As submitted to QEX/Quarterly Communications for Publication

 Published in May-June 2004 QEX

 

A RUGGEDIZED GENERAL PURPOSE 100KHz-2GHz LOW NOISE RF PREAMPLIFIER

by

Glen E. Gardner Jr.

AA8C

 

Introduction

Some time ago, I was experimenting with a home-brew VHF receiver for use with weak-signal over-the-horizon voice and satellite communications. Out of that project, came a number of small projects that I have since found to be very useful for my other amateur radio interests. One of these little bits is is a wideband preamplifier that I have found to be a good performer that is very inexpensive and easy to build.

 

The circuit is based on the use of the MAR-6 monolithic amplifier integrated circuit made by Minicircuits. It is inexpensive, readily available from a number of sources, and extremely easy to work with. The preamplifier features high gain, low noise, a regulated power supply for good isolation from external noises on the supply side, transformerless construction for good bandwidth, and reverse polarity protection on the power supply input.

Additionally, the low component count makes the project simple and easy to complete. The use of double-sided, copper-clad glass-epoxy board material for the enclosure assures low cost, light weight, and high mechanical strength while offering excellent RF integrity. A schematic, board templates, enclosure templates, and component list are available in electronic format.

 

The specifications for this preamp are taken to be those of the MAR-6 amplifier as stated by Minicircuits. It's needless to say that, the performance that the builder achieves depends a lot on construction practice, and individual variations in component tolerances. Even though the equipment to perform exhaustive testing of the preamplifier was not available to me, I have learned experimentally that the performance claims for the MAR-6 are reasonably close to truth and that the repeatability of the circuit is quite good. My experience with building and using several of these preamplifiers suggests that rigorous testing of the preamplifier ought to reveal these claims to be reasonably close to truth.

 

Theory of operation (please refer to the schematic)

An operating voltage, +V, ranging from about 9.5 to 15 VDC (13.8VDC nominal) is applied to the anode of diode D1.

Diode D1 provides reverse-polarity protection for U2 and C5. U2 is an AN77L07/ZR78L07/LM78L07 linear voltage regulator. The regulator features 200 mA current capacity with current limiting and thermal overload protection. It requires a minimum operating input voltage of 8.7Vdc and tolerates a maximum input of 20Vdc. The regulator is very quiet, with a nominal noise output of 75 microvolt rms, thus providing an extremely quiet power source for the mmic preamplifier.

Operating the preamplifier with input voltages that are near it's maximum rating can lead to elevated operating temperatures in the regulator, so the circuit is derated conservatively to operate with a reduced maximum +V input of 15Vdc. Operating the regulator near it's minimum input specification will reduce the regulator's noise immunity, potentially spoiling the noise floor of the preamplifier. For these reasons, a nominal input voltage of 13.8VDC was chosen to assure that the regulator operates at an acceptable temperature as well as providing low-noise output. U2 provides low noise, regulated +7VDC to R1 and also acts to isolate the mmic amplifier (U1) from the external power source. C3, C4, and C5 provide adequate DC transient filtering and RF decoupling to assure quiet operation of the voltage regulator.

R1 delivers power to U1 while acting to limit current and drop the supply voltage for U1 to acceptable levels for proper operation. U1 is a Minicircuits MAR-6 MMIC amplifier circuit. Normal operation requires 3.5V at 16 mA. According to Minicircuit literature, the value of the bias resistor (R1) is calculated as R1 = (Vcc-Vd)/Id. In this case, Vcc is 7VDC, Vd is 3.5V, and Id = 0.016 Ampere. Given these parameters, the ideal value is R1 = (7-3.5)/0.016 = 218.75 ohms. A slight increase in the nominal operating voltage is not detrimental, thus a value of 215 ohm was chosen from resistors in stock. A value of 215 ohm for R1 provides an operating voltage of 3.56 Volts at the rated 16 milliampere current.

