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10MHz-1.5GHz biastee

Introduction

Wideband biastees have two major areas of application: (i) RF semiconductors characterization fixture and (ii) phantom powering of masthead pre-amplifier in wireless reception. The aim is to build a biastee covering 10-1500MHz using components in the junk box.

Material and method

A microstrip transmission line bridges the 'RF' and 'RF & DC' ports to form the biastee's signal path. For economy, the microstrip consists of a 0.8mm thick double-sided FR4 PCB. To minimize reflection at the coax-microstrip transitions, the trace width is scaled for a 50ohm characteristic impedance. To minimize insertion loss, the two RF ports should be positioned as closely as possible because microstrip loss scales linearly with length - hence, it is preferable to house the biastee in a small enclosure. The 10nF DC blocking chip capacitor, a 4.6 uH ferrite ring inductor and a ferrite bead are mounted on the microstrip side of the PCB. A series connection of ferrite ring and bead is used for choking because the former has a relatively low self resonance frequency (SRF). The ring core is not the optimum shape for a wideband choke - commercial biastees use conical solenoids for the highest SRF [1] - but the ring is available at hand. For the widest bandwidth, a small core with high permeability is best - however, a moderate permeability core (ui=125) was used because it is available. A ferrite core enables a higher self resonance frequency than an air core because inter-winding capacitance is reduced by the former's fewer turns and smaller size. The flip side of a small ferrite core is reduced current carrying capability because of core saturation - however, this doesn't pose a problem for phantom powering a pre-amp because of the small current. To extend the high frequency response, a ferrite bead provides choking above the ring inductor's SRF. To improve RF-DC isolation, a 1nF feed-through capacitor, mounted in a PCB hole and soldered to the ground plane, decouples the 'cold' end of the RF choke. Therefore, the PCB ground plane also functions as an RF shield to improve the isolation between RF path and the DC port. A second, non-critical, ferrite ring inductor, which connects the feed-through capacitor to the 'DC' port, improves the RF-DC isolation at low frequencies. A Zobel network consisting of 56R and 22nF in series, improves the DC port's high frequency match. The enclosure is fabricated from PCB scraps and soldered at the seams. Two variants were planned, the first with PAL TV/IEC 169-2 connectors for interfacing with TV equipment and the second with BNC connectors for RF characterization.

Ferrite bead: Sumida/Vogt Fi130 ferrite, OD 3.7, ID 1, L 5.5, ui=30.
Ferrite ring core: Philips RCC6.3/2.5-4C65, ui=125, AL=32, n=12t.

 
Fig. 1 circuit
 
Fig. 2 Two assembled biastees with different RF connectors: PAL TV/IEC 169-2 (left) & BNC (right)

Results & discussion

Less than 0.8dB insertion loss from 10 to 1500MHz. At the -1dB point, the upper frequency limit extends to ~2GHz. The insertion loss drops abruptly below 50MHz; either the blocking cap or the ferrite ring inductor is responsible for the low frequency roll-off. The RF connectors, microstrip PCB and choking inductor contribute to the overall loss at high frequencies. Between 1 to 2GHz, there are some passband ripples of less than 0.1dB peak amplitude and they are likely caused by mismatch in the signal path. 

 
Fig. 3 "RF & DC" to "RF" port insertion loss

Better than -13dB return loss (RL) over 10-1500MHz. Up to 2GHz, the RL is better than -12dB. Both 'RF' and 'RF & DC' ports exhibit the same RL response. The DC blocking cap limits the RL at the low frequency end. Above ~70MHz, the RL degrades with frequency.

 
 
Fig. 4 "RF & DC" port return loss

Greater than 40dB isolation between RF & DC ports. Peak isolation occurs around 200MHz - this likely coincides with the the ferrite ring inductor's self resonance. At the lower frequency limit, the ferrite ring inductor's decreasing reactance degrades the isolation. Above 200MHz, the slowly degrading isolation is probably caused by insufficient choking and RF shielding.

 

Fig. 5 Isolation RF to DC


Postscript - areas for improvement

1.       Low frequency insertion loss (<25 MHz) can be improved by increasing inductance above 4.6 uH.

2.       Low frequency isolation ("RF & DC" to "DC") can be improved by increasing the inductance nearest to DC port.

Reference

[1] T. A. Winslow, "Conical inductors for broadband applications," IEEE Microwave Magazine,  Mar. 2005.

(c) July 2012. This work is protected by copyright and may not be reproduced in any form without the expressed consent of the owner.
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