adaptation filter  from 330 ohm to 50 ohm - Page 1
Forum section: electronics / rf-microwave

author: nenea dani
author_rank: Regular Contributor
timestamp: January 26, 2019, 12:01:36 am
content: Hello colleagues,I have found the attachment scheme that comes from a spectrum analyzer and I want to use it. It is a ceramic filter 10,7Mhz with Zi = Zo = 330 Ohm. How did the designer calculate the adaptation to get 50 Ohms at the input and output. I see that the two LC circuits resonate on approx. 10.4Mhz, possibly 10.7Mhz, but I do not know how the capacitive divider turns 330 in 50 Ohm.

author: Marco
author_rank: Super Contributor
timestamp: January 26, 2019, 02:01:55 am
content: Quote from: nenea dani on January 26, 2019, 12:01:36 amBy swinging up the voltage. The matched impedance behaves as 50 Ohm load power wise, since the 330 Ohm equivalent section is dissipating that power it requires a higher voltage for that to happen.I no longer know how to do the math to design the network, AWR's iMatch wizard does though

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2019, 07:48:03 am
content: This is technically how it works,http://analog.intgckts.com/impedance-matching/tapped-capacitor-matching/though if you aren't comfortable with math, you may be just as lost...Suffice it to say, capacitors don't have loss, and loss is where the signal goes (hopefully to a useful sink, like a radio detector). Capacitors do affect the impedance, so by tuning out the reactance we can change the resistance. Thus, it's an impedance transformer or matching network. Figuring out when it applies, and over what ranges (bandwidth, ratio), is where math applies.Tim

author: radiolistener
author_rank: Super Contributor
timestamp: January 26, 2019, 10:00:38 am
content: Quote from: nenea dani on January 26, 2019, 12:01:36 amI think this LC circuit is a Band Pass Filter with input impedance 50 Ohm and output impedance 330 Ohm. This BPF is just calculated for 50 Ohm input and 330 Ohm output impedanceAt least simulation shows that it works like bandpass filter with center frequency at 10072 kHz and 14732 kHz bandwidth at 3 dB threshold. On 10.7 MHz this filter has voltage gain +7.86 dB. I think this noise free voltage gain is possible due to the higher impedance on the output and inductance.You can do that calculations with a small and easy tool - RFSimm99

author: nenea dani
author_rank: Regular Contributor
timestamp: January 27, 2019, 10:14:12 am
content: With calculated data, it is somehow checked at 5.7 = 6. Thanks Tim for opening my eyes with the capacitive divisor. I was going on the wrong idea that it was the same as resistive. So far, so good. I selected a set of COG capacitors, a red torrus with Al = 6.5 (1, 033microH) and built the adapter on the left side of the filter. On the right I plugged only a 330 Ohm resistor. At the input of the adaptation circuit I connected a VNA. I attach the results. I tested with several filters and the results are pretty much the same. First, it seems like I need a variable coil that resonates at 10.7 Mhz since the filter has only BW = 180Khz or even 140Khz. Something is wrong .

author: nenea dani
author_rank: Regular Contributor
timestamp: January 27, 2019, 10:15:58 am
content: I've attached another photo.

author: whalphen
author_rank: Regular Contributor
timestamp: February 11, 2019, 01:46:59 am
content: Don't try to understand it as a filter. It's an impedance matching network. Download the Iowa Hills Smith Chart software and plug in the values. You'll find that it matches 50 ohms to 330 ohms just as it's supposed to. With the Smith Chart software, you can easily find other values and topologies that will work, also. For example, a series 120p capacitor followed by a shunt 2.2uH inductor will do it.