Of particular interest is the lack of a series choke between the bias resistor (R1) and the amplifier output/bias pin. This arrangement leaves the output with a minor impedance mismatch. Given the tendency of most chokes and many RF transformers to have limited usable frequency ranges and unwanted resonances, the small loss due to impedance mismatch without a choke seems trivial. Minicircuits gives the loss in power gain due to the output impedance mismatch to be 20log[(2Rbias+50)/2Rbias]. In this case, 20log[430+50/430] = 0.95 dB. Thus the 1 dB performance loss is easily tolerated in favor of simplicity, lower component count, and good wideband performance.

C1 and C2 provide DC blocking for the input and output of the RF amplifier circuit to assure that external DC loads or current sources do not adversely affect the proper operation of the amplifier. The 50 ohm nominal input and output impedances of U1 are low enough that the reactance of C1 and C2 rolloff the low-end frequency response of the RF amplifier is conservatively rated at 100 KHz. For this application the 6dB cutoff point is taken as: Xc = 50 ohm, C = 0.1uF, and f = [(1/Xc)/(2 pi C)]*2 = [0.02/(6.28*1E-7)]*2 = 63.69 KHz.

 

Making the board and enclosure

The preamplifier circuit board was made photographically from single-sided, 1/16 inch, glass-epoxy circuit board stock.

Full size negative templates for the board are available electronically in 300 dpi resolution, for those wishing to photfabricate their own boards. It is strongly recommended that the preamplifier board be made as a single sided board. Using a double layer board with this design will likely degrade the noise figure and ruin the frequency response of the amplifier.

The original prototype enclosure was cut from surplus double sided glass epoxy board stock without the benefit of templates and etching. For those wishing to have a nicer finished appearance, 300 DPI templates for the enclosure panels have been made. The use of good quality double sided board stock for the enclosure is important to assure adequate mechanical strength. Full size negative templates for the enclosure sides and enclosure top and bottom are also available in electronic format.

 

Construction Practices

The semiconductors used in this project are easily damaged by electrostatic discharge, so be sure to observe proper ESD precautions during construction and testing of the preamplifier. The components are very small, and a little precision is required to get everything to fit together as it should. Some magnification will help. The use of a temperature controlled esd-rated soldering iron with suitably small tips is recommended. A pair of nonmagnetic tweezers is useful for handling and holding small parts. If you are new to surface mount technology, you might find that using a tiny amount of paste flux to adhere the parts to the board prior to soldering helps reduce the tendency of the components to skate around and "tombstone" when soldering. For surface mount resistors and capacitors, tinning one pad first, then holding the opposite end of the component with nonmagnetic tweezers while soldering it in place is a preferred method of avoiding these problems. Newcomers to smt might wish to practice with some scrap pcb stock and spare parts. Once you get the hang of it, SMT is easy to work with, but learning to solder components that are just a couple of millimeters wide can be intimidating at first.

 

 

Building the preamplifier board

Manufacture the preamplifier circuit board and cut out the clearance notches for the BNC jacks at both ends of the board. Test fit the end panels against the ends of the board by temporarily installing the jacks on the end panels. Work slowly, this is a cut-and-try operation. Carefully trim excess board material from the circuit board cutouts (the large end areas with no foils). Rough cut with a dremel tool or a nibbling tool. Fine trimming can be accomplished with a small file. Some BNC jacks may be a little longer than others and will require that the center pin be trimmed in order to avoid clearance issues with the 0.1 uF coupling capacitors at C1 and C2. Also, the drop-in package of the MAR-6 requires that a 5/64" hole be drilled at the mounting point so that the amplifier can be mounted flush with the surface of the board. The surface mount version of the MAR-6 does not require a mounting hole.

