[amsat-bb] Re: AMSAT-BB-digest V2001 #460

[G0MRF@xxxxxxx]

In a message dated 7/17/01 1:12:30 AM GMT Daylight Time, 
owner-AMSAT-BB-digest@AMSAT.Org writes:

<< Date: Mon, 16 Jul 2001 20:08:18 edt
 From: Jesse Morris <w4mvb@juno.com>
 Subject: Re: [amsat-bb] Drake instability & Dielectric Filter Simulation
 
 Hi Bill and All,
 
 I can confirm that the noise that I (and others) have encountered with
 the G0MRF modified Drakes is indeed instability.  My tests have not been
 nearly as detailed as yours but I have been focusing on the amplifier
 input circuit.  I have two units and one is worse that the other
 (naturally it was the one I had put up to test).  Both of the units seem
 to be sensitive to input terminations.  As long as I have a good antenna
 on the good one it seems to be OK >>

Hello Bill, Jesse and group.

Re Drake / filter simulations.

Simulating the drake front end with the filter in place is a difficult task 
and will probably raise more questions than it can answer.  When the Drakes 
were imported into the UK a while back I bought several and have used them to 
try out a variety different front end mod's.  Out of about a dozen I have 
two, both with the MGA86576 mod but less filter, that have been unstable. Of 
these, one was stable into a 50R load while the other oscillated like a 
banshee all the time. 
Without a consistant problem to examine, it's difficult to recommend a course 
of action which will be guaranted to work 100% of the time.
However here are some thoughts which may help.

1)  I think that the most likely cause of instability is the additional 
inductance due to the 1mm dia pins in the ground leads of the LNA. This 
problem is made very obvious in the data sheet which has warnings about the 
device being unsuitable for use with normal 1/16 inch PCB's as the plated 
through holes would have too much inductance. As the position of the pins 
will vary with constructors, this may explain why some are fine while others 
are not.

2) I've used the plastic encapsulated version (86563) in my S-band to 70cm 
converter. In that design I mounted the device on the ground plane side of 
the board so that the ground connections have no 'lead length' at all. As in 
the drake, the output feeds a Toko filter. That arrangement is 
unconditionally stable.

3) I'm not sure that the Toko data for the filter is accurate enough to allow 
simulation of signals passing through on the lower band edge. ( The filter is 
a 2450MHz device with a +/- 50MHz passband.)

4) The 33R resistor in the design was chosen because it was a reasonable 
value and 
there was one in the drake which could be re-used. Agilent suggest 10R to 
100R. This resistor affects gain and stability although its effect will 
depend on the arrangement used for the series RFC in the supply.

Overall, I would suggest that the grounding pins are the probable cause of 
instability.
In kits being sent out now I'm suggesting that the pins are omitted and the 
grounding is achieved by bending the ground leads down 90 degrees. If a small 
slot is cut in the board each side of the device the leads can be passed 
through the board and soldered on the underside.
Another technique is to again make two small slots and to use a small length 
of copper foil to form 'wrap round' connection to bring the ground up through 
the board. The 86576 can then be fitted to the board without bending any 
leads.

I will change the pins for copper foil in my two drakes and report back.

I hope some of this is of use. The Amsat BB is a good medium for exchanging 
ideas and for highlighting and resolving problems of this type. I hope anyone 
with constructive comments can share them with all of us.

BTW. As technology advances, the MGA86576 is probably not the optimum 
modification to the drake. Up until now the PHEMT has had the best 
performance but always had the disadvantage of requiring a negative bias 
supply. However that all changed recently with the new Agilent ATF-54143.  
This is a PHEMT with a 0.5dB noise figure which is half the price of the 
'86576. What makes ths device very special is that it is an enhancement mode 
device. Like a normal bipolar transistor, it needs a positive bias supply, 
making it an ideal replacement for a drake or other converter front ends that 
use a single positive supply. The data sheet shows how it can be used with 
small molded inductors. - So you can fit it into almost any space.
I have some on order but they are on a 4 week waiting list.

73

David  G0MRF 
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