Bredhurst Receiving and Transmitting Society

Part 3

4e.3 Recall that a filter is a device that blocks some frequencies and passes others.

Understand the effects of low pass, band pass and high pass filters. Interpret their frequency/amplitude diagrams.

Understand that a low pass filter and a band pass filter can minimise the radiation of harmonics.

Transmitters can produce unwanted signals even when care has been taken with their design. If these unwanted signals are radiated strongly enough to cause interference to RF users one or more filters is needed to eliminate or at least reduce the level of the unwanted signals. The filter needs to be designed to pass as much of the wanted signals as possible.

There are three filter you need to be familiar with for this course:-

• Low Pass

• High Pass

• Band Pass

for the purpose of this course the filters are constructed from inductors  (L) and capacitors (C) and hence are referred to as LC filters. The filter is in fact a "tuned circuit"

Filters can be described as passing some frequencies and stopping other hence the expressions "pass band" and "stop band". In a perfect world the filter would:-

• pass unhindered all frequencies within the pass band but in fact they are reduced slightly and

• stop all frequencies within the stop band but in fact they are only much reduced.

The frequency where the pass band and stop band meet up is called the "cut-off frequency" (fc) of the filter.

A low pass filter (LPF) passes frequencies lower than the cut off frequency

A low pass filter (LPF) is designed so that it passes frequencies that are lower than that of the stop band so in effect signals above the cut-off frequency (fc) are reduced. The Low Pass Filter is therefore use with a radio transmitter to reduce as far as possible harmonics of the transmitted frequency.

In the next animation a plot of the harmonics is overlayed the output frequency to show how the fundamental frequency is hardly affected by the low pass filter but the harmonics 2 through to 5 show significant reductions in output level (amplitude).

You will be pleased to learn that many VHF and UHF transceivers have a low pass filter built in.

A high pass filter (HPF) passes frequencies higher than the cut off frequency

A high pass filter (HPF) is designed so that it passes frequencies that are higher than that of the stop band so in effect signals below the cut-off frequency (fc) are reduced.

A band pass filter (BPF) passes frequencies between the cut off frequencies

A band pass filter (BPF) is a combination of low pass and high pass filter that will passes a range of frequencies in the pass band, any frequencies above or below this range are reduced. Unlike the LPF and the HPF, the BPF has two stop bands and two cut-off frequencies (fc) at the meeting points of each of the stop and pass band.

Remembering which circuit diagram refers to which can be a daunting task for some students. So try to remember just one and the other should then fall into place.

Think of the inductors at the side of the filter as springs on a lifting bridge that can operate to allow a high bus to go through the HIGH PASS filter.

Then look at the low pass filter and the capacitors could be like bricks of a low bridge that cannot stretch and so will not allow the bus through.

Note: That the use of filters on all external leads from a transmitter can on occasions reduce harmonics from a transmitter.

4e.4 Recall and understand that too fast a rise and fall time of the transmitted RF envelope of a CW transmitter may cause excessive bandwidth and that this can be minimised by suitable filters in the keying stage.

When a morse key is opened and closed to operate a CW Tx too fast a rise and fall time of the transmitted RF envelope of a CW transmitter may cause excessive bandwidth a problem known as key clicks. The clicks are a large number of audio frequencies caused by the sharp edge of the wave form shown below in RED.

Key clicks can be minimised by suitable filters in the keying stage.

Note : In the red diagram the very sharp corners to the shape which cause key clicks whilst in the blue diagram the corners have been rubbed off giving slower rise and fall times and reduces / eliminates key clicks.

The animation merely show the change from the original to the improved wave shape.