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.

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.




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)

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.



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 ...

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.

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.

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