At 5/6/2009 09:41, you wrote:
>A Home Brew 224 MHz Repeater Project.
>May 2009
>Part 2 The Receiver
>First Post May 2, 2009, this post May 6, 2009
>
>This text is part 2 of a description of a recently completed 224
>MHz Repeater Project. One could easily apply the same techniques
>toward a repeater project in different frequency ranges. Pictures of
>the completed repeater project reside in the group photos section.
>
>http://groups.yahoo.com/group/Repeater-Builder/photos/album/1157128983/pic/list
>
>
Thanks for posting this, Skipp. It's always fun to see another angle on
repeater building. Threaded are my $0.02:
>I selected a Hamtronics R302-6 Receiver for the project for a few
>of the following reasons. The receiver is frequency synthesized,
>relatively low in cost (about $229 each at the time of this post),
>respectable in performance and fairly straight forward to interface.
>Hamtronics normally requests the frequency of operation at the time
>of purchase so the receiver arrives "pre- tuned" and ready to
>interface after mounting.
Any idea what Hamtronics chose for the loop bandwidth of the PLL (I assume
it's not a direct DDS synth.)? If very low, it could have very good phase
noise characteristics. Not that that's important on 220, but could be good
for a 2 meter version @ 15 kHz spacing.
>New old stock ("NOS") Com Spec TS-32 CTCSS (PL) tone boards are
>very nice "Ebay sleeper deals".
Yeah, but I don't like them. They do decode reliably, but the encoders
have harmonics that need to be externally filtered if fed to a phase
modulator. They also don't respond to reverse burst. Again, not a big
deal if we're talking 220 & a direct FM TX.
My favorite is the Sigtone C1116. Unfortunately they've been out of
production for a few years, but fortunately for me I made a bulk purchase
at a local swap meet many years ago & still have a couple of them left
today. They decode & release faster than the TS-32 & have a better HPF
(see below).
>Repeat Receiver audio is routed through the TS-32 on-board CTCSS
>tone filter to provide fairly clean filter flat audio output to
>your external controller.
The problem with the TS-32 tone filter is that it has poor transient
response, "ringing" around 400 Hz. To me, a system that uses one in the
audio path sounds "boxy". What's worse is that if more than one is used in
a linked system, the audio at the other end will quickly degrade as that
400 Hz region begins to dominate the frequency response.
>Don't panic, the TS-32 also provides a separate tone generation
>"encoder" section for your transmitter CTCSS requirement. A
>shielded audio quality wire is routed out of the receiver box
>to the transmitter board at the proper CTCSS connection point.
I assume this is done using feedthrough capacitors - you don't want to
plumb any wires straight through the case.
>The Hamtronics Receiver COR/COS output is active high, which I
>don't like one bit.
When I first started building repeaters, I used active low CTCSS as
well. The main reason was that it was convenient to use as a cheap way to
key the TX when a controller wasn't available back in those "lean" college
days. When I started using G.E. radios with their active high CAS & RUS
outputs, I switched to active high as my standard.
> So I simply routed the receivers COS/COR Output
>logic line through a 120 ohm resistor to the gate of a 2N7000 FET
>(also known as a VN10K and similar device). The FET source lead
>is grounded and the drain output lead becomes the main active low
>logic output source routed to your external repeater controller.
That works. If you don't have a 2N7000 available, any decent NPN BJT
(2N2222, 2N3904) will do too - just use a higher value resistor on the base
like 4.7k. In fact, you can use a bigger resistance on the 2N7000 as well
since the gate is high impedance. I mention this because around here BJTs
are easily obtained whereas I had to order 2N7000s via mail.
>DB-9 Connectors are very popular with repeater builder types. I
>for personal preference and experience tend to move away from
>using them in for this type of repeater chassis through hole
>connections. I have returned to using through hold feed-through
>capacitors
So what do you use for a connector? In my latest RX boxing project I
decided to put a DB9 "doghouse" on the box. The doghouse is a cheap
plastic box since shielding isn't necessary. I plan to install the
feedthroughs in a manner similar to what you describe, but then mount a DB9
& Anderson PowerPole on the doghouse so all the connections are
"connectorized". I may even add some switching circuitry inside the
doghouse so this receiver can be grafted into a existing system using a
single-port controller (2 RXs on one port).
> and while the value is not ultra critical, you don't
>want the capacitance value large enough to impact the information
>passing through. I found and used surplus 100pf (pico farad)
>feed-through capacitors although I'm sure higher values will work.
To make sure that the shunt capacitance blocked any RF, I added some series
resistance to form an RC. On lines like the RX audio, adding 470 ohms in
series with a source that was already several kohms didn't affect the audio
at all. Same for COS & CTCSS decode logic outputs if the controller's
input impedance is much higher than the series resistance used. Also
serves to protect those outputs if you accidentally short them to 12 V or
ground.
>In my opinion the 100pf value is enough to bypass troublesome stray
>RF, but not greatly impact or modify the audio information passing
>through to the external controller.
However, it might not be enough shunt C for 2 & 6 meter RF. At 2 meters,
the reactance is 11 ohms. Who knows what the impedance of the hanging wire
is at RF? But add the 470 ohm series R with the 100 pF & you get over 32
dB of attenuation.
Bob NO6B
