Blog posts of '2014' 'May'


Well, I had a query from Vicky today who was interested in buying a watch from us but  was concerned that the dial might be radium luminous.  Most pre war watches are of course radium lumed however, post war, we moved towards dials using the 3H isotope of Hydrogen, Tritium, to give luminosity through a Jablonski decade emission, today, on watch dials we use a proprietary compound known as Luminova which is a strontium aluminate photoluminescent compound invented in the early 1980s.

Anyhow, Vicky, just look at how well rated radium was in the 1920s and the health benefits so claimed - simply super stuff: -



As early as 1952, Mullard top brass were considering how best to promote their product range with the expounding of an idea which became all too prominent some 10 - 15 years later.      The idea was of course a calendar and not just any calendar but a glamour calendar, much board room discussion was made of the merits of Roedean-esqe girls showing cheeky smiles, a shapely ankle or even a svelte swimsuit with a plunging back line but the eventual conclusion was that such an approach might sully the wholesome Mullard image - the phrase 'sex sells' wasn't yet coined by Madison Avenue though I am sure Don Draper and Roger Sterling must have thought of it!!!!!!

Instead, a compromise was reached, the calendar format was used with a rather fetching presentation of Sid the Serviceman on the front cover and handy valve equivalents and operating data inside for common valve types.  Here we see the Mullard Top Brass at the Mullard Dealer event in 1954 perusing an advance copy of the wall chart with the chap on the right, Mr D M Hall, Manager of the Mullard Valve Sales Department appreciatively commenting, sotto voce, "I say, phwoarrr, look at the getter bloom on that!"

Today, these wall charts do occasionally turn up with their page edges crinkled and worn, a testimony to their use as an aide-memoir in some long gone radio service workshop.



I had a call from Chris who was asking how his Leak Troughline worked so I decided that maybe it was time for a quick blog article on FM.  Frequency Modulation or FM is a system of radio transmission where the carrier amplitude is kept constant whilst the frequency varies about the nominal value.  

The frequency variation has to convey two pieces of information: - 

i) Audio frequency which is the number of times per second the carrier frequency oscillates or wobbles either side of it's nominal value.

ii) Audio amplitude which is the amount the transmitted frequency shifts above and below the carrier frequency fc and is known as the frequency swing fs where fs is proportional to the amplitude of the original sound and has a maximal value of 75KHz

The simplest method of recovering the audio frequency oscillation is shown below: - 

Above you can see that an RF circuit has been set 'off-tune' to the carrier frequency such that the oscillations correspond to the straight part of one half of the response curve.  As the frequency of the FM signal oscillates from fc-fs to fc-fs and back again the response changes from point A to B and thence back to A.  This completes one cycle at audio frequency.

The amplitude of the AF component in the output depends on the magnitude of fs and also on the slope of the response curve. The repetition periodicity of the waveform (= 1/frequency) is the time taken for the signal to swing fc-fs to fc+fs and back again.

To give a worked example consider a carrier frequency of 90 MHz being modulated by an AF signal of 1KHz showing a frequency oscillation of 50KHz - this will show a signal oscillalation of 1000Hz between the limits of 90.05MHz to 89.95MHz and will reach these maxima 1000 times per second.

The bandwidth required for FM transmissions is much larger than that needed for AM transmissions.  If the modulating (audio) frequency is denoted by fm then sidebands are formed at fc+/-fm dependant upon the modulation index fs/fm.  Theoretically, there will exist an infinate sequence of sidebands but the range can be reduced by operating with minimal frequency oscillations, the maximal allowable of which is denoted the frequency deviation fd.   In broadcasting, the magnitude of fd is typically several times greater than the highest transmitted audio frequency.  fd can be characterised further by the deviation ratio a value of 5 or higher being preferred where the frequency deviation of 75KHz results from a maximal modulating frequency of 15KHz.

Consequently, medium waves are unsuitable for FM transmissions as the output would be ruined by phase selective fading,   conversely at very high frequencies (VHF) fading and selectivity concerns are much reduced as tropospheric bounce has little effect as only the ground wave reaches the receiver.