EXTRA-2 REV-5
Introduction
The large number of extremely high signal levels at VHF contests causes bigger and bigger problems (spreading of multi-PAs, multi-antennas) nowadays. Low noise figure is easy to achieve with modern devices, so the primary design requirement is the strong signal performance (IMD characteristics). Transistor selection is the first and most important step in an LNA design. You can find a very interesting review about devices with high dynamic range in [1] but unfortunately the most promising one, ATF-53189 by Avago Technologies is not included. The main data for this GaAs FET can be found in Table 1 [2].
The preamplifier described here was produced to replace circuits built with ATF-54143 pHEMT FET [3]. The ATF-54143 is very sensitive to static voltages and TX power, and although the noise figure is good, the device is not unconditionally stable. This can cause many problems in the case of a nearby broadcasting or cellular base station.
|
Design
Avago Technologies’ ATF-53189 is a high linearity, medium power, low noise E-pHEMT FET in a low cost surface mount SOT89 package. Packaging of the ATF-53189 can be seen in Figure 1. The package has two source leads with large surface areas for efficient heat dissipation and low-inductance RF grounding. The enhancement mode technology provides superior performance while allowing a DC grounded source amplifier with a single polarity power supply to be easily designed and built. The enhancement mode PHEMT requires about +0.6V bias potential between the gate and source (VGS) for the target drain current, IDS.
Table 1 ATF-53189 Absolute Maximum Ratings, Electrical specifications
Modern design methods require that the ATF-53189 pHEMT should be analysed with a simulator and then followed by measurements. The most favourable DC setup (UDS = 4 V and ID = 135 mA) results in a significant heat dissipation, which the SOT-89 package can provide by soldering to the groundplane. However, this means that source feedback is not applicable, even though it could provide the best matching and stability factor.
The S parameters of the chosen FET between 50
Ohm ports can be seen in Figure 2. The input and output matching is
clearly not adequate, especially the input return loss (S11). Figure 3
shows the stability factor, K. When K is greater than unity, the circuit will be
unconditionally stable for any combinations of source and load impedance. It can
be stated that in the examined frequency range the ATF-53189 cannot be
used directly between 50 Ohm ports. The source inductance of the SOT-89
package is so low that no improvement can be obtained by that method.
Voltage feedback from drain to gate (Figure 4) is another way to improve
the K stability factor, and this type of feedback need not cause a significant
increase in noise figure. The gain of the FET at 145 MHz is approximately 29 dB,
which proves to be too high. The amplification can be reduced to 23 dB with a 6
dB attenuator at the output of the preamp, and this attenuator further increases
the K stability factor. It can be changed to 10dB if needed.
The detailed circuit diagram can be found in
Figure 5.
A band pass
filter was added at the output, which improves the out-of-band selectivity. The
FM broadcast rejection is better than -60 dB at 100 MHz. The insertion loss of
the filter further reduces the unnecessarily high gain to 20 dB.
The simulated S parameters of the preamplifier without the band pass filter can
be seen in Figure 6. Figure 7
shows the stability factor and Figure 8 shows the noise figure.
The losses of the input matching circuit are
estimated values, so some differences are possible in the practical
measurements. The input circuit consists of a ‘T’ match with suitable low loss
microwave capacitors and spiral inductor. The K1 security relay needs to be
built in if TX power is higher than 200 W. If you connect ground potential onto
the PTT connector, the K1 relay will short-circuit the inputs, and this
increases the isolation from the TX power by 28 dB. The circuit layout of the
built prototype can be seen in Figure 9.
Construction
The professionally made PCBs are 1.5mm, FR4, double sided with plated through holes. The boards are solder resist coated and silk screened to show component designations, i.e. R1, C1 etc. I recommend soldering the SMD components with a hot air soldering tool (Figure 10). The bottom side of the PCB must be soldered to the walls of the box at several places: IC1, the spiral inductor, the band pass filters and the two BNC sockets.
Adjustments
Connect +12…15 V to the amplifier (observe DC polarity!) and adjust the potentiometer for the needed 135 mA drain current (135 mV can be measured between the TP1 and TP2 test points).Then tune the L3 L4 TOKO inductors to the maximum signal from a beacon. For minimum noise figure, the CT capacitor at the input should be set to a position 20 degrees from maximum as shown in Figure 11. The preamplifier does not need further adjustments.
Measurements
The wide-band selectivity of the preamplifier can be seen in Figure 12, and the detailed results are summarised in Table 2. You can see the complete list of figures of the measurement diagrams below.
Fig. 12 Wide band selectivity |
Fig. 13 1dB bandwidth |
Fig. 14 3dB bandwidth |
Frequency [MHz] |
Rejection data [dB] |
51 |
-72,7 |
100 |
-71-3 |
145 |
0 |
432 |
61,95 |
Table 2 Reference frequency: 145 MHz G=20,10 dB
|
![]() |
Fig. 19 First prototype TOI -Id=100 mA (Tnx HA3KZ!) | Fig. 20 Noise figure of first prototype, G=16 dB (Tnx HA3KZ!) |
![]() |
![]() |
Fig. 21 Output signal with open input, without the top of the box (local signals are visible) | Fig. 22 Output signal with open input (between 100 MHz and 6 GHz), box covered |
Results
Several contest stations used the recently built 20 prototypes with good results during the IARU VHF contest in 2010. I have stocked components for 100 preamplifiers for HAMs who wish to build or use this EXTRA-2 Contest Preamplifier.
The Please KIT or the completed and tested EXTRA-2 144 MHz Contest Preamplifier can be ordered.
Please contact me!
I wish great success for all users!
Literature
Special thanks to GM3SEK, Ian for reviewing the English text!
Other photos
Copyright HA8ET 2010-11