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 many applications 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 build a 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 you have 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 like noise between 0 Hz and

• 1 Hz
• 5 Hz
• 10 Hz
• 50 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.

• Input filter (5 kHz),
• switched capacitor filter,
• clock spike filter (5 kHz),
• output driver,
• clock generator circuit and, finally,
• a simple power supply.

The clock generator is a bit more tricky - since the filter has eight different switch selectable bandwidths, 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:

The filter is shown in the next picture. The input signal from the RG-1 can be terminated with a 600 Ohm resistor. After this it is fed into a discrete low pass filter with a cutoff frequency of 5 kHz before 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).

• LED showing that input signal termination 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 the output signal.