ALEXIARES
Contents
ALEX INTRODUCTION
ALEXIARES (or ALEX for short) is a combination RF Preselector for use with MERCURY or any other SDR, as well as a transmitter low pass filter bank for a transmitter such as PENELOPE, and optionally, with an associated RF power amplifier up to 100 watts peak.
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.
As a receiver preselector, the purpose of ALEX is to reduce the level of out-of-band signals at the input of a receiver, and importantly, to supress any signals at the sampling image frequencies.
As a transmitter low pass filter, ALEX will suppress the harmonic energy typically generated by an RF power amplifier. The transmit low pass filters will also be used for additional MERCURY receiver input band limiting.
PRESELECTOR ISSUES WITH DATA CONVERTERS
There are at least three major issues to consider with a preselector for a data converter type receiver such as MERCURY.
First is protecting the receiver from overload. The data converter in MERCURY is a highly linear and robust converter, but it does have limits. The maximum input level that the converter can accept before degrading overall performance is 1.5 volts peak to peak, or +8 dBm which is S9 plus 81 dB. This is after any gain that appears between the antenna and the data converter. A local shortwave broadcast transmitter or nearby amateur transmitter might generate this kind of level. This is the traditional purpose of a preselector, to lower the level of out of band emmissions. They do not need to be eliminated, just reduced so that the data converter is not overloaded.
Second is linearity. The native linearity of the data converter is excellent, something on the order of an IP3 of +50 dBm. So high that it is difficult to measure with traditional test equipment, and so high that the non-linearity of the toroid inductors or electronic switches can limit receiver performance. You don't normally expect switches or inductors to be "non-linear," but at these levels, they can be. The IP3 of the overall receiver will typically be lower than the +50 dBm of the data converter by any preamplifier gain (typically 10 or 20 dB with MERCURY), so a preselector IP3 target performance around +40 dBm is being used.
Third is image supression. The MERCURY receiver uses a sampling frequency of 125.0 MHz for the analog to digital data converter. This means that the MERCURY receiver will directly sample signals in the 0 to 62.5 MHz band, but signals above those frequencies can also appear in the data converter output. For example, a signal at 10 MHz is to be monitored by Mercury. A signal at 125 MHz plus 10 MHz and a signal at 125 MHz minus 10 MHz will also appear to be at 10 MHz as "images," if not prevented from getting to the input of the data converter. Those are just examples. Of more concern are the VHF TV and FM signals that will "fold" back into the HF and 6 Meter receiver band if not eliminated. The 88 to 108 MHz FM band could appear as images at 17 to 37 MHz. The data converter has NO native selectivity or rejection at the image frequencies, so the total system selectivity of the antenna system, matching networks and preselector frequency rejection must add up to something on the order of 120 to 140 dB, depending on how strong the TV and FM signals are in your area. This also has implications for requirements for some shielding contribution from the overall housing and some shielding of the ALEX filters themselves.
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 similar preselector.
MERCURY is an extremely robust receiver in its native performance.
Measured MERCURY Prototype Specs:
MERCURY with no net preamplification. -
Noise figure 27 dB
IP3 +50 dBm
Max signal +8 dBm (S9 + 81 dB)
MERCURY with 10 dB net preamplification - [Typically 1.8 to 13 MHz.]
Noise figure 17 dB
IP3 +40 dBm
Max signal -2 dBm (S9 + 71 dB)
MERCURY with 20 dB net preamplification - [Typically above 13 MHz.]
Noise figure 7 dB
IP3 +30 dBm
Max signal -12 dBm (S9 + 61 dB)
Note: MERCURY will likely be configured as a 0 to 31 dB variable attenuator,
which will be in front of an (always on) +20 dB gain preamplifier, which drives
the input of the data converter. The above represent attenuator settings
of 20 dB, 10 dB and zero, respectively.
DESCRIPTION
ALEX consists of two PC boards and a third shield/cover board. The physical size is 10 cm by 16 cm (Euroboard size) by 4 cm thick. It will not plug into the ATLAS bus, but is intended for separate mounting. There are nine BNC connectors that should extend through to the outside of the HPSDR housing (PANDORA), for external access.
