Home Forums Electronics Restoration How to Avoid Aligning on the Image Frequency

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    Ed Stone
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    When aligning a receiver, a common problem encountered (particularly in single conversion sets using a low intermediate frequency (IF), eg. 460 kHz), is identifying which is the desired signal and which is an image. The problem is worse on the short wave bands. If the set has been ‘fiddled with’ by someone in the past, it is often the case that the set has been set up to tune the image. The following problems can be created by someone that does not know what they are doing/has poor (or no) test equipment (or cannot use it):

    1 The local oscillator (LO) is set on the low side of the IF instead of the high.
    2 The LO is set on the low side of the IF at the high frequency (HF) end of the band
    and the high side of the IF at the low frequency (LF) end of the band
    3 The LO set on the high side of the IF at the HF end of the
    band and the low side at the LF end of the band.

    These scenarios will produce apparently ‘accurate’ tuning at the band ends and haywire tracking in the middle of bands and it is very easy to get very confused as to what is going on when faced with these problems. But in this modern day and age there is an easy answer! Use a cheap modern digital radio as a ‘monitor’ and listen to the LO whilst tuning up and down the band. This technique will identify a problem at once: say you are tuned to 1.410 MHz, then the LO in a set with an IF of 460kHz should be heard on the monitor receiver at 1.870Mhz if the IF is tracking correctly on the high side of the IF (usually the case – but check for the receiver you are working on). The same technique can be used for multiple conversion receivers.

    To illustrate this, the photos below show an Eddystone Model 1830 dual-conversion communications receiver that has a first IF of 1350kHz. The receiver is tuned to 21MHz (see the cursor at the top of the Eddystone tuning scale). The LO in this set tracks high of the IF and therefore should be oscillating at 22.350MHz. If you look at the digital display on the small monitor receiver (here an Eton E100), sitting atop the Eddystone sets coilbox to ensure good coupling, you will see the Eton is tuned to 22.350MHz and the signal strength (bar scale) meter on the Eton is indicating that it is receiving the LO signal loud and clear – the LO stage in the Eddystone is therefore aligned corrrectly at this particular frequency.

    By the way, the Eton cost me about $40 and was money well-spent even just as a useful monitor in the shack – you can of course use it as a portable tranny and even as a digital frequency meter with an older signal generator.

    Ed

    Download DSC00043 [1024×768].JPG. (Caution: This file may not be virus scanned.)

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