[hpsdr] ALEX - Call for comments

pvharman at arach.net.au pvharman at arach.net.au
Mon Mar 5 22:56:29 PST 2007 

Hi Henry,

To answer some of your questions. 

The prime use of Alex is at the front end of a receiver. It may also  be used 
to filter the low level stages of an associated transmitter - see Penelope on 
the Wiki to see how this works - it is not intended to take more than a few 
Watts of Tx power. 

The insertion loss of the filters at the lower frequencies, whilst not an issue 
on receive given the high noise levels on these bands, will not be acceptable 
for a Tx output stage - for this reason you normally use LPF's on a PA output 
rather than a BPF.

There is provision on the block diagram to fit a over voltage protection part 
at the input - the device discussed on the SoftRock reflector looked suitable - 
I can't seem to find the reference, could you please pass it on.

Hope that helps with the understanding.



73's Phil...VK6APH 



Quoting Henry Vredegoor <henry_vredegoor at hotmail.com>:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Hello Graham,
> 
> Thank you for starting this project!
> 
> My first thoughts for Alex at this moment:
> 
> - Could you/anybody please provide links/references to or add
> diagrams/info/core-specs here or to the WIKI of the original filter designs?
> 
> - About shielding:
> In a "computer-signals infested environment" like the HPSDR/Atlas card cage
> with al its processors, clocks, data/address lines etc., I would suggest
> putting the individual filter sections in closed compartments, if possible,
> avoiding noise pick-up by the RF-coils?
> Be it at the cost of more space required. (see expandability)
> 
> - For this reason also no SMD components (of the filter sections at least
> that is) outside on the back side of the board?
> 
> - top-side = ground-plane/shield?
> 
> - Would using relays lower the losses/noise and allow for more TX-power?
> 
> - What would be the estimated TX-power allowed?
> 
> - Max. Expected/targeted usable frequency?
> 
> - Expandability, using more daisy chained boards for more bands?
> 
> - Guidelines to change/make custom band pass-filter sections?
> 
> Maybe with the help of something similar like this I found on the net:
> 
> http://www.aade.com/filter.htm
> 
> It looks like a handy (and FREE!) tool to design custom (band pass-)filters,
> gives you amongst other things the number of turns on a standard type
> ferrite O-core (like I think used in the band pass-filters of the
> SoftRocks). 
> Not tested/used this though...
> 
> - There was a discussion on the SoftRock Yahoo group about some static over
> voltage protection circuitry in front of the receiver which could be a good
> thing to include here also.
> Or maybe this should be part of a separate (sturdy) mounting bracket for the
> antenna- (BNC socket('s) ) and earthing connections?
> 
> (I made the following suggestion there to call it Cyberus (Cerberus) .......
> Defender of the gate that leads from RF-space to Cyberspace.........  ;-) )
> 
> Regards,
> 
> Henry.
> 
> - 
> 
> 
> 
> 
> > -----Original Message-----
> > From: hpsdr-bounces at hpsdr.org 
> > [mailto:hpsdr-bounces at hpsdr.org] On Behalf Of Graham Haddock
> > Sent: dinsdag 6 maart 2007 3:31
> > To: hpsdr at hpsdr.org
> > Subject: [hpsdr] ALEX - Call for comments
> > 
> > 
> > ***** High Performance Software Defined Radio Discussion List *****
> > 
> > All:
> > 
> > Hello, I am Graham, KE9H, the project leader and PCB layout
> > person for ALEX which is the RF preselector filter pack for use
> > with Mercury or other receivers.    Phil, VK6APH, is the designer.
> > 
> > ALEX is a multi-band receiver preselector intended to
> > reduce the level of out-of-band signals at the input of a
> > receiver, but it can also be used as a low level transmitter
> > filter.
> > 
> > ALEX is an Atlas bus compatible card.  The Atlas bus is used
> > for power and control, and RF in and out will be on coax
> > connectors on the card, separate from the Atlas bus.
> > 
> > The card is relatively "dumb" compared to the other HPSDR cards.
> > It will set the switches as it is told, via an I2C command
> > coming from the bus.
> > 
> > Switching will be via Hittite HMC226 RF switches, rather than
> > relays.
> > 
> > If I try to stay with a 100mm by 120mm card, (same size as
> > Janus) then I suspect that we will be limited to about ten
> > filter sections.
> > 
> > I propose that these ten filter sections would be:
> > 160 Meters
> >  80 Meters
> >  40 Meters
> >  30 Meters
> >  20 Meters
> >  17/15 Meters
> >  12/10 Meters
> >   6 Meters
> > User Defined #1
> > User Defined #2
> > 
> > There is also a bypass feature, providing no
> > filter or frequency selectivity.
> > 
> > Each "user defined" option would be an un-populated filter area,
> > with circuit runners laid out to use the same family
> > of inductors and capacitors as the others, and could be
> > populated by the user to tune the 60 Meters ham band, Medium
> > Wave, AM Broadcast, 160 KHz or whatever. A three section
> > bandpass or lowpass could be placed in each "user defined" area.
> > 
> > A block diagram of Alex and other information is on the Wiki.  
> > The current plan is to use an I2C I/O expander chip to control
> > the RF switches, not the full CPLD bus interface as shown.
> > 
> > http://hpsdr.org/wiki/index.php?title=ALEXAIRES
> > 
> > 
> > Specific questions for the group.
> > 
> > 1.) Is ten filter sections enough?  If more is needed, will
> > there be any issues going to a longer card, such as the 160 mm
> > length card proposed for Penelope?
> > 
> > 2.) Does anyone care if I put SMT chip components on the
> > back side of the board, and perhaps a shield?
> > 
> > 3.) Any preference for SMA verus BNC connectors?  I propose
> > to use BNC for "external" connections such as antenna(s), and
> > SMA for "internal" or card to card connections.
> > 
> > 4.) Should we allow for multiple antenna inputs? - we could
> > select each one on a Tx/Rx and band basis using a GUI on the PC.
> >  
> > 5.) Using Mercury as a base line, we have an MDS of -140dBm in
> > 500Hz without Alex in circuit. Alex will degrade this by say
> > 2.5dB on 6m due to the loss through the filter and RF
> > switches.  Is an MDS of -137.5dBm acceptable on 6m or should we
> > consider adding a low noise preamp after the 6m filter?
> > 
> > 6.) Any comments you want to make?
> > 
> > Best regards,
> > --- Graham
> > 
> > ==
> > 
> > 
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