The first PC board is the Receiver/HPF board. It consists of a 33 or 55 MHz 7th order Low Pass Filter that is always in line with the receiver. If the user is going to operate on 6 meters, the 55 MHz version should be used, otherwise, the 33 MHz version is recommended (for better image supression.) There is a 6 meter bandpass filter and a low noise preamp; a 13 MHz High Pass filter, one or two (space permitting) user definable High pass filters, and a 1.7 MHz high pass filter. It has five external BNC connections, described below.
The second PC board is the Transmitter/LPF board. It consists of seven relay switched low pass filters for transmitter harmonic supression, that are also used in conjunction with the receiver high pass filters to provide a flexible variable bandwidth receiver input filter function. It has four external BNC connections, described below.
The third PC board is a simple shield board, with no circuits or connections. All three boards are assembled into a three layer "sandwich" mechanically connected by 0.625 inch (1.6 cm) metal standoffs at ten locations around the edge of the boards. The board to board spacing of 0.625 inch (1.6 cm) allows for the vertical mounting of wound T-50 toroids.
ALEX is controlled by an SPI bus. This SPI bus should NOT be shared with other devices, in that it is intended that the data and clock lines should not be transitioning except when a command is being sent to one of the ALEX boards. There are two analog outputs providing an indication of transmit forward and reverse power. There are no oscillators or continuously running clocks anywhere inside of ALEX.
A block diagram of ALEX is shown below.
Specifications:
On frequency insertion loss - variable according to frequency, but typically will not exceed 2.5 dB total for receive paths, and 0.5 dB for transmit paths.
Power handling capability - The transmit harmonic filter banks and associated relay switches are intended to handle up to 100 watts, maximum peak power.
Contribution to Receiver IP3 performance - To be measured, but expected to be in the range of +40 dBm.
Total power consumption for ALEX should be less than 2 watts, in any configuration, all from the +12 volt supply.
External Connections -
Receiver Board - five ea. BNC female connectors to extend through the case.
Receiver Out (to MERCURY input)
Transverter Input
Receiver AUX input 1
Receiver AUX Input 2
AUX Output (for looping in an external filter or preamp)
Transmit Board - four ea. BNC female connectors to extend through the case.
Antenna 1 (transmit-receive)
Antenna 2
Antenna 3
Transmitter In (From PENELOPE or RF PA)
Internal RF connections -
Receiver Board - one SMA female connector
Internal receive input from Transmitter card
Transmit Board - one SMA female connector
Internal receive output to the Receiver board
Control - 8 pin (0.1 inch header)
+12V, GND, SPI Data, SPI Clk, SPI Load1, SPI Load2, Forward PWR, Reverse PWR.
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 Hercules (Herakles), born after the Hercules' ascension to Olympus and his marriage to the goddess Hebe. Alexiares and Anicetus helped serve as the gatekeepers of Olympus, assisting their father in a role which was commonly assigned to him.
http://www.theoi.com/Cat_Olympioi.html
CURRENT STATUS
July 24, 2007 - Phil, VK6APH released measured performance for prototype MERCURY in an email to the group dated June 18. A preliminary design for ALEX is being released for review and comment before first pass PC board layouts are completed. The information released for comment on July 24 is at revision level 6-2.
RELATED DOCUMENTS AND LINKS
If you want to look at the PCB Outlines or the proposed filter sets, you can download the following files.
ALEX Top Level - Media:ALEX_PACKAGE_6-2.pdf
Receiver board Filter Set - Media:ALEX_RX_FILTERS_6-2.pdf
Transmitter board filter set - Media:ALEX_TX_FILTERS_6-2.pdf
The PCB cards and schematics are being designed in EAGLE Layout Editor
All of these filter plots, plus the original design of these filters were done in ELSIE. The FREE "student version" of ELSIE is sufficient to design and analyze this order of filter. If you want to implement one of the user defined filter positions, or modify one of the filters, this is a good tool.
