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Application Note AN-1e
Copyright November, 2000

Broadband Coaxial Bias TeesJames R. Andrews, Ph.D., IEEE Fellow
frequency cutoffs down to ultrasonic frequencies of tens ofkHz or lower.

Bias Tees are coaxial components that are used whenevera source of DC power must be connected to a coaxial cable.
When properly designed, the bias tee does not affect the ACor RF transmission through the coaxial cable. The usualapplication is to provide a means of powering an active device
such as a transistor, laser diode or photodiode. Other uses
would be to provide power to operate remotely located coaxial
relays or amplifiers, or to transmit low frequency analog ordigital signals on the same coax cable along with RF signals.
Bias tees have been around and used for a long time. There
are several microwave and RF manufacturers that have built
bias tees for many years. Most of these tees were designed
only for specific octave frequency bands. However, in todaysmodern digital and analog world, very wideband systems are
now being used. For these systems, octave bandwidthcomponents are grossly inadequate and cause extremely
severe pulse distortion. Telecom and CATV companies are
now fielding extremely wideband 10 GigaBit fiber optic digitaland > 1 GHz analog systems. These higher bandwidth
systems are using laser diodes and photodiodes withrisetimes of 20 ps or less. To support these fast 20 ps
risetimes requires system components with -3 dB bandwidths
in excess of 20 GHz. An approximate equation relating
risetime and bandwidth is: Tr (10%-90%) * BW (-3 dB) =0.35. Digital telecom systems also require very low frequency
responses to reduce bit error rates due to sagging pulses. If
a very long duration square wave pulse is passed throughan AC-coupled circuit (such as a bias tee), the topline of thepulse will eventually decay exponentially back down to zero.
This can happen in a digital system with an NRZ digital code
whenever a long string of either 0s or 1s occurs. In orderto maintain acceptable bit error rates for GigaBit datasystems, the low frequency cutoffs of various systemcomponents must extend well out into the microsecond
domain. Thus, system components must have -3 dB low

Figure 1 shows the basic schematic diagrams of the twomost common bias tee designs. Capacitor C is a DC block
installed in the center conductor of the 50 Ohm coaxial line.
It prevents the DC power from flowing out the AC port. For
low-current applications, such as biasing photodiodes, aresistor R is used to provide the connection between theDC input and the coaxial center conductor. To avoid loading
the coax line, the resistor value is chosen to be much greaterthan the coax impedance. For higher-current applications
in which the potential drop across R or the power dissipatedin R would be too great, it is necessary to instead use aninductor.

Figure 1: Bias Tee Schematic Diagrams
The low-frequency performance of a bias tee can be very
easily calculated from elementary circuit theory. For the RC bias tee, the DC blocking capacitor determines the -3 dB
low-frequency cutoff (fo).
fo=1[2 * P * (Rg + RL) * C]

Where Rg is the signal generator source impedance andRL is the load impedance. For example, with 50 Ohm source
and load impedances and a 0.22 mF blocking capacitor, the-3 dB cutoff is 7.23 kHz. For the L-C bias tee, the -3 dB low
frequency cutoff is determined by the rising series
impedance of the DC-blocking capacitor C and also thedecreasing shunting impedance of the DC feed inductor L.

PICOSECOND PULSE LABS P.O. BOX 44 BOULDER, CO 80306, USA TEL: 1.303.443.1249 FAX: 1.303.447.2236
WWW.PICOSECOND.COM

AN-3041e, Revision 1, 11/00

Page 1 of 3

Application Note AN-1e
Copyright November, 2000

The microwave performance, high-frequency roll-off andrisetime of a bias tee are limited by the parasitic reactancesand losses associated with the components R, L and C andthe care in which they are mounted within the coaxialenvironment. Any impedance discontinuities from 50 Ohms
created by their mounting or the bias tee package will also
limit the high-frequency performance and bandwidth.
The mid-band frequency response and also the time domainstep response, pulse fidelity and flatness are primarily
determined by the behavior of the DC-feed inductor L. In
reality, it is impossible to use a single inductor to cover all
frequencies from audio to microwaves. Real inductors areprobably the worst components in that they never perform
according to textbook theory. Any physically realizable inductor
always is plagued with parasitic capacitances, losses, etc.
that cause multiple resonances and prevent it from behaving
as the ideal textbook inductor. Manufacturers making narrow,octave-band bias tees can get away with using only a single
inductor optimized for that band. One cannot use a single
inductor for broadband bias tees. Thus, for broadband bias
tees, the single inductor L shown in Figure 1 is, in reality,
several inductors connected in series. Each inductor is
optimized to cover various frequency bands from microwave,
UHF, VHF, HF, and MF down to audio frequencies. The
smallest nanohenry, microwave inductor is connected directly
to the 50 Ohm coax-line center conductor. Progressively larger
inductors in mH to mH values are then connected in seriesout to the DC port. Additional R, L and C components arealso needed to ensure controlled Qs and smooth frequency
crossovers from one inductor to the next. Thus, a goodbroadband bias tee is a very complex R-L-C network.
Picosecond Pulse Labs (PSPL) was originally founded in 1980
to produce extremely fast picosecond risetime pulsegenerators. In the design of these generators we needed bias
tees to power various semiconductors. We found that thenarrow-band bias tees then available commercially weretotally unacceptable for pulse applications. Thus, we wereforced to design our own bias tees. Our bias tee designs
were so successful that we decided to offer them for sale.
Today, PSPL is a major supplier of bias tees to most of themajor telecom companies. PSPL now offers a wide selectionof bias tees for various bandwidth, voltage and current
requirements. PSPL is continuing its development of additionalmodels for higher bandwidths and higher currents. PSPL alsooffers to design and build custom bias tees.