 

Install the MAR-6 amplifier first, following the recommended layout. You may need to trim the leads of the MAR-6 in order to get proper clearance for the pads at C1 and C2. Next, install the resistor and the capacitors, being careful to allow enough pad space for the tips of the chassis mounted BNC jacks near C1 and C2. Install the voltage regulator (U2) last. Be careful to install the voltage regulator close enough to the surface of the board so that the top cover can be installed when the enclosure is assembled. To prepare the regulator IC for installation, cut the leads to an appropriate length, then bend them to form small "feet" that can be soldered to the circuit board pads. Be sure the regulator's package will not interfere with the placement of the feedtrough capacitor and diode D1 when they are installed later. Don't install the BNC jacks, enclosure panels, feedthrough capacitor, or the diode (D1) until the board is fully stuffed and the enclosure is ready for assembly.

 

 

Building the enclosure

The enclosure for the prototype preamplifier was cut from surplus, double sided, 1/16" G-10 glass-epoxy circuit board stock using a small sheet metal shear. FR-4 glass-epoxy laminate is equally suitable. Phoenolic resin board stock proved to be too brittle to be usable for the enclosure. Two-ounce copper board stock was used on the enclosure panels and cover plates but 1 ounce per square foot copper clad material is likely to be equally suitable because the peel strength for copper-clad board material is about the same regardless of the copper thickness. Use good quality material for the enclosure and strength won't be a problem.

 

 

Final Assembly

Install the BNC jacks on the end panels prior to assembling the enclosure around the completed board. Test fit the end panels to the circuit board and carefully trim the preamp circuit board cutouts to allow proper clearance for the BNC jacks per the instructions above. Be sure that the circuit board is centered properly and sits flush against the end panels. Also, be sure the center pins of the BNC jacks are in contact with the oversized foil pads provided on the circuit board and do not interfere with C1 and C2 as discussed earlier. When done, square up the end panels with respect to the board and solder the seams where the boards meet the end panels. You can tack spots temporarily to hold things together to make the soldering easier, but be sure to establish a good solder connection at every seam where one copper panel meets another. Likewise, install the side panels and solder them in place. Also, be sure that the inside and outside corners where the side panels meet the end panels are also soldered. Fillet all seams and don't leave unnecessary gaps. If copper meets copper, solder it. This is important to both good low noise operation, and good mechanical strength.

In order to reduce mechanical stress on the connections at the BNC jacks, do not solder the center pins of the BNC jacks to the board until the preamplifier board and enclosure panels have been completely assembled.

When done, install a feedthrough capacitor or a through-hole filter in the hole provided for power in the side panel and install D1 being careful to allow sufficient lead length for stress relief (fiberglass enclosures flex a lot), now solder the center pins of the BNC jacks to the board and inspect all solder connections. The preamplifier is now ready for testing. If the test is a success give yourself a pat on the back and complete the enclosure assembly.

After inspection and testing is complete, clean the board as-needed and install the top and bottom covers then solder all external seams carefully. For those who want to be able to get the cover off easily, I suggest tack soldering the corners of the top cover for easy removal in the event that servicing is needed at a later date.

 

 

Caveats

Very strong signals (greater than 13 dBm) will damage the MAR-6 amplifier. The addition of a pair of front-to-back small signal diodes on the input of the amplifier to limit input amplitude ought to remedy this issue at some cost to performance at VHF frequencies and above. For those needing good performance at VHF and above I would suggest arriving at some other means of resolving this issue.

Don't apply power without first properly terminating the input and output of the amplifier. The author has used these preamplifiers hard and long and has not experienced any troubles with instability, but the MAR-6 is not guaranteed to be unconditionally stable. Connecting/disconnecting the input or output of this preamplifier when the power is applied could lead to instability and sudden failure of the amplifier. The same precautions should be taken with any high performance preamplifier, because many high gain preamplifiers are not unconditionally stable and may oscillate destructively if the input impedance is badly mismatched. Be sure you power down the preamp before connecting/disconnecting and you will avoid this potential issue completely.

 

73

Glen Gardner

AA8C

 

 

 

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