White Noise Generator - Is there a chip combining LFSRs and analog output?

[radiolistener]

Read the HP Journal April 1971.
The analog output is generated by summing binary digital outputs.  Each binary is weighted (gain) with a resistor.  It is a non-linear DAC. 
In a DSP you could achieve the same by ANDing the binaries with numbers (gain), then summing to produce a single output value.  Apply this to the DAC.
This approach gives the option of staying digital, or going analogue.

This clarifies the functionality of the LFSR but doesn’t address the rationale for placing a DAC between the LFSR and the output.

Do I understand correctly that the DAC is intended to act as a filter to shape the flat noise response of the LFSR into a specific curve?

What specific frequency response are you aiming to achieve using the DAC as a filter?


For example, here is the spectrum of digital noise (direct output from LFSR):



And here is the spectrum of analog noise (using DAC between LFSR and output):



I don't see significant difference, except a little ~3 dB noise power offset. So, I'm trying to understand the reason to use DAC for noise generator...

[dazz1]

Do I understand correctly that the DAC is intended to act as a filter to shape the flat noise response of the LFSR into a specific curve?

What specific frequency response are you aiming to achieve using the DAC as a filter?

All is explained in the HP Journal attached to an earlier post.  My minimum requirements are to emulate an HP 3722A Pseudo Random Noise Generator.  I am a long way from starting any serious design work.
The HP 3722A implements a non-linear DAC with resistors and op-amps.  As mentioned above, this also implements a Finite Impulse Response (FIR) filter.

[dazz1]

I don't see significant difference, except a little noise power offset. So, I'm trying to understand the reason to use DAC for noise generator...

You need to look at the probability distribution function, and read the HP Journal. 

[radiolistener]

You need to look at the probability distribution function, and read the HP Journal.

Does it really matter when the LFSR is clocked at a sufficiently high sample rate from a non-synchronized clock source? The output will naturally undergo low-pass filtering through the output driver, which should result in a good natural distribution, which will be even better due to asynchronous clock. Isn't it?

I'm interested in this, because in such cases I usually just use digital noise with the maximum sample rate, but maybe I'm wrong and this can lead to some disadvantages?

[dazz1]

Does it really matter when the LFSR is clocked at a sufficiently high sample rate from a non-synchronized clock source? The output will naturally undergo low-pass filtering through the output driver, which should result in a good natural distribution, which will be even better due to asynchronous clock. Isn't it?

I'm interested in this, because in such cases I usually just use digital noise with the maximum sample rate, but maybe I'm wrong and this can lead to some disadvantages?

It depends on the application.    The HP 3720A Spectrum Display has a frequency range of 0.005Hz to 250kHz.  Some more reading attached.

[Kleinstein]

If an asynchronous clock is good or bad depends.
A 250 kHz max. bandwidth suggests some 5 MHz as maximum clock. So not expecially fast. HP3722 even uses a reduced clock to get a reduced BW part, so that more of the dynamic range is used. This could be in part also to have the same clock for the delay to use in the correlator. With the full clock one would need more memory / shift registers for the delay function there. A modern correlator would be more using 2 ADC channels and math to get the correlation function for many delays at the same time.

The coeffcients / resistors of the semi digital delay line filter should not be that critical. Any error there can effect the filter roll off a little, but not that much. The sinx/x scaling gives the brickwall type filter, but even with small errors in the filter one still gets a good low pass. One still has the analog LP filter anyway, that is not that ideal in the transition region.  I would consider some 16 odd resistor values (2x used each) not that bad. Even if 10% off, the respose curve of the filter would not be that much different from the ideal case and much would be towards the higher frequencies that are of little interest. There is anyway a none ideal response from the limted length of the sequence (32 bits in the HP7322).

[dazz1]

Using code rather than resistors has a number of benefits.  One of which is to significantly reduce the parts count.  Also easy to add self calibration feature.  Also software control of the FIR response including using an IIR filter.  All software selected.

By modern standards 250kHz is positively pedestrian.  It should be relatively easy to use a low spec, low cost DSP to emulate the HP 3722a. Such an instrument is likely to be of interest the modular audio synth crowd.

[dazz1]

If an asynchronous clock is good or bad depends.
A 250 kHz max. bandwidth suggests some 5 MHz as maximum clock. So not expecially fast. HP3722 even uses a reduced clock to get a reduced BW part, so that more of the dynamic range is used. This could be in part also to have the same clock for the delay to use in the correlator. With the full clock one would need more memory / shift registers for the delay function there. A modern correlator would be more using 2 ADC channels and math to get the correlation function for many delays at the same time.

The sample period is in the range 1s to 1us with a processing time of 136us.
An external clock can be applied to extend the clock period.
250kHz can be used with real data for the Correlator at a higher data frequency so long as the statistics of each sample remain the same.  Nyquist does not apply.
The Correlator can only process 100 samples but it can step along the noise signal in the time domain to extend the number of samples.    This is how the system can get down to the 0.005Hz freq.  Nyquist does apply.
The pre-GPIB 50 pin interface allows for control and export of data streams. 

The HP3722A, HP 3721A and HP3720A were designed and built run together so their specs compliment each other.