Hi MF,
Some of you are using the old Telefunken mechanical filter in homemade
receivers for 630m. I use them i all my MF receivers.
There have been discussions about how to match the antenn to that
filter.
So far, i used this matching method:
https://dl.dropboxusercontent.com/u/19882028/MF/MF%20Filter%20schematic.png
(recently using the 500 Ohm input rather than the 18k input) i.e. an
active impedance converter stage.
This has a few advantages:
-The input of the filter can be matched perfectly to the jFET, giving a
minimun pass band ripple (less than 1 dB)
-The matching is independent of the impedance of the voltage source
connected to the input of the jFET
-You can use a 50 Ohm resitor on the input of the jFET, giving a
perfect matching of the cable for all frequencies
-The input can even be used as an active antenna (very high Z)
-Many receiver inputs can be switched in parallel! For example a LF RX
parallel to a MF RX plus one 50 Ohm resistor...
-The input of the mechanical filter must not be exceeded to a voltage
above 2 V rms (inside the passband or on any frequency?????) So, if the
jFET is driven at 5V supply voltage, the filter is protected against
overvoltages!
These were the arguments why i decided to use this technique so far.
Here is an image of the RX on my remote site, captured today:
https://dl.dropboxusercontent.com/u/19882028/MF/MF%20mechanical%20filter%20active%20input.jpg
But now, on my remote RX site i have a relatively large RX antenna (T,
5m V, 20m H) with a 50 Ohm matching. Furthermore i want to reduce the
power consumption of the RX system. And i want to try to improve the RX
performance even more. So i've made a modification, a simple
transformer matching (nothing new!!!!!!!) as recently discussed.
Here the advanatges are:
-No active parts in front of the mechanical filter, just wire,
Ls, Cs, cable and ferrite cores!
-One active stage less, i.e. less power consumption, about 10 mA at
12V, i.e. 0.24 Ah per day!
-A 'gain' (relative to the active stage which has a voltage gain
of - 6 dB due to the resistor matching) of 16 dB (confirmed by
measurements on SpecLab) because the transformer makes 10 dB when
transforming from 50 Ohm to the 500 Ohm input of the filter
-This gain makes one of the following amp stages obsolete, i.e. likely
i can remove one more active stage which will result in even less
power consumption, maybe 0.5 Ah per day, which is much in winter!
-Maybe less RX noise?
-Maybe less unwanted IM products??
Here is an image of the simple new input matching
https://dl.dropboxusercontent.com/u/19882028/MF/MF%20mechanical%20filter%20passive%20input.jpg
One of the advantages of the active input matching is missing now: the
perfect matching. The pass band ripple is now about 2 dB but this is
still acceptable for me if there are some advantages on the other side.
The second amp stage is not yet removed in my currently running RX, so
there is to much gain in the moment, i.e. a smaller dynamic range. I
just wanted to confirm the 16 dB to decide the next steps.
Now my question: Is it better to remove a BF981 amp stage
(about 16 dB to 18 dB gain!) in front of the mixer (SBL-3) to get a
lower RX noise or do i better reduce the gain on the LT1028 stage on
the AF side? The AF pass band is 11...18 kHz (LO= 461 kHz)
73, Stefan/DK7FC
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