Categories
Statistics
Flag Counter
Since 08.08.2014
Counts only, if "DNT = disabled".

Your IP is 44.211.31.134
ec2-44-211-31-134.compute-1.
Info
Valid HTML 4.01 Transitional Creative Commons Lizenzvertrag
rss
เราจะทำแบบวิศวกรผู้ยิ่งใหญ่
We love the King
24. July 2024
Your valuable opinion :
5 stars

Avg. 5.20 from 10 votes.



RF-T-DC-Block.php    8889 Bytes    28-11-2020 08:45:11


DC-Block


RF TOOLBOX 3599 EDITION





DC-Block

The assembled prototype




✈ Motivation




The available DC-Blocks (from the usual suspects) very often are designed to reach 12.4 GHz or even more. This at the cost of insufficient low frequency performance. We now can tailor the DC-Block to the special needs of the experiment.




✈ CP-DC-Block




This is the "QO-Kit". A standard Sucobox with SMA Connectors. PCB is made from FR-4. It can be hacked together in less than an hour. In case you need ultralow cutoff frequency, assembly may go up to 220 µF, 6.3 V when using 1206 size.


Lower Cutoff, 47 µF and 10 µF
3 GHz Span, 47 µF has only 1 dB loss @ 2 GHz


Calculation is super-easy, just keep in mind, that in a 50 Ω System, the two (source and load) resistors add up, as they are in series. (Therefore choose Xc = 100 Ω)
DC-Block

The so found value for the capacitor is very likely not available. Choose the next larger value to extend the cutoff frequency to the lower side. Reactance Calculator

This Capacitor leads to an insertion loss of S21 = 3 dB (or better). Please note, that the matching S11 is also 3 dB - which is considered poor. Increase the requirement by a factor of 10 i.e. lower the cutoff frequency to 10 % to have a S21 = 20 dB. This means that only 1 % of the Power is reflected. This is true when using a 50 Ω system. It is very likely, that low frequency systems use higher Impedances !

And yes, using the largest value you can get your hands on will yield the lowest possible cutoff frequency. But there is a drawback ! The upper cutoff will decrease, as capacitors are not ideal, meaning that they start to behave like an inductor, above the self-resonant frequency.


Construction of a (leaded) Capacitor. [1]
Impedance vs Frequency. [2]

Source : [1] Drawing courtesy of Kemet, [2] Drawing courtesy of Johanson.


In order to overcome this, nifty Homebuilder use several capacitors in parallel. See the picture below, the one from the Crystek DC-Block. In case you need ultawide bandwidth, stack the capacitors on top of each other ...




✈ Mini-Circuits BLK-18-S+




This solution from Mini Circuits reaches 18 GHz (as the name suggests). The datasheet specifies a lower cut-off frequency of 10 MHz. As we all know, MCL specifies very conservative, and so it does go lower. We measured about 400 kHz or 4.1 nF (on a randomly picked unit). Unfortunately this is still far away from audio frequency.


One capacitor, bounded to the inner conductor
... has a high lower cut-off




✈ Crystek CBLK-300-30




This solution from Crystek uses a very small pcb with two capacitors (100 nF, 10 nF) in parallel. The specified maximum frequency is much lower (3 GHz), but also the lower cut-off frequency and the price. Even so the datasheet speaks of 300 kHz, our measuremnt shows a much lower frequency.


Two capacitors in parallel, on a pcb
... can cover a larger range




✈ Downloads








✈ Share your thoughts



The webmaster does not read these comments regularely. Urgent questions should be send via email. Ads or links to completely uncorrelated things will be removed.


Your Browser says that you allow tracking. Mayst we suggest that you check that DNT thing ?

 
t1 = 6586 d

t2 = 260 ms

★ ★ ★  Copyright © 2006 - 2024 by changpuak.ch  ★ ★ ★

Impressum