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MAR6/MSA0685 wideband gain block as masthead amplifier

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Add references to J. J. Carr's Elektor and K. Wagner's Oz articles - Sep. 2012.


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 MAR6/MSA06 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, its stability and noise performance have been questioned in constructors' circles [5-6].

This article documents the implemention and the testing of a MAR6/MSA06-based amplifier which can be supplied from the coax feeder (phantom power). Its intent to address the concerns highlighted above.

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.


To realize a phantom-powered masthead pre-amplifier, the circuit below was assembled on the PCB shown below. A 390R resistor is used to drop the 12V phantom supply voltage to the device's specified 3.5V operating voltage. The DC blocking capacitors are dimensioned to support operation down to 3MHz (80m).  The PCB is 0.8mm thick FR4. The microstrip width is sized to provide 50R Zo. The ground pads have three viaholes each. 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 the BNC connectors.


The amplifier's output is connected to a Minicircuits ZFBT-6G biastee for inserting the 12V supply. The current drawn is ~21mA. 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 amplifier has a 3dB gain bandwidth of ~780MHz. Over this bandwidth, the noise figure measures below 3.2dB and the gain above 17.5dB. This noise performance is comparable to the newer ERA-series MMIC. The noise figure's spikes at 200MHz and 960MHz are due to external interference. 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.


Input and output return losses are better than -21dB and -13dB, respectively in the 3dB gain bandwidth (3-780MHz). The gain is greater than 15dB up to 1GHz.


The evaluated prototype is unconditionally stable over 10MHz to 6GHz range. 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. 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].

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#HF_VHFPreAmp
[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.

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