[Lowfer] AMRAD Converter and Lowpass Filter

[Ben Hall]

Good evening all,

Today I assembled my versions of the AMRAD LF Converter and Lowpass filter.

The converter is the AMRAD design as modified by Todd Roberts to accept 
a 3 MHz OCXO input for high stability.

My modifications consisted of:

1) replacing the thru-hole parts for surface-mount
2) the inclusion of a 24VDC to 12VDC regulator stage for the output 
amplifier
3) a 12VDC to 5VDC regulator stage for the remaining circuitry
4) plus instead of the 74HC4053, I'm giving the 74LVX4053 a try instead 
of the 74HC4053.  The LVX has the same pinout and much lower switch 
on-resistance.  We'll see how well this works.  I do have a supply of 
HC4053's so I can always pull the LVX and replace it.

The OCXO runs off of 24VDC, which is why I added the regulators so I can 
use one power supply.

I have yet to test the converter on the air, as I wanted to get the 
lowpass filter built first.

The lowpass filter mostly follows the AMRAD design.  The Toko slug-tuned 
coils are no longer available, so they were replaced with Coilcraft 
Slot-10 units.  The Coilcraft units have a lower Q than the original 
Toko parts...but seem to work just fine (more later).  Instead of 
thru-hole capacitors which are hard to find in 2% or better values, I 
again used surface-mount poly-film devices.

I did omit several parallel capacitors to get the trap tuning 
frequencies up a bit.  The article mentions:

"With the values shown, all three circuits tune from about 530 kHz to 
700 kHz.  To reject stations above 700 kHz, use capacitors of lower 
value..."

Being that I have a really strong station nearby on both 730 kHz and 770 
kHz, I needed to move the tuning range up.  So C1A, C2A, and C3A were 
omitted.

Using my trusty URM-25D signal generator and 'scope, I was able to 
easily adjust the traps to put a very healthy dent at 730 and 770 kHz.  :)

Tomorrow I'll get the lowpass filter mounted in a box, get the 
converter, lowpass filter, OCXO, and 24VDC linear mounted on some sort 
of plate (so they are not banging around) and will get them on the air.  :)

thanks much and 73,
ben, kd5byb