ALEXIARES

ALEX INTRODUCTION

Alexiares (or ALEX for short) is an RF Preselector for use with MERCURY or any other SDR. The project leader for ALEX is Graham, KE9H, who is also laying out the PCB. Design work is being shared between Graham and Phil, VK6APH.

The purpose of ALEX is to reduce the level of out-of-band signals at the input of a receiver.

DESCRIPTION

ALEX has three filter sections, a continuously tunable 1.8 to 30 MHz bandbass filter, a 50 to 54 MHz bandpass filter, and provision for a user defined filter. In addition, a bypass function is provided. An on-board noise generator is provided as "built in test equipment" for alignment of the filters.

Protection from potentially damaging high signal levels is provided in the form of a transient suppressor (details TBD) at the antenna socket.

ALEX has provision to be switched between two paths; either in the path from the antenna to the receiver, or to filter a low level transmitter signal.

A CPLD is used to interface the filter selection and routing to the Atlas bus to provide maximum flexibility in Alex’s control and configuration.

A block diagram of ALEX is shown below.

DESIGN GUIDELINES

The primary purpose of ALEX is to be a preselector for MERCURY, and should not materially degrade MERCURY's native performance. There should not be any restrictions that would prevent ALEX from being used with any other receiver that required a preselector.

MERCURY is an extremely robust receiver in its native performance.

  Proposed MERCURY Specs: 

  MERCURY with preamp OFF - [Typically OFF below 13 MHz.]

              Noise figure            23.4 dB
              IP3                     46.8 dBm
              Max signal              s9+77 dB

  MERCURY with preamp ON - [Typically ON above 13 MHz.]

              Noise figure            10.3 dB
              IP3                     34.8 dBm 
              Max signal              s9+62 dB 

  Note: MERCURY also includes a 0 to 30 dB attenuator, which can
  be set independent of the preamp On/Off.  It will increase all
  of the above figures by the amount of attenuation selected.

ALEX Performance specs:

On frequency insertion loss - variable according to frequency, but typically will not exceed 2.5 dB.

Off frequency rejection - The 1.8 Mhz to 30 MHz HF filter will be "continuously" tunable in steps that are typically in the 2 percent range, with pass bandwidth on the range of 6 to 16 percent. At the low end of the band, the passband is deliberately widened to 200 kHz, so as to allow the receiver the opportunity to process waterfall displays 100 kHz either side of center. It will be narrow in the 3.5 to 14 MHz region, and widen out again above 20 MHz.

Power handling capability - TBD, but expected to be less than +20 dBm, so only suitable for low power exciter clean-up, NOT high power PA filtering.

Contribution to Receiver IP3 performance - TBD, but in the range of +40 to +50 dBm.

ALEX is intended to be a single module suitable for plugging into the ATLAS bus/backplane, and extract the control commands from the ATLAS bus. Depending on shielding and bus noise interference isolation issues, it is possible that ALEX could end up as two modules, a filter "brick" that gets bolted to the housing away from the ATLAS bus, and a controller card that plugs into the ATLAS bus. The filter "Brick" and the controller card would be connected by a three wire SPI bus (plus power and ground).

Total power consumption for ALEX should be less than 4 watts, in any configuration. Probably less than one-half of that.

  External RF connections - three ea. BNC female connectors to extend through the case.
		Preselector In 1
		Preselector In 2
		Preselector Out

  Internal RF connections - two each SMA connectors.
		(Low level) Transmit In
		(Low level) Transmit Out 

WHY ALEXIARES?

Alexiares and his twin brother Anicetus - were the Greek Gods of Defense, in particular the defense of fortified towns and citadels.

Both were sons of Herakles (Hercules), born after the hero's ascension to Olympus and his marriage to the goddess Hebe. The pair likely served as the gatekeepers of Olympus, assisting their father in a role which was commonly assigned to Herakles.

http://www.theoi.com/Cat_Olympioi.html

CURRENT STATUS

ALEX is still somewhat a moving target, in that final filter size and performance is still being determined.

May 5, 2007 - Per the email from Phil, VK6APH, titled "Penelope's progress" of May 3 to the HPSDR mailing list, it has been determined that the use of iron core toroids, even at the T200 size, in the preselector resonators causes measurable degradation of the IP3 performance of the Mercury receiver. Further investigation reveals that the use of small SMT chip inductors in Mercury and Penelope input filters results in even worse impacts. Receive path filters for both these modules are being moved off-board, and may require changes in the ALEX requirements. The likely result is that all inductors used in receiver input paths will have to be air core, with the resulting impact on increased size and shielding requirements.

April 5, 2007 - The original concept of fixed filters for ham-bands-only has been set aside in favor of a continuously tunable HF filter, in part because there was a lot of interest expressed in general coverage, and also the IP3 performance of the coils in the PicAStar type filters would degrade the native performance of the Mercury receiver. Phil, VK6APH is preparing to run a series of IP3 tests on I/O transformers and various sizes of toroids as would be used in a multiple resonator continuously tunable filter.

RELATED DOCUMENTS AND LINKS

Watch this space for developments.