Montag, 6. Juni 2016

A Preselector for the Icom IC-7300

The behavior of direct sampling receivers is different from superhets. In many respects they are superior to legacy receivers, but direct samplers have also a "dark" side. When the ADC runs out of bits it can't no longer work properly.
At -10dBm the ADC of the IC-7300 begins to clip. Heavy distortions are the result and the operator has to reduce the RF-Gain to get a clear reception. Unfortunately this reduces also the sensibility of the receiver and weak signals will get lost.

A level as strong as -10dBm can be the sum of all signals in the passband of the receiver or it can be produced by a single signal from a nearby Ham radio or a strong BC station. The bandpass-filters in the IC-7300 are rather large. In the case of the 20 and 30m bands they include also the 19, 22, 25 and 31m BC bands.  In Europe, the BC signals on these bands are sometimes very strong and the new Icom may run into troubles, especially with good (and broadband) antennas.

In this case an additional preselector in front of the IC-7300 may help.
I’m using the following circuit with my IC-7300:

This is my 2nd prototype with improved selectivity and it covers now all bands from 10 to 160m. It works with a single LC-circuit, but the selection is sufficient  sharp to suppress BC signals even on frequencies in the vicinity of the amateur radio bands, which is especially important in the case of the 40, 30 and 20m ham bands. On the other hand the attenuation in the passband is low, on the higher bands no more than 1 to 2dB.

The preselector may be activated or deactivated by a switch and a relay takes the preselector out of the game when transmitting. It is controlled by the “send” line of the IC-7300.

The schematic should be self-explaining. The sharpness of the passband peak and the passband attenuation can be adjusted by positioning the coupling windings on the toroid coil. Both coupling windings should be close together in order to get the best results.

The selectivity at 1.9 MHz is high enough to suppress local BC stations in the medium wave band. Horizontal span is 0 - 10 MHz, Vertical 5dB/division.

Selectivity at 3.6 MHz. Passband attenuation -4dB.

And here the 40m band. Passband attenuation has dropped to 2dB.

30m Band: Now the damping pole is in on the low side, due to another position of the coupling winding's. Still 2dB passband attenuation.

20m band: the passband attenuation drops to 1.5dB.

17m band.

15m band: The higher we go, the broader the peak is. But the attenuation of out-of-band signals is still high enough. The passband attenuation is now around 1dB and therefore in most cases irrelevant. With the preselector I can even use the preamp of the IC-7300 without driving the ADC into clipping (from BC stations).

Keine Kommentare: