# Impedance Matching Network / Band Pass Filter

#### Nomi 1114

Joined Dec 4, 2017
124
Dear all,

Hope you all are working well good health. Today i am asking for one important question, where i am stuck, and need your kind suggestions and help.
I have designed a Power Amplifier for RFID applications. The output power of RF PA is 15W ( and frequency of signal is 7.7MHz ~ 8.7MHz, Short band communication), and i need to fed this signal at the antenna input (where the input impedance of antenna is 50 ohm), now i need to design a impedance matching network (or band pass filter for the said frequency) which will be used between Power Amplifier and Antenna. I am failed to designed this matching networking for short band ranging from 7.7MHz to 8.7MHz. I have designed a matching network for specific frequency, because of lack of experience i am unable to design broadband/short band matching network.

Regards

Joined Mar 10, 2018
4,057

#### Papabravo

Joined Feb 24, 2006
14,859
The design of a bandpass filter has several steps, but the first step is to come up with an appropriate specification:
1. You need the source impedance of the Power Amplifier
2. You need the allowable attenuation in the passband
3. You need the desired attenuation in the stopband
4. You need the bandwidth of the upper and lower transition bands
5. If the filter will not be maximally flat in the passband you need the allowable ripple in the pasband AND the stopband
This information will determine the type and the order of the filter. Once we know the order of the filter it is plug and chug with an online calculator. Then we select real component values and check to see if the actual realization matches our requirements. Then we can look at the effect of component tolerances. Can you take the first step and rustle up a set of requirements?

Here is an example of a 3-pole Butterworth Filter with 6 dB insertion loss at 50Ω using ideal components

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#### Janis59

Joined Aug 21, 2017
1,245
Nomi1114: Thanks for wishing the health, but due I answered to that telling two sentences what I see here on the streets my mail was deleted by AAC censors. So the answers main thing was lost - it is - the instrument You need have name RF-Sim-99. That is freeware, Just Google.

#### Papabravo

Joined Feb 24, 2006
14,859
Nomi1114: Thanks for wishing the health, but due I answered to that telling two sentences what I see here on the streets my mail was deleted by AAC censors. So the answers main thing was lost - it is - the instrument You need have name RF-Sim-99. That is freeware, Just Google.
Yah. That is really going to help him if he does not know what his requirements are. BTW -- what censors are you talking about?

#### Janis59

Joined Aug 21, 2017
1,245
RE: Papabravo - 1)Yesterday 8:50 PM by JohnInTX; 2)""That is really going to help him if he does not know what his requirements are."" - I answered for people may be one day will read and understand the things only dont know the ropes. If he today isn’t grew up enough, that not mean we should sink down to equalize with his level here and now. Most of people are growing in knowledge whole life and by the way, the FAQ (frequently asked questions) in that soft isn’t bad, its quite educative.

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#### Nomi 1114

Joined Dec 4, 2017
124

Thank you very much for your kind help and cooperation. Here i wanna inform you that you shared some link those are good , actually i already used visit these sites, these are helpful/used for specific frequency not for a band of frequency. but if you have some better option you can share with me. again thank you very much for your concern.

With humble regards

#### Nomi 1114

Joined Dec 4, 2017
124
The design of a bandpass filter has several steps, but the first step is to come up with an appropriate specification:
1. You need the source impedance of the Power Amplifier
2. You need the allowable attenuation in the passband
3. You need the desired attenuation in the stopband
4. You need the bandwidth of the upper and lower transition bands
5. If the filter will not be maximally flat in the passband you need the allowable ripple in the pasband AND the stopband
This information will determine the type and the order of the filter. Once we know the order of the filter it is plug and chug with an online calculator. Then we select real component values and check to see if the actual realization matches our requirements. Then we can look at the effect of component tolerances. Can you take the first step and rustle up a set of requirements?

Here is an example of a 3-pole Butterworth Filter with 6 dB insertion loss at 50Ω using ideal components
Dear Respecetd Papabravo,

Thank you very much for your kind help and cooperation. you really helped me lot. i will use this cct with my RF PA, i feel any problem then i will contact you again. Once again thank you very much for your concern.

With humble regards

#### Papabravo

Joined Feb 24, 2006
14,859
Dear Respecetd Papabravo,

Thank you very much for your kind help and cooperation. you really helped me lot. i will use this cct with my RF PA, i feel any problem then i will contact you again. Once again thank you very much for your concern.

With humble regards
I was hoping you would respond with your actual requirements. The example I worked up is a 3-pole Butterworth filter. The skirts are not very steep, the component values DO NOT correspond to real parts that you can buy, and the stopband attenuation could be much better. I can't tell you what you need UNLESS you can define your requirements. Why won't you DO THAT?

#### Nomi 1114

Joined Dec 4, 2017
124
Nomi1114: Thanks for wishing the health, but due I answered to that telling two sentences what I see here on the streets my mail was deleted by AAC censors. So the answer`s main thing was lost - it is - the instrument You need have name RF-Sim-99. That is freeware, Just Google.
Dear Respecetd Janis59,

Thank you very much for your kind help and suggestion. I have downloaded RF-Sim-99, it's really very good and useful tool for designing of impedance network. If i feel any problem in future then i will contact you again. Once again thank you very much for your concern.

With humble regards

#### Papabravo

Joined Feb 24, 2006
14,859
To help you with your design task, I should mention that the first step in designing a bandpass filter is to compute the center frequency. The way you do this is to take the geometric mean of the two frequencies that determine the band edges. The filter response will be 3 dB down at these two frequencies. The stated requirement was a bandwidth of 1 MHz., from 7.7 MHz. to 8.7 MHz. The geometric mean of those two frequencies is the square root of their product:

$\sqrt{(7.7\times10^6)(8.7\times10^6)}\;=\;\sqrt{66.99\times10^{12}}\;=\;8.18\times10^6$

This also happens to be the resonat frequency of the series leg of the 3-pole bandpass filter.

#### Nomi 1114

Joined Dec 4, 2017
124
I was hoping you would respond with your actual requirements. The example I worked up is a 3-pole Butterworth filter. The skirts are not very steep, the component values DO NOT correspond to real parts that you can buy, and the stopband attenuation could be much better. I can't tell you what you need UNLESS you can define your requirements. Why won't you DO THAT?
Dear Respected Papabravo,

Thank you very much for quick response. Actually i have designed Power amplifier by using IRFR-110 MOSFET. and it is used for amplification of signals (having frequency 7.7MHz ~ 8.7 MHz and holding 5Vp-p) after amplification frequency is same as defined before but magnitude is about 77 ~ 85Vp-p and power is 15 Watt. Frankly speaking i don't know the source impedance of PA but one requirement that behavior of matching network is flat and corner/strike should be steep with width of 3-dB.

Please help me by using this information and if you need my schematic then i can provide you. I shall be really highly grateful to you for this act of kindness.

With Humble Regards

#### Nomi 1114

Joined Dec 4, 2017
124
The filter response will be 3 dB down at these two frequencies. The stated requirement was a bandwidth of 1 MHz., from 7.7 MHz. to 8.7 MHz.
Dear Respected Papabravo,

exactly , the info is right.

#### Papabravo

Joined Feb 24, 2006
14,859
Dear Respected Papabravo,

Thank you very much for quick response. Actually i have designed Power amplifier by using IRFR-110 MOSFET. and it is used for amplification of signals (having frequency 7.7MHz ~ 8.7 MHz and holding 5Vp-p) after amplification frequency is same as defined before but magnitude is about 77 ~ 85Vp-p and power is 15 Watt. Frankly speaking i don't know the source impedance of PA but one requirement that behavior of matching network is flat and corner/strike should be steep with width of 3-dB.

Please help me by using this information and if you need my schematic then i can provide you. I shall be really highly grateful to you for this act of kindness.

With Humble Regards
I don't seem to be getting through to you. The Butterworth response is as flat as it can be in the passband. No other filter can be flatter. If being down 3dB @ 7.7 and 8.7 MHz is too much then your bandwidth needs to be wider. For example: you could say that the maximum attenuation at the band edges is 0.5 dB. You could also say that the attenuation in the stopband is 60 dB down from the passband, and you could say that the width of the transition band is 500 kHz. These are Examples! Your requirements will probably be different.

You also need to find the output impedance of the amplifier. It is critical.

#### Nomi 1114

Joined Dec 4, 2017
124
I don't seem to be getting through to you. The Butterworth response is as flat as it can be in the passband. No other filter can be flatter. If being down 3dB @ 7.7 and 8.7 MHz is too much then your bandwidth needs to be wider. For example: you could say that the maximum attenuation at the band edges is 0.5 dB. You could also say that the attenuation in the stopband is 60 dB down from the passband, and you could say that the width of the transition band is 500 kHz. These are Examples! Your requirements will probably be different.

You also need to find the output impedance of the amplifier. It is critical.
Dear Papabravo,

I agreed with your opinion that i need to calculate the output impedance of PA, secondly it’s also absolutely right the response is flat. Right now can you suggest me how can we steep sharp in this case ?? With same parameters !!

With humble Regards

#### Papabravo

Joined Feb 24, 2006
14,859
Dear Papabravo,

I agreed with your opinion that i need to calculate the output impedance of PA, secondly it’s also absolutely right the response is flat. Right now can you suggest me how can we steep sharp in this case ?? With same parameters !!

With humble Regards
To get a steeper rolloff in the transition band the filter needs to be of a higher order. In other words it needs more poles, or at the expense of some ripple in the passband you can try a different topology, like a Chebychev or an Elliptical