To All from PA0SE
When I had to decide what type of mixers to use in my new TRX, a doubly
balanced passive diode mixer or the SL6440, I chose the latter for the
following reasons.
1. The SL6440 amplifies so there is no need for an amplifier between the
mixed and the following IF filter.
2. The SL6440 requires very little LO power
3. There is no need for critical terminations of the ports, as is the case
with a passive mixer.
Both PA0KDF and I found that the LO signal appears very strong in phase at
the output pins 3 and 14 of the SL6440 and this signal must be suppressed
to enjoy the full large signal capability. I did this by connecting
a parallel tuned circuit between the pins with the centre of the coil
grounded for RF and connected to 12V. The circuit is tuned to the IF and
the IF signal extracted by a link over the coil.
I also use a balanced circuit at the input of the mixer.
The large signal behaviour of the receiver is very good. Using a 2 x 20 m
dipole on 7MHz there is no trace of IM when listening on a winter
night. Not even when the 12dB preamplifier is switched on. As a test of
course, the amp is not needed below 24MHz. But in this respect the Cohn
bandpass filter (bandwidth 180kHz) at the input of the receiver helps as well.
The SL6440 is no longer in production, though they can still be found. But
if you are going to use one better buy some spares as
well. Fortunately they are not very expensive.
73, Dick, PA0SE
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