Table 1 on page 3 lists the broadband bias tees currentlyoffered by PSPL. These are all 50 Ohm units. The fastest
risetime is 7 ps. Bandwidths up to 50 GHz are available.
Max. current ratings range from 10 mA to 1 Amp. Max.
voltage ratings range from 16 V to 200 V. Low frequency
cutoffs range from 5 kHz to 100 kHz. For two models, 5550B
and 5575A, the low frequency cutoff is a function of the DC
current. For these two models, the magnetic core of thelargest mH inductor saturates at high currents, thus lowering
the effective value of L and raising the low-frequency cutoff.
Detailed specification sheets are available from PSPLsweb site for all of these models. The specification sheets
include plots of frequency domain S parameters and time
domain pulse responses.
PSPLs highest performance bias tee is the new Model
5542. It features a bandwidth extending from 10 kHz to 50
GHz. It is rated at 16 V and 100 mA and is available witheither 2.9 mm or 2.4 mm connectors. A dual inductor versionof the 5542 is also available.
PSPL also offers a family of four high current (8 Amps),
high voltage (100 V) bias tees. They are narrowband
components covering decade bands from 200 MHz to 20GHz.
Figures 2 - 7 on page 3 show the typical performance ofthe PSPL Model 5541A bias tee. The 5541A has a risetimeof only 8 ps. Figure 2 shows the pulse response when driven
by a 10 ps risetime input step. For this measurement anHP-54124A, 50 GHz oscilloscope and a PSPL 4015C, 15
ps pulse generator were used. For 10 ps measurement
details see PSPL AN-5c. Figure 3 shows the TDR
impedance profile of the AC input port. Figures 4 - 7 show
the frequency responses for insertion loss, group delay andreturn loss. These were measured using a Wiltron 37369A,
40 GHz vector network analyzer.
The measured 8 ps risetime of the 5541A would imply a 44
GHz bandwidth (BW = 0.35 / Tr). However, close
examination of Figure 5 shows that there are multiple
resonance spikes occurring in the 5541As insertion loss
for frequencies above 26 GHz. These are due to the 5541AsSMA connectors, which support higher order waveguidemodes above 26 GHz. The minimums of S21 show only 1
dB of loss up to 40 GHz, which correlates well with themeasured 8 ps risetime. Due to the SMA resonances, useof the 5541A for frequency domain applications above 26
GHz is not recommended.

PICOSECOND PULSE LABS P.O. BOX 44 BOULDER, CO 80306, USA TEL: 1.303.443.1249 FAX: 1.303.447.2236
WWW.PICOSECOND.COM

Page 2 of 3

AN-3041e, Revision 1, 11/00

Application Note AN-1e
Copyright November, 2000

Table 1: PSPL Broadband Bias Tees (detailed specification sheets available on request or at www.picosecond.com)
I Max(amps)

R(ohms)L(mH)>2520010m1k---0.082>255010m

1k---0.22S21(dB)

2812200.2RC

28

1270.25541A

LC

8>26800.4

2050100m3.7

1.00.025542

LC

750

100.2>25

16100m5.6

1.5

0.225545

LC

12>20

65

0.7

23

50

500m

1.1

0.34

0.035547

LC

23

15

50.5

23

50

500m

1.5

1.340.445550B

LC

20

18

1000.9>15

50

500m*

0.4

1.00.025555

LC

20

18

100

0.9>1550500m

0.6

0.110.025575A

LC

30

12100.6>18

50

500m*

0.6

80.225580LC2815101.0>1550

1.00.81.1

0.225530A

RC5535

S11(dB)C)

fo(kHz)

TypeTr(ps)VoltsmaxBW(GHz)

ModelTime Domain and Frequency Domain Responses of PSPL Model 5541A Bias Tee

50 Ohms

Figure 2: Response to 10 ps
Risetime Input Step. 20%/div.
Top to Bottom: 1 ns/div, 200 ps/div
and 20 ps/div.

Figure 3: 35 ps TDR of AC Input
Port. 2.5% r/div and 500 ps/div.

Figure 5: Insertion Loss, S21.
Figure 6: Group Delay, S21.
Linear Plot from 40 MHz to 40 GHz. Linear Plot from 40 MHz to 400.5 dB/div and 4 GHz/div.
GHz. 10 ps/div and 4 GHz/div.

Figure 4: Insertion Loss, S21.
Log Plot from 10 kHz to 100 GHz.
Vertical Scale is 1 dB/div.

Figure 7: Return Loss, S11.
Linear Plot from 40 MHz to 40 GHz.
5 dB/div and 4 GHz/div.

PICOSECOND PULSE LABS P.O. BOX 44 BOULDER, CO 80306, USA TEL: 1.303.443.1249 FAX: 1.303.447.2236
WWW.PICOSECOND.COM

AN-3041e, Revision 1, 11/00

Page 3 of 3

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