Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
A METHOD FOR DIFFERENTIAL SAMPLING
Document Type and Number:
WIPO Patent Application WO/1991/012533
Kind Code:
A1
Abstract:
The present invention concerns a method for differential sampling with a fixed sampling frequency fo. When sampling a signal varying between +Vmax and -Vmax, the dynamics of the sampled signal is large close to the ends of the interval but it is small close to zero. The present invention, however, gives a solution to the problem of measuring the fine structure of a signal, with constant resolution over a large amplitude interval. This is done by measuring an analogous signal and on every sampling occasion directly sampling the signal V and forming the difference, Vdiff, between the signal on this sampling occasion and the preceding. The difference is then amplified in order to increase the dynamics in the following sampling and the present signal value is reconstructed from the direct value V of the signal at a starting point, the initial value, and the sum up to the present time of the following differential values Udiff, (6), divided by their amplifications.

Inventors:
ALMSTROEM HENRIK (SE)
Application Number:
PCT/SE1990/000079
Publication Date:
August 22, 1991
Filing Date:
February 07, 1990
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ALMSTROEM HENRIK (SE)
International Classes:
G01R13/34; (IPC1-7): G01R19/00
Foreign References:
GB1208306A1970-10-14
Download PDF:
Claims:
Claias:
1. A method for differential sampling at the sampling frequency f characterized in that an analogous signal is measured, that on every sampling occasion the signal V is directly sampled and the difference, ^diff between tne signal on this sampling occasion and the preceding is formed, that the difference is amplified in order to increase the dynamics in the following sampling and that the present signal value is reconstructed from the direct value V of the signal at a starting point, the initaial value, and the sum up to the present time of the following differential values Ud^^, (6), divided by their amplifica¬ tions.
2. A method according to claim 1, characterized in that the signal difference is amplified by a constant amplification which is chosen as large as possible taking into account that the absolute value of the amplified signal difference must not exceed the maximum signal voltage V_3V of the equipment more than a limited number of times per timeunit and that every time the absolute value of the amplified signal difference 1^.^ has exceeded V the next direct signal V is used as a new initial value.
3. A method according to claim 1, characterized in that the signal difference Vd^ is simultaneously amplified with a number of fixed amplifications, after which that amplified signal difference U « is chosen whose absolute value is less than, but closest to, the maximum signal voltage V of the equipment.
Description:
A method for differential sampling

The present invention concerns a method for differential sampling with a fixed sampling frequency f .

When sampling a signal varying between +V_, V and -V m3V , the dynamics of the sampled signal is large close to the ends of the interval but it is small close to zero. Of course, the dynamics could be generally increased by sampling the signal with a larger number of bits, e.g. 16 bits instead of the usual 8 or 12. Close to zero the signal is still given with a small number of significant figures. Besides, a 16-bit A/D-converter is normally slower than an 8-bit or a 12-bit A/D- converter. In attempts to overcome this it has been suggested to amplify the signal so that its absolute value is only a little less than V mav . This can be done by simultaneously amplifying the signal with a number of fixed steps and sampling the most suitable. Informa¬ tion about the amplification together with the sample value gives all necessary information.

This method, however, does not solve the problem of measuring a small signal which is riding on a larger signal. When the absolute value of the large signal is close to V the signal can not be amplified and the small signal is not noticeable. In certain cases when the signal is periodic it would be possible to filter away a slowly varying signal of high amplitude and only retain the interesting fast signal of low amplitude and then amplify and sample it with good dynamics. In other cases, and especially in transient processes, such filtering can not take place with preservation of the information in the signal.

The present invention, however, gives a solution to the problem of measuring the fine structure of a signal, with constant resolution over a large amplitude interval by designing it as is evident from the following claims.

In the following, the invention will be described in more detail, with references to the enclosed drawings where

fig 1 shows the invention schematically,

fig 2 shows schematically a device for achieving increased dynamics, that is included in one of the embodiments of the invention, fig 3 shows a diagram of a measuring sequence, fig 4 shows a diagram of another measuring sequence and fig 5a-c shows the different parts of another embodiment of the device to achieve increased dynamics.

