|
23 October 2010
One of the revised printed circuit boards I mentioned a
few days ago is a new kit, the Z10042AV Norton amplifier. You can read the
instruction manual by clicking here
or through the Z10042AV page.
I've had few requests for a Norton amplifier usable well
into the VLF range, from 3-30 KHz. The Z10040B Norton amplifier has a lower -3
dB point around 60 KHz and the Z10042A improves this to 45 KHz.
The Z10042AV drops the lower -3 dB point to around 16 KHz.
This should meet the requests for a lower -3 dB point.
Reducing the lower -3 dB point turned out to not be a
simple task. Several seemingly simple changes are needed to move the lower
corner frequency down:
- Feedback transformers T2 & T4 need more primary
inductance
- RF choke impedance must be increased
- Input and output transformer magnetizing inductance
must be increased
Modifications 1 and 3 are not too difficult.
Modification 1:The Z10042A Norton Amplifier uses a
1:11:4 transformer turns ratio for 12 dB nominal gain (11.4 dB realized in
practice). Doubling all turns to 2:22:8 preserves the gain factor, but
quadruples the primary inductance. The negative side of doubled turns is that
the feedback transformers have increased turn-to-turn capacitance, which reduces
the high frequency response, with an upper -3 dB point of 20 MHz, compared with
46 MHz typically seen in the Z10042A.
Modification 2: The Z10042A uses physically small
1mH shielded RF chokes, API Delevan's SDS-680 series. To improve low frequency
performance, the emitter chokes should be 2.2mH and the collector chokes (at a
higher impedance point than the emitter) should be 3.3 mH. Although SDS-680
series parts are available in both values, their inductance drop with DC current
produces unsatisfactory results. Hence the Z10042AV uses Coilcraft LPS-6235
shielded chokes with a higher DC current rating. Since the LPS-6235 parts are
physically larger than the SDS-680 inductors, I found it necessary to revise the
PCB layout. While revising the layout, I made a few other changes to the board,
including:
- Almost all parts are now surface mount.
- DC power is now provided with a female connector to
avoid exposed DC pins
- Modified DC power circuit to always use the 1mH
isolation inductor whether power is provided over the coaxial output (duplex
power) or directly.
- Cleaned up a few untidy traces and pad locations and
added silk screen connector information.
Modification 3: The input and output transformers
are increased from 10 bifilar turns to 18 bifilar turns, more than tripling the
magnetizing inductance.
The new PCB layout also is easier to build as it reduces
parts crowding around Q1 and Q2 by moving more components to the bottom surface
and revised trace routing.
The Z10042A has identical physical dimensions with the
Z10042A and uses the same mounting holes. Hence it fits the custom powder coated
enclosure developed for the Z10042A and Z10046A amplifiers.
The Z10042AV is the same price as the Z10042A and is
available for immediate shipping.
|
