[Premium-Rx] Improving 1st Image and IF rejection.

[Michael O'Beirne]

Hi everyone,

This is a simple idea to improve the 1st image and IF rejection of your premium receiver at relatively small cost.

Nearly all premium receivers will have a high 1st IF, typically the 40.455 MHz of the RA1792 and Cubic R-3030.  The first image will lie at twice the first IF (80.910 MHz) beyond the tuned frequency, ie well into the VHF region.  

These potentially troublesome responses are reduced drastically in the receiver by means of a low pass filter in the antenna and/or mixer inputs.  You can reduce the response further by means of adding an external low pass filter cutting off just beyond 30 MHz in the antenna lead.  In theory every additional dB of attenuation at the IF or image frequency reduces the response by the same dB.  If your receiver's image rejection spec is 70dB bareback, adding, say 60dB of LPF at the image should theoretically give one 130dB rejection.  Of course other effects take over well below that level, such as considerable intermodulation.

There are plenty of ham LPFs for HF txs that will do nicely. The Drake TV-3300-LP has an attenuation of about 60dB at 40MHz and an ultimate attenuation at the image of about 75dB.  I use a Racal MA4115A designed for green radio which gives an ultimate attenuation of about 70dB at 38MHz. Used with the RA1792, measuring the IF rejection accurately is all but impossible.

An external LPF will also reduce any troublesome strong local VHF transmissions, eg from commercial broadcasters or fire brigade repeater stations.  This could be useful if you use an active aerial multicoupler which does not itself have an internal LPF and which might be prone to overload.

However, the LPF will do nothing for the 2nd IF and 2nd image responses (and the responses from any further conversions).  In the case of the RA1792, the second IF is 455kHz, and thus the second image lies at 910kHz away from the passband.  All that could be done would be to insert a better roofing filter (if one exists) or to use an image rejection second mixer (as used initially by Drake in I think the R8 and now in the Icom IC-7800).

73s
Michael
G8MOB
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