DE1766425B2 - SAMPLE PROCEDURE FOR AN N-PATH FILTER - Google Patents

Description

The invention relates to a sampling method for an N- path filter with pulsed sampling.

As is known, N-path filters are circuit arrangements which consist of several parallel channels which in turn an input modulator, a filter network that is the same in all channels and an output modulator included, the modulators of the individual paths (preferably switches) by control signals specific frequency and phase position can be controlled.

Circuit arrangements of this type have the property of reducing the transmission behavior of the parallel Paths existing filter networks in the area around the sampling frequency with which the modulators or Switches are controlled, transform.

The advantages of this filter technology (tunability, high constancy of center frequency and bandwidth and thus very high achievable effective filter qualities, micro-miniaturization) stands among other things considerable disadvantage compared to limited wide selection.

This disadvantage of N-path filters is, for example in the magazine "Frequency" issue 1/1968, pp. 11 to 16, referred to, in particular to 12, 15 and 16. As a result, further passbands occur at multiples of the sampling frequency, which are caused by additional conventional input and / or output filters, for example made up of coils and capacitors of the N-path filter know to be suppressed. The requirements for these additional filters are given in high required wide selections correspondingly high.

The invention is based on the object the technology of the N-path filter with pulse-shaped sampling in terms of its economic outlay to improve far-reaching selection.

According to the invention, the solution to this problem is that the pulse duty factors of the pulse-shaped Scanning programs, i.e. the 1 switching time of a

Channel for the period not equal to ^ (N = number of channels) is chosen such that a suppression passbands other than the desired passband corresponding to the fundamental sampling frequency occurs, with the duty cycles for the input modulators and the output modulators are chosen to be equal or different.

In other words, the invention resides in one Sampling method for an N-path filter with pulsed Sampling in that the duty cycles of the pulse-shaped scanning programs are independent of the Number of paths can be set so that a suppression of other or further passbands occurs than corresponds to the known procedure that Select pulse duty factor with 1: N.

The prior art and the method according to the invention are described in more detail with reference to FIGS.

Fig. 1 shows the basic arrangement of an N-path filter! The input signal SJt) is fed in parallel to the N different input modulators M _te to Mnc and modulated here with the pulse-shaped key function of the frequency ^ _{0 to be described in more detail.} The output signals of the N input modulators then pass through N identical filter networks F (; ώ) and then N further (output) modulators Mu to Mn ₃ , in which they are generally modulated with the same key function as in the input modulators. After summation over all N channels, the output signal SJt) results. At this point it should be pointed out that SJt) and SJt) can be voltages or currents and that the modulators can have multiplying properties in the broadest sense. In the following, the explanation of the invention is limited to the technically particularly interesting case that SJt) and SJt) represent voltages and that the modulators correspond to series switches, without thereby limiting the general applicability of the invention.

F i g. 2 explains the generally used sampling function using the example of a 3-channel filter (N = 3) The main features are that, firstly, a time shift is set between the channels will that look after

2.7

calculates that, secondly, the ratio of the switch-on time of a channel to the period is equal to - ^ and that

Finally, the input and output modulators of each channel are controlled with the same scanning function. When using the scanning method according to FIG. 2 pass bands occur at Ar times the sampling frequency, as shown schematically in FIG. 3a for a 3-path filter and in FIG. 3b for a 4-path filter. The transmission efficiency of the transmission ranges at λ- · ω ₀ calculates ^s: h Narh of the formula

Atk) - A ₁ ,

sin

ky

with k = 0, 1, 2

wob- 1 A " one depending on the filter type and design

represents a different constant factor. It can be seen from FIG. 3 that the transmission ranges of the Ordinal numbers> 1 make high demands on the VGrselection and that the 3-path filter in the With regard to this represents the best case

It is now essential for the invention to deviate from the generally customary scanning scheme to the effect that that certain passbands are omitted. This is achieved by choosing the appropriate duty cycle the sampling pulse trains, namely z. B. separate for input and output of the paths.

= A ₀

A technically very important example of a sampling scheme according to the invention for a 3-path filter shows Fig. 4. Here, the input modulators are sampled with an extended pulse duration, and although the pulse duration is so much longer that the ratio of the pulse duration to the penode duration is 1: 2 results The output ratio is 1: 3 and therefore in unchanged in this particular case.

The transmission efficiency of a 3-path filter operated with the sampling scheme according to FIG calculates to

I.
sin kn
'2 kTi kn 3 k .τ

with k = 0, 1, 2 ....

One can see from FIG. 5 that A (k) has zeros for Jt = 2, 3, 4, 6, 8 etc., so that in particular the pass band for 2ω ₀ , which is difficult to suppress, disappears

F i g. 6 shows an embodiment of a filter arrangement according to the invention:

A 6-stage ring counter RZ controls 6 logic elements G \ to Gs in such a way that the switches S \ to Se in the FIG. 4 can be controlled accordingly.

The logic elements can be used in a known manner as gate circuits (according to the illustration F i g. 6), but also be designed as bistable multivibrators etc.

The additional effort compared to a conventional 3-path filter is the increase in the number of stages Ring counter from 3 to 6 and in the links, i.e. only on the digital side.

For this purpose 3 sheets of drawings

Claims (3)

Patent claims: 1. Scanning method for an N-path filter with pulse-shaped scanning, characterized in that the duty cycle of the pulse-shaped scanning programs, i.e. the switch-on time of a channel to the period duration not equal to γ (N = number of channels) is chosen such that a suppression of other major: Passband occurs as the desired passband corresponding to the fundamental sampling frequency, the pulse duty factors for the input modulators (M \ e-MNe) and the output modulators (M \ _a -MNa) being selected to be equal or different. 2. The method according to claim 1, applied to a three-path filter, characterized in that two Sampling programs with a duty cycle of 1: 2 in the input and 1: 3 in the output can be used. 3. The method according to claim 2, characterized in that the three-path filter of the series modulator type is.