Calculation of the frequency response of narrow-band microwave filters. Scheme, description

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Calculation of the frequency response of narrow-band microwave filters. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In the journal "Radio" in 2003, an article [1] was published on how to calculate a strip microwave filter using the BPF-PP computer program. Radio amateurs who have included it in their album of calculation programs can supplement it with the proposed block, which, together with the BPF PP program, will allow not only to determine the physical dimensions of the design elements of a narrow-band filter, but also to analyze its frequency dependences of the transmission and reflection coefficients.

For better readability of the calculation results, they are displayed on the monitor screen in the form of graphs, which make it easy to assess what changes need to be made to the original information. The results obtained with this program allow the best choice of material for the microstrip design even before the filter is manufactured, as well as the correct "fit" it into the device for which it is intended.

The first thing to do in order for the program to work is to enter line number 495 into the initial block BPF-PP, which will supplement the information about the material of the workpiece. It looks like this:

495 INPUT "Loss tangent of the substrate dielectric tg*e ="; TGD:TGD=TGD/10000.

An additional block for calculating filter characteristics contains information about copper foil, which is sufficient for the vast majority of cases, but changes can be made if necessary. As a rule, in the reference literature, the value of the loss tangent is given, which, for convenience, is overestimated by 10000 times, which line 495 takes into account.

Next, "sew" the BPF-PP program and the additional program block from line 830 into a single unit. It is advisable to change the name of the "linked" program, for example, to BPF-PPGR, in which the letters GR will remind you that it will also present graphic material.

Now, as an example, let's perform filter calculations for two different foil materials.

Let's introduce the filter parameters (decimal commas, as usual, are replaced by dots):

Filter order <2-9>? four
Filter type-? T
Load resistance RN, Ohm? fifty
Bandwidth limits, GHz:
Upper ? 2.8
Lower ? 2.4

Next, the program displays on the screen the central frequency of the bandwidth: F0 = 2.592296 GHz.

The first option is made on the basis of foil fiberglass with epoxy resin filler:

Foil thickness, t, mm? 0.05
Substrate thickness h, mm ?
Dielectric constant E? 4.8
Loss tangent of the substrate dielectric tg*e4=? 250

The program performs the calculation on fifty frequency values that lie within the bandwidth, and on twenty-five values on each slope of the frequency response, which it offers for viewing by a message on the screen:

Chart viewing: Kn - enter '1'; km(log)-'2'; Gvh-'Z'.

The Kn graph displays the frequency response of the voltage transfer coefficient. Its appearance coincides with what we are used to seeing on the curve tracer screen when using a detector head with a linear characteristic. The Km plot is a logarithmic plot of power transfer ratio versus frequency. And the last graph - Gvh - displays the power reflection coefficient from the filter input. A similar image (as an envelope) can be observed if the filter is connected to a sweeping frequency generator (GFS) through a reflectometer.

If the blocks of the program are "linked" correctly, then the graphs shown in Fig. 1-3. They display the results of the calculation for the first option - for fiberglass.

(click to enlarge)

For the second version of the filter - based on the FLAN material - we introduce:

Foil thickness t, mm? 0.05

Substrate thickness h mm ? 2

Dielectric constant E? 3.8

Loss tangent of the substrate dielectric tg*e4=? 12

As a result of the calculation, we get three more graphs - fig. 4-6.

Comparison of the corresponding graphs of both options clearly shows that the use of epoxy-based foil fiberglass leads to poor results in this frequency region. At a higher frequency and a smaller bandwidth, the performance will be even worse. The high attenuation of the signal is due to the low quality factor of the filter resonators - less than 40 (Q<1/tg6), which is why the construction of a filter with satisfactory characteristics on this material will require a lot of work.

The proposed program gives the minimum of what is needed to create a microwave filter. Those who wish to improve it can be offered to create a block that provides for changes in the parameters of the inverters JY (k, k + 1), for example, by making changes to the values of the coefficients A (k), A (k + 1), etc. in order to determine which ones are more acceptable.

You should not widen the bandwidth of the filter response analysis, as the equivalent model is only true in the passband and small surrounding areas. Also, this program should not be used for frequencies higher than 5...6 GHz, since the width of microstrip resonators becomes commensurate with the length and the errors due to the edge effect increase, which are taken into account here in the simplest way.

Cross-linked program BPF-PPGR

Literature

Soldatov O. Calculation of strip microwave filters. - Radio, 2003, No. 6, p. 29, 30.
Microelectronic microwave devices (under the editorship of G. I. Vasiliev). - M.: Higher school, 1986.
Mattei G. L., Young L., Jones E. M. T. Microwave filters, matching circuits and communication circuits, vol. 1 and 2. - M .: Svyaz, 1972.
Fusco V. Microwave circuits. Analysis and computer-aided design. - M.: Radio and communication, 1990.
Hanzel G. Handbook of filter design. - M: Soviet radio, 1974.
Handbook on the calculation and design of microwave strip devices (edited by Volman V.P.). - M.: Radio and communication, 1982.

Author: O.Soldatov, Tashkent, Uzbekistan

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