The method means that a measuring sequence is analogously measured by a sensor 1. The signal is amplified in the usual way in an amplifier 2 and is suitably then passed through a low-pass filter 3. The low-pass filter establishes an upper limiting frequency in such a suitable relation to the sampling frequency f of a following sampling that the sampling theorem is met. The signal is then fed to a device 4 for achieving increased dynamics. In this there is a feed-back coupling from the A/D-converter to a delay circuit in the device which makes sure that the delay,? " , is I/f all the time. In the device for achieving increased dynamics the difference between the signal at one sampling occasion and the signal at the preceding one is formed. The signal difference is then amplified to a suitable extent. This can be done in a differential amplifier 8. From the device 4 two signals emerge: the direct signal 5 as sampled at a given moment and the said amplified signal difference 6. These two signals are fed to a computer 7 where the A/D-conversion is completed by reconstructing the momentary value from an initial value of the direct signal 5 and the sum of the thereafter up to the actual time established signal differences 6, divided by their amplifica¬ tions.

Thus the new feature of the device is that, apart from recording the direct signal, V, (n bits), successive parts of the signal see fig. 3, are measured. In the figure we have

V diff (t) = V(t) - V(t-r).

τ = ι/f 0

f = the sampling frequency.

When the signal has been recorded in the computer it can be recon¬ structed according to the expression

m _. V(mT)-V(0) +2__V d1ff ( ), m = l, 2...

To avoid sampling errors during the A/D-conversion, the signal is low- pass filtered at for instance f /4 Hz before the A/D-conversion. If the signal is filtered somewhat harder, the signal difference V di ^ will well approximate the time derivative of the signal.

V((m - 1/2)1) * V d1ff (mT)/r , m = 1, 2...

The advantage of this expression compared with the expression above is that the absolute accuracy is constant, while it decreases with in¬ creasing m in the previous expression.

The signal difference is amplified before the A/D-conversion with an amplification factor K, where K is chosen such that

l ϋ diffl < v max' where u d1ff '

Vj-rr is then obtained from

V diff = U diff

Then the dynamics increases to

" bits.

If the amplification K occasionally is too large, that is, the resul¬ ting value of is over-modulated (e.g. 12 zeros or 12 ones), the directly registered signal can be used to get a new starting point for the differential registration according to fig. 4.

m V(mT) = V(0) + ∑V diff (iT), m - 1, 2, ..., j

JL ~ V(raf) = V((j+1)T) + v diff (i . m = j+2, ... j+2 αιττ

The device has the property that, with increasing sampling frequency f , the amplification K can be increased, which gives an increased dynamics of the signal. For instance, with K=128, the resolution is given by 19 bits, using a 12-bits A/D-converter.

In one embodiment of the invention, the signal difference V d -^ is amplified with a constant value K in a differential amplifier 8. When choosing the value K, one faces two conflicting interests. On the one hand, one wants the amplification to be as large as possible, on the other hand, one does not want the equipment to be over-modulated, that is to give 12 ones or 12 zeros. One has to balance the amplification K in such a way that the probability that the equipment is over- modulated is acceptably low. If the equipment is over-modulated one uses the direct signal of the next sampling occasion to get a new initial value, after which the following values are calculated from this initial value and the following amplified signal differences ^diff 6 ' - t ^ ιe same way as t ^ ιe P 1 "^ 0115 calculation.

In another embodiment of the invention, the signal difference V -^ is amplified in a number (here exemplified by 3) of parallel amplifiers 10, 11, 12, having different amplifications K j , 2 and Kg where K,=l . These signals are then compared, in comparators 13, 14, 15, 16, with the given maximum value + V of the equipment. With the help of the logical circuit 17, the most amplified signal 18, the absolute value of which is less than V , is fed into the computer 7 together with information about the used amplification 19. At the continued A/D-con¬ version the instantaneous value is reconstructed from an original in¬ itial value 5 and the sum of the thereafter up to the actual time measured amplified signal differences U ^^, 6, divided by their respective amplifications K,, K 2 and K,. Here a new initial value will normally never be used, as a value smaller than V m , v is selected. In the example shown only three amplified signal differences have been used U | = ±K j V d .j , U 2 = ±K 2 dl -^ and U 3 = ±K 3 V diff , where 1 = K j < K«< 3 . In the example, the time delay is caused by two sample-and- hold circuits 20, 21, which makes the U. alternatingly change sign. To

decrease the load of the sample-and-hold circuits 20, 21, a isolation circuit 22, 23, is used before each sample-and-hold circuit. The sig¬ nal is obtained from

Y(mT) = V(0) + (-l) 1 K j; V d1ff (ir)J ,

where 3>

Depending upon the circumstances it is of course in an actual case possible to use any different number of amplified, with different amplification, signals differences U d -^ that is suitable.