[hpsdr] Penelope's progress

[Robert McGwier]

Phil Harman wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> 
> I now have the Alpha prototype of Penelope running and producing RF out.
> Lyle, KK7P,  did a great job of laying out the PCB and kitting for the 
> prototypes -
> many thanks Lyle.
> 
> My experience with the prototype, and the experiments on the  preselector
> (Alex) that I have been doing with Graham, KE9H,  have resulted in a change 
> in
> architecture for Penelope (and Mercury).
> 
> The reason for this is as follows.  We had intended to allow the use of  the
> LPFs from Penelope as front end filters for Mercury and/or QSD etc based
> receivers.
> 
> In testing various filters for Alex what I found was that for receivers with
> a potential IP3 of  +50dBm one needs to be very careful as to  the choice of
> inductors used at the front-end if one is not to significantly degrade the
> IP3 performance.
> 
> I now fully appreciate why the German Hilberling PT8000A  receiver uses a
> massive 2" diameter  T200-2 toroidal inductor in its preselector.
> 
> See the photo of the preselector here
> 
> < http://www.hilberling.de/produkte/produkte_pt8_t9.htm >
> 
>  Even with such a large core I can detect a slight degradation in  IP3 
> performance when using a
> T200-2 inductor as apposed to an equivalent air cored inductor.
> 
> The bottom line is that whilst we could  make LPFs for Penelope that will
> provide adequate harmonic attenuation, given the size constraints of the
> Penelope PCB, the IP3 performance of a high performance receiver will be
> noticeably degraded if we use these filters on receive also.
> 
> We have found the same IP3 issues with the HPF's and LPF on the Mercury
> front-end - the small SMD inductors are simply not good enough for a 
> receiver
> with such a high IP3 performance.


---- snip ----

I just want to caution everyone that it is almost trivial to get 50 dBm 
IP3 with my old Heathkit HW8 without modification to it.   I put a 70 dB 
attenuator in front of it!

What matters in a receive SYSTEM, is THE ENTIRE SYSTEM.   If the Linear 
Technologies through the Cyclone presents a 60 dB noise figure, and it 
takes 40 dB gain to get a usable NF,  then a 50 dBm IP3 just became 10 dBm.

We need to be careful with claims until the SYSTEM AS A WHOLE is measured.

That caution stated, my calculations and simulations show say that with 
usable gain  the Mercury is going to be excellent, possibly even world 
class.  How excellent will be determined by implementation details (as 
is clear from Phil's fantastic note) and it must be measured to be sure 
and measurement at one frequency is not enough.

---- back to our regularly schedule program ----
> 
> I will update the Wiki shortly to reflect the changes we have made.
> 
> 73's Phil...VK6APH
> 
> 

73's
Bob
N4HY



-- 
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"If you're going to be crazy, you have to get paid for it or
else you're going to be locked up." Hunter S. Thompson

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