The blog of a dedicated radio amateur and electronics enthusiast

"Having fun on the air and in the workshop - communicating and creating"

11 April 2011

BF991 receiver RF preamplifier completed

While waiting for the boardhouse to make the printed circuit boards for my frequency synthesiser, ( see post on Mar 8, 2011 ), I reckoned I should have just enough time at long last to build the low noise 144MHz receiver RF preamplifier which I designed months ago, ( see post on June 16, 2010 for circuit schematic ).  For this piece of equipment I again made the pcb; the artwork for it is shown, ( actual size 4.2" x 2.2" ).
I built the circuit on single-sided copper laminated fibre-glass board, all components being mounted on the same side as the tracks and copper ground-plane; in effect surface mount technology even though only the BF991 mosfet, in a SOT143 package, is actually a surface mount device. A piece of double-sided board material was soldered vertically across the circuit board to act as a screen to prevent coupling between the input and output tuned circuits. The preamplifier is housed in a Hammond "Eddystone line" type 27134PSLA die-cast metal enclosure, 111x60x31mm.
Being somewhat larger than a matchbox, it's not the tinyest rf preamplifier that I have ever seen. Small size is not an issue for me, as it will not be installed internally in any other equipment but used outdoors close to the antenna. Besides that, the heavy-duty change-over relays that will be necessary to divert the transmitted signal around the preamplifier will add significantly to the overall size anyway.
Achieving a low noise figure for the preamplifier is strongly dependent on the C-C-L input bandpass filter and matching circuit which should use low loss, ( i.e., high 'Q' ), components. I easily obtained some suitable ceramic trimmer capacitors for C1 and C2, but sadly no silver plated wire from which to make the coil L1 !

3 comments:

Admin said...

Hello Steve,
I was Googling for BF991 pre-amplifier designs, and I stumbled upon your website. I've been studying your design since last night, and I was wondering if your could post more extensive design notes and equations for a newbie?

I would like to change the operating frequency and/or gain of amplifier. Your design is similar to

http://fivedash.com/resource/6m4m2mRXSchematic.pdf

This SDR uses the same dual-gate MOSFET as your pre-amp. This is ultimately the design I want to modify.

Thanks for any help you can offer.

Sincerely,
Joel Crouch, KD4PPA

jc_crouchhotmailcom

Blog Author said...

Hi Joel, I can post the circuit diagram here but it's been a long time since I made the preamp and my calcs were probably scribbled down 'back of an envelope' style. Anyway if I come across anything I'll let you know. However I do remember that the designed preamp gain was chosen to achieve the target noise figure I wanted for my system and I used AppCAD s/w [noise calc] module to make my noise fig versus gain/loss simulations.

Blog Author said...

I've just searched the blog and discovered an earlier post on 16 June 2010, about the preamp, including the circuit diagram and some notes on NF. So please take a look.