Icom IC-740

Additional roofing filter

The first IF is on 39.730 MHz. There are two rather wide 2-pole filters there. I didn't find any specs but it looks they are about 12 kHz wide, or so.
In fact, the 1st IF is not fixed but floating. Any filter must be designed with the center at 39.731 MHz and must be 1 kHz wider.

If there was an extremely strong signal on the band within the bandwidth of these filters the second mixer suffered from overloading. I decided to ease the situation with an extra crystal filter inserted right before the second mixer.

The filter is of McCoy layout and consists of 4 crystals. They work on the third O/T.
I developed the following schematics:

The filter is made on a small PCB:

At the end if the 1st IF amplifier (RF board) there is a switching diode (D4) removed and the filter is inserted as the following image shows:
This way the filter operates only during reception. It has no impact on transmitting.

This is the RF board with the additional ffilter installed:

I tested the filter with a home-made noise generator and the HDSDR software running on FUNcube Dongle Pro+.
The curve I got was quite promissing during the prototyping stage:

This is the curve of the final implementation. Here the filter was measured on 9 MHz IF (9 011.5 kHz). This means the curve includes impact of the other parts including the mixer. Some little ripple occured in the passband. However, it shows that attenuation of at least 40 dB is obtained just about 5 kHz off the center. The center freq exactly matches and the bandwidth is as desired (about 3.5 kHz):


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