CHEMANDY ELECTRONICS

FLEXI-BOX Bias Tee

    The bias tee is built in a FLEXI-BOX on a dedicated PCB. The box has one 50Ω input and one 50Ω output. It is intended for use with the Remotely Powered VHF Pre-amplifier, but its wide frequency response would allow it to be used in many applications that require a medium current bias tee. The DC input can be fitted with a feed through capacitor with a solder tag, or a standard DC Connector as used with most mains power adaptors. The RF connectors can be selected from BNC, N and SMA. They could also be Belling Lee (Television Type 75 Ohms!) or Type F to special order. Other connector types could be fitted if required in quantity. We build and test these at Chemandy with an insertion loss of 1 dB + 0.5 dB from 10 MHz to 3 GHz, a current handling capability of approximately 1 Amp and a maximum DC input of 16 Volts. Other performance specifications can be achieved by changing component values and types on the one common PCB.

Circuit

    The simple circuit is shown in Figure 1. The inductor used for L1 dictates the low frequency response and the B62152A4X30 binocular with 2 turns of 0.71 mm enameled wire, fitted as standard, will give better than 15 dB return loss from 10 MHz to 1 GHz. A larger ferrite would of course give a lower frequency response and a high permeability type is required. L2 takes over where L1 leaves off and can extend the performance to 3 GHz, see Figure 2. We use a 68nH SMD for this, a Coilcraft 1008HS-680T will suffice and the self resonance of the inductor at something over 1300 MHz is unlikely to be a problem in this application. The increasing inductance at greater than 1 GHz for this  particular component is also unimportant because it still fulfills the basic function of producing a high reactance but passing DC current. The Coilcraft 1008HS-680T will also pass 1 amp whereas other manufactures equivalent inductors can go as low as 300 mA.

    The DC input voltage is restricted mainly by the type of electrolytic capacitor used for C1 and a surface mount type is typically 35 Volts maximum. If this voltage is too low, a wire ended type could be fitted, or when the supply is known to be reasonably clean at low frequencies, then the electrolytic can be missed out. The maximum input voltage then becomes dependant on C2 which could be a wire ended type, and C3 which should be kept as a surface mount component because of the very low series inductance required. With the SMD capacitors that are generally available, this means that the maximum voltage for C2 becomes 100 Volts for a 1206 size. The values normally fitted by us, are C2 = 470nF and C3 = 1nF. The values are not normally critical, but a good spread is advisable in order to suppress any frequency of RFI at the DC input. Current can be increased for applications with frequencies up to 1 GHz by fitting a larger diameter winding for L1 and omitting L2, but it should be remembered that the law for an inductor is that the inductance will decrease as the DC current is increased. This will reduce the return loss performance at lower frequencies and measurements should be made with the DC current present in order to assess the working performance in critical designs.

Figure 1

Figure 2

      The above results were obtained with SMA connectors and the performance will most probably be inferior with any other type fitted.

       This test aid is available Ready Built or in Kit form. The Kit contains the hardware only; i.e. box, pcb, connectors, screws etc. Components are not included because they are surface mount and  it is impossible to supply the correct amount because of the 'ping' factor. Anybody who has fitted surface mount components will know that when a component is being placed with a pair of tweezers there is often a 'ping' and the component disappears into thin air. We would have to supply at least two of every component thus doubling the cost.

A selection of escutcheon drawings are available for downloading as .zip or .bmp files.

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