Tuesday, 28 August 2012


Components, pcb, schematic, board layout and parts list laid out.

The pcb: already printed, acid etched and drilled.

Removing coating.

Pcb ready for assembly.

Small components first (usually resistors).

I like to solder one leg of each device and check none came loose and are riding high before soldering other leg.

Stage 1 complete! Continue fitting components in ascending size order.

Sandwich loose fitting components between two boards before flipping to solder.

When all finished check for shorts, bridges and splashes in the solder.

And for my efforts hersm here's a wideband rf amp (for an oscillator stage) ready for testing.

Friday, 3 February 2012

Mini FM transmitter (500mW - 1w) 88-108MHz VCO

Here is a little project i have been working on; A miniture FM transmitter (my favorite of all electronic fields RF!).

This transmitter puts out between 500mW and 1W (maybe more!). It runs off 12-15VDC and draws between 100 and 150mA.

Tuning is voltage controlled (using a variable capacitance diode (varicap) and a 10k 10 turn pot).

I've used 2 off MC120 type coils in series for the oscillator coil in a effort to minimise drift.

The borad was only a prototype and unfortunately i slightly misaligned the top copper layer which means it had to be cut by hand in place, still ,works ok. Because of it's prototype nature ground pins are soldered on both copper planes.

There is a single resistor and capacitor that are no longer required (i changed the design a little), there is also a small piece of ground plane i forgot to connect, and a low value ceramic capacitor i ahd to solder to the bottom of the PCB. There is a couple of surface mount components, one (the cap) i used through choice, the other (an inductor) was always designed to be SMA to save space.

All these errors have been corrected in the rev.2 PCB.

I used 4 off 220 ohm 1/2 watt resistors to make a 55 ohm dummyload (becasue i had a load of them laying around).

I may make kits available through ebay if there is any interest.

Sunday, 29 January 2012

Low Pass Filter (Chebyshev) for 88-108 FM Broadcast Band

Here is a little project i have been working on today, it is a low pass filter for the FM broadcast band (88-108 in the UK, although the filter as shown would be suitable for use in the extended (e.g. Japanese) band too.

A low pass filter is a very important piece of kit when using broadcast transmitters of anything over a few hundred milliwatts. NO PIECE OF TRANSMISSION EQUIPMENT WILL PASS TECHNICAL REQUIREMENTS WITHOUT PROPER FILTERING, AND A TRANSMITTER WITHOUT THIS WILL CAUSE INTERFERENCE TO OUT OF BAND SERVICES.

Filters do exactly that, they filter. In this case they filter frequencies above and beyond 108MHz (actually about 115MHZ to allow plenty of headroom). There are several different characteristics/design parameters for filters, the one shown here is a LOW PASS FILTER, meaning it passes frequencies lower that a given level, and attenuates frequencies above. Three important design considerations are the frequency which the filter begins to act, how hard it filters and pass band ripple (the amount of effect the filter has on frequencies within the pass band).

The design shown here is Chebyshev architecture, known for it's good cutoff and pass band characteristics. It is a 7 pole filter (meaning there are 7 different sections to the circuit).

I have included schematic as well as some of the software analyses of the filter itself. The inductors can be wound of wither enamelled copper, or tinned copper wire, if you have an LCR meter you can adjust the spacing to get the value exactly right, if not just wind them carefully and measure them as accurately as possible. Capacitors can be standard ceramic up to about 70 watts, anything over that i would recommend mica capacitors, or metal clad for really high powers (unlikely! If you need this much power you will need to recalculate the coils using thicker wire). I have included a Monte Carlo analysis which randomly adjust the component values within a given tolerance and shows the changes in filter characteristics (shown as red lines with the original design shown as a black line in the middle).

Input/Output characteristic impedance is 50 ohm, this is closest at the top and bottom of the band and worst in the middle (about 58 ohm), this is perfectly normal for a filter of this type and will not affect the output power too much.

M.A.S.T. Chebyshev Low Pass Filter for the 88-108MHz FM Broadcast Band, 0.1dB pass band ripple, 3rd Harmonic -60dB or better:

Tuesday, 17 January 2012

Current project: Audio compressor/limiter/processor optimized for FM braodcast

Here is a picture of the prototype of a limiter/compressor for FM broadcast.
This is the PCB (after etching, before tin plate).

And this is the finished unit (prior to testing). In/out audio connections to the left. It has pots to adjust various timing/level parameters.

I will post results of testing shortly.


This is where i will post details of any (but maybe not ALL) electronics projects i am currently working on (there are a few of them!!).

For some i may post scematics/PCBs, but again not all.