacitors have the same value. At the
central frequency, the phase rotation is zero, because the two networks have
equal phase shift, but in opposite direction from each other. The slowdown
amounts to three times at that time. So by, as drawn in Figure 1, a simple
3x-amp to add to the circuit can oscillate.
Filament-SYSTEM
Two RC
networks and an amplifier stage, which is all we need for a sinus generator
according to the Wien-bridge recipe. That sounds simple attractive, but due to
the fact that in this case more than average demands on the shape of the
produced sine up, there are a few points that deserve special attention. Thus,
the amplifier used must be of impeccable quality. Furthermore, a sophisticated
control system is desirable, which ensures that at all times there is enough gain
to oscillate to launch and maintain, but simultaneously prevents the amplifier
at any time could be overdriven. That would be the desired waveform fine
because irreparable damage.
The first of
the above issues can be resolved as a good opamp amplifier to apply. The second
point, the control shows fine in practice achievable with the help of an
ordinary incandescent bulb.
The actual Wien Bridge
here consists R1, R2, C1 and C2 is at a frequency of 1 kHz dimensioned. The two
resistors are 1% types. The MKT-capacitors C1 and C2 you will have them
selectable on a tolerance of less than 1%, otherwise the chances that the
circuit is not oscillating. Would you exakte a frequency of 1 kHz, these must
also capacitors a value of 119.67 nF.
If amplifier
is an OPA627 opamp of the type used. The reinforcement is required, as said,
about a factor of three and this is also the dimensioning of the network
against link P1/R3/R4/La1 tuned. What is the function of La1? Well, that bulb
plays here the role of PTC. Because the filament has a temperature dependent
resistance, cold state, the resistance around 25 Ohm in warm condition that
value is a factor two higher. This property is gratefully used here, by the
light directly into the network to flock to incorporate. Is it cold light, then
the strengthening of IC1 mainly determined by the ratio between R3 and R4. Is
the output voltage however, it increases the flow through the network and
torque against the resistance of La1 far. The ratio of the network will change,
such that the coupling increases and against the strengthening of IC1 (and
hence the output voltage) decreases. This in turn is the current through the
LED, so the versterkins of IC1 increases something ... etc., etc. Of course,
after a short time balancing act, so the output voltage to a stable value
remains "hanging".
To balance it
in the lower branch of the torque against network using standing lamp is in the
upper branch instelpotmeter one (P1) included. This is a fine adjustment of the
"balance point" as possible. Furthermore, a system as stable as
possible to get an additional current drive of the lamp used in R5. The latter
resistance will be slightly adapted to the type of bulb used. In our prototype
showed a value of 3k3 for R5 properly comply. Shows the output voltage in your
case after a few minutes to run something, it's wise to R5 what value should be
reduced. A decreasing output voltage requires slightly larger value of R5. As a
matter of experimenting so!
SPEC'S GOOD
The presence
of the light was of course necessary to the relatively low-ohmic resistors
around IC1 dimensioning since there will be a reasonable flow through La1 flow to
the PTC effect noticing. Such dimensioning impedance has the advantage that the
noise contribution of the resistors is very low. This does have an opamp as
konsekwentie that should be applied in a symmetrical supply voltage of + / -15
V can control at least 600 ohms.
The OPA627 is
against this type of task more than cope and provide for an opamp with FET
inputs also a very low input noise (5.6 nV / sqrt (Hz) at 1 kHz), low input
offset (100 μV max) and an extremely low distortion. The actual THD consists solely of the second and third harmonic, which together
no more than 0.00014% size.
Unfortunately,
the relatively expensive OPA627 opamp. If you have no problem with a slightly
higher distortion, then instead of this type is also a cheap (pin-compatible)
NE5534 be taken.
CONSTRUCTION
Given the
simplicity of the circuit is deliberately refrained from a print design. The
title picture illustrates the sine generator to a fairly simple piece of plate
hole is to accommodate.
Since the
current consumption of the circuit is limited to approximately 10 mA, the
(balanced) diet very modest in scope. The smallest transformer that delivers 2
x 15 V, a three-legged brugcel and two controls (78L15 and 79L15) will ease
this chore done. Very suitable in June '92 Elex described "low-power
balanced diet", which number 926,061 are still in print in the Elektor
service available.
The adjusting
of P1 one must take time. In practice, it proved beneficial to those in
instelpotmeter to maximize output voltage. This yielded the lowest distortion,
and so also prevents the output voltage after the adjustment of the system to
an unexpected (and unwanted!) High value would increase.
Specifications
frequency: 1
kHz
output
voltage max: 8 VRMS
Distortion
(THD + noise): <0.0003%
voltage: 15 V
V/-15
current:
approximately 10 mA
