TrxAVR-Picastar
- Preamp, 4xJ310amp and BPF switch for PA3AKE front end

Martein Bakker, PA3AKE has developed a superb HF transceiver front end.

It consists of:

• A band pass filter unit with 10 band filters, an IF notch filter, an attenuator and an IC2 controller.
• A front end unit containing an H-mode mixer and SSB (2.6kHz) and CW 500Hz roofing filters.

A full description of the project is at:

http://www.xs4all.nl/~martein/pa3ake/hmode/index.html

These units are expensive to build but probably represent the ultimate in performance.
They result from a large amount of experimental work.
For example:
- Martein's H-mode mixer development evaluated 13 transformer types and 7 different Fairchild switches.
- He found that roofing filter crystals were a significant source of IMD and so uses high specification crystals
from EQZAL.
- The BPFs for 160m - 20m use T94 toroids.

I quickly came to the conclusion that there was no point in building this front end unless it closely followed Martein's
design. To do otherwise would make nonsense of the huge effort that is gone in the optimising the design.

I have installed a PA3AKE BPF and H-mode mixer / roofing filter unit in my TrxAVR Picastar rig.
A few others have done, or are doing or are considering doing the same.

Hobcat / Trxavr-Picastar has full software support for the I2C controlled Pa3AKE units.
see PA3AKE software control

I have designed, built and documented three modules which are used in the PA3AKE installation:

• A 4xJ310 IF amplifier unit to replace the 16dB gain lost from removing the Magic roundabout
  (which has a 4x J310 amplifier)
• A BPF switch to switch the BPF 'input' between antenna input (from the LPF unit) on receive and
  the PA input on tranmsit. (The G3XJP BPF has this switch. The PA3AKE BPF doesn't have it)
• An optional switchable RF pre-amp between BPF and mixer.
  (The G3XJP magic roundabout switches the 4xJ310 preamp before the mixer in best NF mode
  and after the mixer in best IP3 mode.)

The PA3AKE BPF and roofing filter are switched byAxicom IM06GR relays.
These are Farnell part no 1175070.

My policy in designing boards for this installation was:

• Use IM06GR relays for all signal switching upto the roofing filter.
• Relay contact wetting current is provided as exactly as in the PA3AKE boards.
• With the pre-amp switched out there is no gain until after the roofing filter.
  Martein, PA3AKE put a huge amout of research into achieving the best possible IP3 performance.
  When BPF and mixer IP3 had been optimised, he found poor quality roofing filter crystals to be the weak
  point in the signal path. He located a source of very clean produced low loss crystals with which it proved
  possible to build roofing filters with low insertion loss (around -1dB) and very high intercept point.
  To place the 4xJ310 amp before these filter would degrade the IP3 performance achieved by using
  these filters.
• The 4xJ310 amplifier, located after the roofing filter can and does use FST3126 solid state
  directional switching.

PCB design
I have provided for each module: a schematic, a layout drawing and a mirrored top side etching mask.
All the boards use the underside as a ground plane. Only the top needs etching.
No mounting hole positions are provided as I intended the PCBs to be soldered into brass boxes.
However, mounting holes can be made if the board is made appropriately larger.

BPF switch
Switches the BPF 'input' between antenna input (from the LPF unit) on receive and the PA input on tranmsit.

  
The underside of the board is a ground plane and is soldered to the brass screening box.

BPF switch schematic (pdf)
BPF switch layout (pdf)
BPF switch layout (pad2pad)
BPF switch top copper etch mask (mirrored)

4xJ310 amplifier
This amplifier is identical to that in the Picastar magic roundabout.
(see Picastar data for transformer details)
Directional switching is done by a FST3126 switch.

The amplifier IF port conencts to the birectional J310 amplifier on the Picastar IF board.
The connection is to transformer T1.
The correct turns ratio for T1 is 3t:10t. (50ohm coax to approx 600ohm amplifier input)

4xJ310 amp schematic (pdf)
4xJ310 amp layout (pdf)
4xJ310 amp layout (pad2pad)
4xJ310 amp top copper etch mask (mirrored)

Pre amplifier
This uses a MAX2611 RF amplifier chip. See: http://datasheets.maxim-ic.com/en/ds/MAX2611.pdf
The IC has 19dB of gain. A 6dB input attenuator reduces the gain to 13dB.

The amplifier has to be switched out on transmit.
In my rig it is controlled from one output a PCA9555 I2C chip and switched by a touch button on the
colour TFT display panel. The Hobcat / TrxAVR system for control of a PCA9555 has provision for the
output to be optionally active only on receive. (this preamp mut be bypassed on transmit)
See PCA9555 button tasks

This board is not quite the final layout. The 6dB (bottom right) attenuator was added here as a modification.

preamp schematic (pdf)
preamp layout (pdf)
preamp layout (pad2pad)
preamp top copper etch mask (mirrored)

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