MAR6 / MSA-0685 wideband gain block as masthead amplifier
[For a discrete BJT gain block, go to Wideband pre-amplifier using discrete transistor ]
Created: July 2012. Modified: Apr. 2019.
Introduction
Masthead pre-amplifiers are indicated when the receiving aerial is small, hence, inefficient or when increasing cable loss at higher frequencies degrades system sensitivity. Although the application of Mini-circuits MAR6 / Avago MSA-0685 as a pre-amplifier for wideband reception has been extensively described in hobbyist publications [1-3] and personal websites [4], the emphasis has been on the construction. Few authors have presented measured results. Due to the dearth of quantitative data, this device's stability and noise performances are frequently questioned in constructors' circles [5-6].
This post documents the implementation and the testing of a MAR6 / MSA-0685 amplifier which can be supplied from the coax feeder (phantom power). This post also intends to address the concerns mentioned in the previous paragraph.
Materials & methods
This section first describes on-chip circuit, then the external circuit and finally, the fabrication.
The MAR/MSA MMIC series is based on a Darlington pair with resistive feedback. The input and output mismatch are relatively stable over frequency because they are set primarily by resistive divider networks. A combination of series (RE) and shunt (RF) feedback desensitizes against variations in active device parameters.
Fig. 1 MMIC's internal circuit
Phantom-powering conveniently allows the masthead pre-amp to be supplied via the coaxial cable connected to the RF output. The application circuit is similar to the datasheet's reference circuit with the exception of the bias resistor's connection. The MAR6 / MSA-0685 nominal device voltage (Vd) is 3.5V. To enable the phantom-power function, the bias resistor is connected in parallel with the output DC blocking cap. The resistor value is dimensioned to drop the 12V phantom supply voltage to Vd. The resultant current draw is ~21mA, which is higher than the nominal 16 mA (fig. 2). The higher operating current is expected to beneficially increase the 1dB gain compression (P1dB) by 2~3 dB.
Fig. 2 Selection of operating current to optimize gain compression (P1dB)
The DC blocking capacitors (10 nF) are dimensioned to support operation down to 3 MHz; i.e. shortwave 80m band (fig. 3). The 390R resistor is soldered to the top of the output blocking cap. All chip components are 0805 size.
Fig.3 Amplifier circuit
The PCB is 0.8 mm thick FR4 (fig. 4). The microstrip width is sized to provide 50R Zo. The ground pads on both sides of the MMIC have three viaholes each.
Fig. 4 PCB
To house the assembly and to provide some measure of screening, a 40x30x23mm enclosure is fabricated from 1.6mm thick single sided PCB scraps - this type of enclosure is popularized by the ARRL Handbook for the step attenuator project ("Low power step attenuator," ARRL Handbook 1992, ch. 25). The side panels are soldered together at the edges, while the top and bottom panels are held by 25x3mm machine screws. RF connections are made through two BNC female connectors.
The assembled PCB is directly soldered to the BNC connectors (fig. 5).
Fig. 5: Assembled amplifier inside case without lids
Fully boxed up
The amplifier's output is connected to a Mini-circuits ZFBT-6G biastee for inserting the 12V supply. The biastee's influence on the test results is removed by calibration. We did not perform nonlinear tests, such as OIP3 and P1dB, because they are not expected to differ from datasheet's specifications.
Results & discussion
The prototype achieved a 3dB gain bandwidth of ~780MHz (fig. 6). This result is slightly lower than the datasheet's 800 MHz; the PCB is likely responsible for the slightly degraded high frequency roll-off. Over this bandwidth, the noise figure measures below 3.1 dB and the gain above 17.5dB. This noise performance is comparable to the newer ERA-series MMIC.
The noise figure spikes at 200MHz and 960MHz are likely due to external Radio Frequency Interference (RFI). The presence of these artifacts indicate the PCB-type enclosure is not as RF-tight as anticipated; apparently, fine gaps between the lid and the side panels form a slot aperture that degrades the shielding. Wrapping copper foils over the sides' edges so that it forms an overlap joint with the lid may improve the shielding, although this has not been verified.
Fig. 6 Measured gain & noise figure (NF) vs. frequency
The amplifier has good input and output match over its usable bandwidth. The input and output return losses are better than -21dB and -13dB, respectively in the 3dB gain bandwidth of 3-780 MHz (fig. 7). The gain is greater than 15dB up to 1GHz.
Fig. 7 Measured gain, input and output match vs. frequency. The prototype achieves good matching and gain over 3-780 MHz
The evaluated prototype is unconditionally stable over 10MHz to 6GHz range (fig. 8). The MAR6/MSA-06 has a 10GHz fT and so, ideally, its stability should be evaluated to to this frequency. However, this was not done due to equipment limitation. Although the manufacturer's data indicates potential instability at the device-level, i.e. Rollett stability factor, k<1 over 1.0-1.5GHz, the prototype satisfies the stability criteria k>1 and B>0 over 10-6000MHz. The reason behind this difference could be different substrate materials; the manufacturer characterizes the device using low loss ceramic PCB (ICM fixture) whereas the actual prototype is constructed with lossy FR4 PCB which may have fortuitously stabilized it. Another possible reason for the better than expected stability is that the 390R bias resistor acts as parallel resistive stabilization [7].
Fig. 8 The Rollett stability criteria point to the prototype's unconditional stability
Conclusion
Simple masthead pre-amplifiers for HF-UHF can be easily realized using the MAR6/MSA06 device. Unconditional stability can be achieved by minimizing ground inductances. We anticipate that the results shown here will encourage constructors to use the device without fearing for instability or poor noise figure.
References
[1] A. Ward, "VHF and microwave applications of monolithic microwave integrated circuits," in The ARRL UHF/microwave experimenter's manual, Newington, CT: ARRL, 2000, ch. 7.
[2] K. Wagner, "Bredbåndsforstærker: En MAR, en Monotolithic Amplifier," [Online] Available: http://techdoc.kvindesland.no/radio/audiforradio/bredfandsforsterker.pdf
[3] J. J. Carr, "Using the Mar-X series of very wideband monolithic microwave integrated circuits (MMIC)," [Online] Available: http://techdoc.kvindesland.no/radio/audiforradio/marex_mmic.pdf
[4] ON6MU, "RE-HFA1MAR6 Wideband VHF/UHF/SHF monolithic PreAmp based on MARx-series" [Online] Available: http://users.belgacom.net/hamradio/schemas/preamp_HF_VHF_UHF_SHF_wideband_MAR6.htm
[5] Reddit discussion list, "Wideband Preamp - LNA for the rtlsdr. Schematics inside. RTLSDR" [Online] Available:
http://www.reddit.com/r/RTLSDR/comments/vekxf/wideband_preamp_lna_for_the_rtlsdr_schematics/
[6] Reddit discussion list, "We should get serious about making the direct conversion receiver conversions work great RTLSDR" [Online] Available: http://www.reddit.com/r/RTLSDR/comments/vq5q3/we_should_get_serious_about_making_the_direct/
[7] L. Besser, "Avoiding RF oscillation," Applied Microwave & RF, Spring 1995.
(c) July 2012. This work is protected by copyright and may not be reproduced in any form without the expressed consent of the owner.