07/02/13 A filter for a noise generator

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A filter for a noise generator

Some computations involving analog computers require a random noise source - think, for

example, of a system consisting of several masses which are coupled to each other using springs

and dampers (like a car suspension system). Such a system may be excited by a random noise

source, thus simulating a bumpy road.

Other uses involve the study of filter simulation or stochastic processes. There are manyapplications requiring a good random noise source. Since real time simulations of mechanical

systems require quite low frequency random signals as excitation values, it is rather difficult to builda random signal generator for this purpose.

Professional devices will amplify the Nyquist noise of a resistor, mix it with a fixed frequency

(normally about 300 kHz) to create an IF down to 0 Hz and amplifiy and filter this. When youhave a look at the drawings of a professional random noise like the Wandel und Goltermann RG-

1, it becomes clear that building a good random noise source is a major project which will not be

completed in a weekend.

Fortunately, I have a Wandel und Goltermann RG-1 noise generator which seemed ideally suited

as a random source for my analog computers. The problem to be overcome is the frequency range

of the device. Normally it delivers white noise between 0 Hz and 100 kHz (there are some filters

like a speech filter, but these filters are not satisfactory for my plans), but I need something likenoise between 0 Hz and

1 Hz5 Hz

10 Hz50 Hz

100 Hz

500 Hz

1000 Hz

5000 Hz

So it was clear that I had to build a high order low pass filter to be switched between the analog

computer and the random noise source. I decided to employ a switched capacitor filter to facilitate

the overall design - after having a look at modern integrated circuits I decided to use the MAXIM

MAX293 - an elliptic low pass filter of eighth order which requires a TTL clock signal of 100

times the cutoff frequency.

The picture on the left shows the overall circuitry of

the completed low pass filter. It consists of the

following part:

07/02/13 A filter for a noise generator

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Input filter (5 kHz),switched capacitor filter,

clock spike filter (5 kHz),

output driver,

clock generator circuit and, finally,

a simple power supply.

Let us start with the power supply which is

rather simple since the 19 inch frame thecard mounts in already has a power supply

delivering +/-15 V, +5 V (for digitalcircuitry) and +/-10 V. Since the switched

capacitor filter requires a +/-5 V supply, Iderive these voltages from the +/-15 V

supply using to constant voltage regulatorICs.

The clock generator is a bit more tricky - since the filter has eight different switch selectablebandwidths, the clock generator must be capable of delivering eight preset frequencies.

I decided to use the rather well known XR2206 function generator IC since it is easy to use and

does its best to generator a 1:1 duty cycle. The clock generator is shown in the following picture:

07/02/13 A filter for a noise generator

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The filter is shown in the next picture. The input signal from the RG-1 can be terminated with a 600Ohm resistor. After this it is fed into a discrete low pass filter with a cutoff frequency of 5 kHzbefore being used as input for the MAX293. This IC does not only contain the switched capacitor

filter but also a dedicated operational amplifier which is used to implement another discrete filter

with a cutoff frequency of 5 kHz.

The output of the MAX293 is then fed into a non-inverting amplifier with an amplification rate of

about 10 (11 to be exact :-) ). This is the place where the drift compensation occurs - the switched

capacitor filter introduced quite a lot of drift.

The output of this operational amplifier is then used as the input signal for a voltage divider before

being amplified another time by a factor of 10 (11).

07/02/13 A filter for a noise generator

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The picture on the right shows the front ofthe random noise filter. From left to right the

following elements can be seen:

LED showing that input signaltermination is in effect,

BNC input jack with termination

switch,eight position switch for selecting the

desired bandwidth,

output amplitude potentiometer and

the 4 mm banana jack delivering theoutput signal.

07/02/13 A filter for a noise generator

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The picture on the left shows the output frequencyspectrum of the filter being fed with a 100 kHz

random noise signal from the RG-1. The filter has

been set to a cut off frequency of 1 Hz only (which

is what I need for my moving mass excitationcomputation) - it is incredible how sharp the cutoff

point in fact is (and it shows how good the RG-1 is

- even at very low frequencies is is a good white

noise signal) - the vertical hairline has been set at 1Hz.

ulmann@analogmuseum.org 21-JAN-2007