SETI LEAGUE UK - Technical scrapbook

Modification of mobile radio equipment for SETI receiver use

Introduction

There is no shortage of FM narrow-band radio equipments around at minimal cost, that can be used for SETI work. These range from single channel crystal controlled to multi-channel synthesised units - and all are suitable for being fed from a converter for the appropriate frequency of interest. Such a converter could be built from scratch, or bought from one of the SETI league suppliers. By far the most common receiver approach in these mobile radio equipments is to use a limiting IF chip with quadrature detection, usually at 455KHz. This will have followed a 1st IF of 10.7, 21.4 or occasionally 45MHz. The conversion to 455KHz may or may not be performed in the same IF chip.

The FM detected output from these receivers is not appropriate for SETI purposes. However, it is interesting to note that some of the IF chips used provide an RSSI (Received Signal Strength Indication) output, and this can be used directly to indicate the presence of a wideband signal (such as the 1420MHz hydrogen line) within the overall passband. In such summing detection of the passband content however, spectral information is lost, and with it the ability to search deeper into the noise for a narrow band signal by bandwidth reduction later on (performed by the PC, in our case) .

So if software processing is envisaged to break the receiver bandwidth down into much narrower 'bins' (increasing the system sensitivity), then a frequency translator or mixer is required in place of the RSSI detector. A standard PC, soundcard and software combination can then process this output, and should be able to handle 30 KHz or so of bandwidth. Most mobile radios will be fitted with filters that have a 7.5 or 15 KHz bandwidth. These can be retained, or a replacement 30 KHz filter obtained to gain the most out of the processing software.

A frequency translator mixer (sometimes called 'product detector') is not difficult to arrange, though at first glance, the FM receiver may seem quite inappropriate, since this is fitted with an unsuitable FM detector(!). In practice though, it is likely to be quite easy to modify existing circuitry for our purpose by changing the 2nd conversion stage to act as the product detector, the description of which is the main purpose of this article.

Block diagram of typical FM mobile radio receiver

The first mixer is being driven at 122.600MHz, so signals at 144.000 and 101.200MHz (ie, +-21.4MHz) will result in a mixer output of 21.400MHz, which will be passed by the crystal filter. An image filter prior to the mixer ensures that the receiver response at 101.200MHz is insignificant. As the following typical filter response plots show, crystal filters at 21.4MHz have very well defined windows - the width amounting to approximately 7.5KHz for a unit fitted to a 12.5KHz channel spaced equipment, and 15KHz for a 25KHz channeling version. In the UK, nearly all currently issued licenses are for 12.5KHz spacing.

 

FM detection could be carried out directly at this frequency, but in practice a further frequency translation to a much lower frequency is nearly always undertaken (it's easier to obtain the high amplification gain required for good limiting) . Most receivers use 455KHz for this final frequency, because cheap ceramic filters are readily available here - but it is academic in our context. All we need to be aware of is the existence of this additional mixer. To translate the incoming 21.4MHz signal to 455KHz requires a local oscillator signal at either 20.945or 21.855MHz. Image rejection is not a problem this time, since the crystal filter response easily meets this secondary requirement, as the plots above show.

Although the reasons for converting the 21.4MHz first IF to 455KHz in the FM receiver are not relevant in our case , we do still need to translate the 21.4MHz signal to something lower in frequency that the PC sound card can handle, so this second mixer is still very useful and can be employed by changing the local oscillator frequency applied to it to something close to 21.400MHz, as the following drawing shows.

 

Here, the local oscillator frequency has been changed to 21.385KHz. If the filter is a 30KHz width filter, the lower edge of the window will coincide with the local oscillator frequency so that a signal at 21.385MHz will mix with the 21.385MHz local oscillator to produce a difference frequency of 0MHz, whilst a signal at the upper window edge (ie, 21.415MHz) will result in a difference frequency of 30KHz - and this range of 0 - 30KHz can be handled by the PCs soundcard.

To take advantage of the reconfigured second mixer, all we have to do is to take the audio output from the pin that previously fed the 455KHz filter. If this filter is one of the ceramic types, it can be left in circuit because it will not adversely load the 0 - 30KHz required audio output. This can be brought directly out of the receiver to drive the soundcard, if the latter is sensitive enough, or a simple op-amp gain block used to increase the level. With some ingenuity, it should be possible to use some of the audio amplifier stages later on in the receiver circuit, but check that the response is flat up to 30KHz - these receivers not only use de-emphasis, but also frequently add low and high pass filters to limit the response outside of the range 300 - 3kHz.

Changing the second mixers local oscillator frequency

Replacing the second conversion crystal is shown below for a typical receiver. There is no need to struggle with the procuring of a replacement crystal here. A dual monolithic element from a/the discarded crystal filter can be used and pulled down in frequency to that required using series inductance (about 4uH) as shown.

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We can supply a number of 15 KHz bandwidth crystal filters also, but not 30 KHz. For these, if you consider this a worthwhile option, you will need to contact a firm like Cirkit. They currently advertise the following 30 KHz band width filters:

At 10.7 MHz, the 2 pole 10M30A at 6.25 and the 8 pole 10M30D at 30.14

At 21.4 MHz, the 2 pole 21U30A at 7.30 and the 8 pole 21U30D at 30.90

At 45 MHz the 2 pole 45U30AF at 8.45 and the 4 pole 4530BF at 20.50. Or contact us for assistance, if necessary.