1. Installation
When I installed the receiver on my boat, I found it could not get it to
work properly on the boats 12vdc power system. I finally concluded
that this was due to ground loops resulting from years of haphazard electronics
installations by a collection of technicians. I finally switched
to powering the receiver from a "wall wart" -- a simple 12vdc power supply
which plugs into a 110 vac outlet, which I plugged into a convenient outlet
on my flybridge. I then ran a single heavy ground wire to the grounding
plate, which is a 5" x 15" bronze plate under the boat in contact with
seawater. In this configuration, the receiver works great.
2. Front end protection
I am fairly sure that I blew out the input fet on the antenna preamp the
first time that I transmitted on my boat's vhf marine-band transceiver.
The receiver still sort of works, but I have not pulled it off the boat
to check it out (fishing is very very good right now -- I'll play with
electronics after the season is over). For anyone who will be operating
their receiver in near a transmitter, I recommend putting a series resistor
with a value of 2k - 5k in series with the input FET gate, and putting
a pair of diodes, in opposite directions, to ground from the FET gate,
so as to limit the input voltage swing to the FET gate to a diode drop
above and below ground. This will increase the input capacitance
a little and thus reduce the gain a little, but the receiver seems to have
plenty of gain to spare.
3. Don't substitute a different part for U3
U3 is a standard CMOS hex inverter used in a linear mode for the First
IF filter and for the LO Loop Filter. Do not use an HC version of
this inverter -- it has different open loop characteristics.
4. A case of overdesign?
One builder has reported squirrely operation of the LO Loop Filter.
In email correspondence, about all I can come up with is that he is experiencing
some sort of crosstalk between the LO Loop Filter and the First IF Filter,
both of which are in U3. To be safe, you might consider using two
hex-inverter chips instead of one, for U3. Put the LO Loop Filter
in its own chip. Remember to tie unused inputs to ground.
5. Schematic error -- LCD signals
The schematic has an error: the wires going from processor (U9) bits RB0-RB3
to U8 are reversed. That is, the connections should be:
RB0 U9-6 U8-17
RB1 U9-7 U8-15
RB2 U9-8 U8-13
RB3 U9-9 U8-11
Also, resistor R70 was missing from the schematic.
The schematic (Processor.pdf) has been modified
to Rev. A to reflect the corrections.
6. Parts List Corrected
There were a few errors in the Digikey part numbers in the parts list.
They have now (8/2/98) been corrected.
7. Power for U3, U7, and U8
The power connections for these three chips are missing from the schematics.
All are powered from +5 volts.
8. Parts list: U2/U3 reversed
I had U2 and U3 interchanged on the parts list. Now corrected (9/13/98)
9. Parts list: R14, R15 missing from parts list
And added to the parts list 9/15/98
10. More Parts Lists Corrections
The following changes were made on 10/18/98:
C33: wrong value/part number on parts list
R34: wrong value on parts list
R46, C43, R60-62, C1 missing from parts list
C106, C107 reversed on parts list
C36, C37 part number wrong -- changed to 22 pf and corrected part number.
(Schematic still says 20 pf -- close enough)
C35, C39, C41: corrected part number
R66: corrected price
A number of resistors were shown at $0.01 ea. Changed to $0.10
ea.
Corrected price on R67
Updated cost summary based on these corrections.
A reader noted that L4 is missing from the components list on the RF
Amp Schematic, and
that "C1" should be "D1" on the Receiver Schematic components list.
The reader is correct,
and I have not entered these particular corrections.
11. Schematic Error Corrected
In doing #5 above, I inadvertently moved the data pin on the freq synth
from processor pin RB1 to RB2 on the schematic, which is wrong. The
Rev B Processor schematic (posted Nov. 28, 1998) corrects that.
12. Component designators
The errors noted in the last paragraph of #10 above are now corrected,
and Rev A versions of the RF Amp and Receiver schematics are now posted
(Nov. 28, 1998)
13. Error in instructions in Hardware.pdf
Step 6 in section 14 of the hardware manual calls for adjusting the LO
freq to 308 khz with 2.5 volts applied to the varactor. It should (and
now does) read 430 khz, corresponding to an input frequency of 308 khz.
Also, I correct my email address in Software.pdf and Hardware.pdf.