Intermediate Frequency Bandwidth Reduction for TV DX Video Signals


Introduction

DX TV enthusiasts often have to deal with relatively crowded VHF bands. In these instances, reduced TV receiver bandwidth results in greater selectivity. The other advantage of bandwidth reduction is improved signal to noise ratio. This results in less noise and distortion of the DX TV picture. Bandwidth reduction is especially useful for weak signal modes, such as meteor scatter. Although bandwidth reduction is equally beneficial for TV and FM audio, this article concentrates on TV video.

Technical

The IF (intermediate frequency amplifiers) strip accepts the ~35 MHz IF output from the VHF (or UHF) tuner and amplifies the required signal to approximately one volt at the detector prior to feeding the video amplifiers(s). It is within this section that the main IF response is determined; the VHF tuner consists of effectively wide band circuits.

In passing, some earlier UK dual standard TV receivers utilised the VHF tuner in whole or part to act as an IF preamplifier stage when operating at UHF. If a valve receiver is used then 3 vision IF stages are ideal and if possible 4 stages in the case of a solid state receiver fitted with discrete devises - though with present day IC technology distributed in the mass produced TV receiver an additional IF gain/shaping stage could possibly be fitted prior to the main IF strip input - see later. This number of stages if necessary to obtain the required gain/bandwidth performance; bearing in mind the transmitted signal information may have a bandwidth up to 8 MHz.

Although weak signals produce problems undoubtedly an equal problem is interference from high power transmitters, thus effectively limiting the use of high gain aerial amplifiers due to various forms of adjacent and co-channel interference. One way of surmounting such a problem is to include various filters and traps within the IF strip, to remove the offending interference from adjacent channels and frequencies. Generally most receivers will have insufficient filtering and additional traps can be added to improve the adjacent channel rejection. If one particular transmitter causes a problem, an aerial notch filter can be fitted, but it should be borne in mind that during periods of enhanced reception, strong signals are likely to appear on otherwise unoccupied channels - producing further problems. Consequently the IF strip should incorporate all the filtering and traps necessary to give good adjacent rejection either side of any channel in use.

It had become practise within the United Kingdom to make use of suitably modified System A receivers, with their narrower bandwidth video IF strip (3 MHz bandwidth). Such a narrow IF bandwidth results in an increased gain with considerably lower noise figure, allowing reception of extremely weak signals, which on a System B receiver (5 MHz video bandwidth) would be marred by noise. This does tend to show a lack of HF detail if the signal becomes extremely strong but the loss is well worthwhile with marginal signals. An additional advantage is that with the multiplicity of transmission standards within Europe and closely adjacent channel allocations, a narrow IF bandwidth receiver is able to tune to each signal, whereas with a system B receiver, adjacent channels tend to float over each other (an example being chE2/R1). The advantage of a narrow bandwidth is only too obvious during a prolonged Sporadic E opening! Where possible the provision of adding wide/narrow bandwidth switching within the IF strip should be investigated. With a narrow video IF bandwidth the sound channel of System B and other similar standards using intercarrier sound will of course be lost.

Printed circuit IF coils are now common, with little or no means of adjustment, and the fitting of multi-purpose integrated circuits in IF strips had dispensed with many of the tuned circuits once associated with this part of the TV receiver. IF shaping in this latter type of receiver is usually accomplished with an IF preamplifier stage fitted between the tuner' s IF output and the input to the main IF strip. Such a stage may comprise one or two transistors and several tuned circuits, if these latter circuits are variable then it is usually possible to re-shape the IF response to a sharper curve in the interests of greater selectivity. It may be advisable to fit a second IF preamplifier stage and adjust the latter rather than disturb the receiver's main response, then either the first stage (wide selectivity) or second stage (narrow selectivity) may be switched into circuit. Such a preamplifier stage may be fitted into a receiver having printed circuit IF coils, thus improving both the IF selectivity and overall gain figures. With the increasing use of Surface Acoustic Wave Filters (SAWF) in modern IF receivers, adjustment of the receiver curve will be extremely difficult and in such receivers it may be possible to construct an IF preamplifer stage and obtain the required selectivity performance.

In modern receivers IF circuitry may be contained in one or more discrete modules and in certain of the Philips range these are designated as an IF selectivity unit and an IF gain board. The former contains several tuneable coils and a single transistor stage whereas the latter features the bulk of the IF gain and with minimal coil adjustment. Since the IF coil selectivity unit adjusts the overall shape of the IF bandpass, it follows that the fitting of this module into any receiver of similar IF will enable modification of the IF bandpass by careful adjustment of the module's coils. It is possible to fit a module of this type in series with the IF input feed from the receiver's tuner to the main IF strip, and peak up the module to provide a restricted bandpass and hence improved selectivity. In certain circumstances, two such modules have been inserted in series to give sharp selectivity. It is then possible to either insert into circuit for narrow selectivity or to by-pass for wide selectivity with a simple slide switch or pin diode switching. The Philips G8/U800 selectivity module may be still available from TV spares dealers.

U800 G8 Selectivity Module Schematic.

Philips U800 G8 selectivity module.

FET selectivity module designed by Paul Barton.

BF196 selectivity module designed by paul Barton.

The reduced IF bandwidth does give considerable help with reception of marginal TV signals, the trade-off for an improved signal to noise ratio figure against that of a reduced video bandwidth. Experience has shown that a signal not visible on a receiver using an IF bandwidth of 5 MHz, can be resolved using a receiver with its bandwidth restricted to 2.5 MHz.

Reduced vision IF bandwidth operation will lead to a loss of intercarrier TV audio. If a scanner or FM radio is available, it may be used as a tunable IF to cover the various TV sound carriers. Alternatively, a splitter can be used to direct one output to the TV tuner, the other output going to a VHF scanner for DXTV audio monitoring.

For many years the writer used two Philips U800 G8 selectivity modules, connected in series, for vision bandwidth reduction and IF notch filtering. The U800 module contains four IF shaping L/C tuned circuits, and can be used to notch unwanted adjacent video carriers. For example, my local 64.25 MHz ch2 video often spreads down to 62 MHz. By using two U800 modules in series, it is possible to completely remove all traces of ch2 video on 62.25 MHz NZch3 video. The four tuned circuits provide a high Q at 36 MHz IF, hence adjustment is very sharp.

I currently use a D100 varicap TV tuner/RF convertor. The D100 uses a Toshiba EG522F MOSFET VHF/UHF varicap tuner, covering 40-860 MHz. The D100 provides switchable IF bandwidths between 6 MHz (wide) 4 MHz (narrow) 2 MHz (super narrow). Experience has shown that attempts to reduce the bandwidth to less than 1.5 MHz, will result in very poor image detail resolution and smearing, etc.

System M signals have a total bandwidth of 4.5 MHz. This means that 2 MHz bandwidth reduction on a system M signal, will give clearer resolution compared to a 5.5 MHz System B TV signal.

Example of TV varicap tuner, with switchable IF bandwidth, and IF to UHF upconvertor schematic.

References and acknowledgements

The material in this article was based on Roger Bunney's book A TV DXers Hanbook, published in 1986 by Bernard Babini BP176.



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