Fig, 1. Schematic diagram of the preamplifier. Numbered components are identified below. Fixed-value resistors are carbon composition, 1/4 W. Fixed-value capacitors are disc ceramic, 50 WV. Numbers in parentheses identify Radio Shack numbers. C7 through C6-5-60-pF miniature trimmer (272-1340) Q1 -MRF901 (276-2044) Q2—2N3819 (276-2035)

., _._ ... L2, L4,15, L7 — 5 turns no. 14 solid wire, 3/8 inch dia., 7/16 inch long (formed on 1/4-inch-dia.


L3, ¿,6 — 3 turns no. 14 solid wire, 5/16 inch ma,, 1/4 inch long (formed on 3/16-inch-dia. rod) L1 — 2 turns no. 24 p/asi/c-/nsu/afed stranded hookup wire formed tightly over the end of L2 L8 — 1 turn no. 24 plastic-insulated stranded hookup wire formed tightly over the end of L7 RFC—approximate!y 30 turns no. 30 wire on 1/2-W, 1k resistor (two layers)

ter is such that it can be used with any type receiver, including the SSB/CW receiver. Both assemblies are simple do-it-yourself projects, All parts can be bought from Radio Shack.

Etched PC-board circuitry is avoided in the fabrication by using small PC pads glued to the primary PC mounting board with ordinary household cement. Components are soldered directly to the pad foil for interconnection and support. If a mistake is made, simply lift the pad with a knife and make the necessary change. The method is simple and effective; I found no disadvantages in its use for this particular application.

Preamp Circuit

Choosing the preamp active device becomes easy when you restrict your choice of parts to those which are readily available. The only one that is competitive from the low-noise consideration is the MRF901, Actually, it is not a bad compromise, as the typical noise figure is less than 1,6 at 2m. It has two disadvantages as compared to a good FET: One is that bipolar amplifiers are more subject to

overload, and the second is

' t that it is difficult to stabilize


Fig: 2. ¿ayouf details. A —PC mounting board, 4 X $-3/4 inch, Gj one side 0 — Paris-moun f/ng pads, 7/76-/ncb PC board, Cn erther one or both sides (preferably glass insulation). 1/4-inch-wide separate pads formed by filing T/f6-inch-wide grooves fo remove Cu. Clued to mounting board with household cement. Length in inches: B1-1 2, B2—1-14, 63-7-7,4, B4-1/2, BS-3/8, B6-1/2, B7-1-1/2, B8 -1-1/4, 69-7/2.

C—Ql emitter connections, So, 14 solid wire shaped as tight

U, 1/4 inch long, soldered to mounting boardI D— Power-supply connections. Wires connected through holes in the mounting board, reamed on the foil side to prevent shorting of the wire to the PC foil. E — Cable clamps F-RG-58/U input-output cables

C — PC-board mounting holes. Require insulated shoulder washers. H —Shield mounting holes when used with narrowband circuitry.

The hybrid cascode circuit shown in Fig. 1 optimizes the MRF901 performance while addressing these potential problems. The cascode 2N3819 FET common-gate second stage accommodates most of the voltage swing, improving the bipolar overload performance Also, its broadband low-impedance load to the MRF901 reduces the instability tendencies. Stability is further encouraged by connecting the MRF901 emitter leads and all other components referenced to common directly to the PC foil The one disadvantage of this method is that it becomes necessary to isolate the foil from dc ground when mounting the assembly. The input-output inductive coupling provides the required cabling isolation- Operating current is established mainly by the MRF9Q1 bias voltage. The bias voltage has been set to make a collector current of 7 mA, the maximum current allowable that will maintain an optimum noise figure.

The input filter, while not a particularly good filter when the preamp is trimmed for optimum signal to noise, provider some overload protection to the MRF901. The output filter is a narrowband T network made up of tuned circuits L5-C4 and L7-C6, adjusted for critical coupling by C5. The T-filter fow-pass characteristic is discouraged by adding the high-pass inductor, L6; its inductance value is established to not after the critical coupling value of C5.

0 0

Post a comment