Motorola Spectra S-meter for RDFing...

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Doug_G

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This is my first post on the forum & I just want to share how impressed I am with the conduct of everyone & the help which is provided. Everyone's input is greatly appreciated. I did a search & I don't see that anyone else has posted these questions.

I'm getting into radio direction finding due to an issue at work. From what I am reading in addition to what I am being told, I should be utilizing the best receiver that I can find. We just had a couple of Motorola Spectra 110w units removed from some of our fire apparatus due to narrow banding. Reading the manuals, these receivers have some impressive receiver specs. The Spectra receivers are just short of what the newer Kenwood TK790H receivers have in sensitivity & selectivity.

Researching on the installation of an analog & or LED S-meter for my RS Pro-2006 scanner leads me to believe that an external S-meter can be added to the Spectra. I just want to be sure I tap into the right place for the S-meter. I plan to build a combo analog LED S-meter. I want the LED capabilities in order to monitor the reading from the driver's seat. The analog meter will be utilized for more accurate readings.

1.) Can anyone tell me where exactly to tap into the Spectra's receiver circuit to install an external S-meter? I plan to run a piece of 174 coax to a connector on the back of the radio for an external plug.

2.) When I tap into the RF signal on the receiver board, will this deteriorate the quality of my received signal?

3.) I hope to run the Spectra audio out of the radio into my Ramsey DDF1 but, I don't want to ground the audio as warned. One side of the audio input for the Ramsey DDF1 is grounded. The information regarding the use of isolation transformers on the Spectra web sites are somewhat confusing to me with my limited electronics background. If someone can help guide me with diagrams & possibly pictures I would really appreciate it. I don't want to waste the time that I've put into building the DDF1 by burning it up or lose the audio PA in the Spectra.

If the Ramsey isn't able to fit the bill I am researching the use of the Montreal Doppler. The RS-232 output to a laptop for mapping would be nice.

Any input would be greatly appreciated.

Tnx,

Doug_G
 

zz0468

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Pin 5 of U301 on the RF board is labeled RSSI, which is the Spectra receive signal strength indicator. I've never tried it, but that's where I would start. Also, if the MLM has 6.xx firmware, a front panel display RSSI indicator is available.
 

Doug_G

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Thanks for the response zz0468.

I had seen the RSSI mentioned before on the various Spectra forums and I thought the RSSI was only sending a signal indication out if the radio was utilized in a trunking situation. I need to check it out to be sure. If it gives me what I need for a relative RF signal that's were I plan to go. That would make things so much easier. I will try to post my findings ASAP.

Tnx again.

Doug_G
 

Doug_G

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Spare time is getting hard to come by.

I finally had a little bit of time to pull the Spectra out of the case and open it up. Based on your suggestion and what little I’ve been able to research I was able to put an old analogue meter off of what appears to be a test point of Pin 5 of U301. With no modulated signal coming in I was getting .69 volts at the RSSI out from U301. Switching to a full quieting signal from NOAA weather radio station just 7-10 miles away, and utilizing a scanner antenna, the meter goes up to 1.45 or 1.5 volts. I was able to get this same response on an open unused channel by keying a 5 watt portable radio in the next room.

With the Spectra case open I would receive RFI audio on the portable radio when both the Spectra and the hand held are on the same frequency. Does the Spectra put out that much RFI with the covers off? Is the .69 volts that I am seeing with no incoming signal due to RFI from the Spectra? The Spectra speaker does not have any hiss or noise coming from it, it sounds normal. Needless to say, none of the test leads that I am using have shielding so I figured that was issue. If I run a piece of RG174 to the case and install a plug for an external meter will I still see that .69 volts on the meter? Additionally, will this reduce the quality of my received signal?

The Spectra does such a good job on the receive side, I don’t want to deteriorate the quality of the receiver.

Any input would be greatly appreciated. Thanks for the help.
 

zz0468

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Spare time is getting hard to come by.

...With no modulated signal coming in I was getting .69 volts at the RSSI out from U301. Switching to a full quieting signal from NOAA weather radio station just 7-10 miles away, and utilizing a scanner antenna, the meter goes up to 1.45 or 1.5 volts. I was able to get this same response on an open unused channel by keying a 5 watt portable radio in the next room.
Sounds about right. The test point does not go to zero. In Spectras with the RSSI option enabled, you never get a measurement below a certain point, regardless of how weak the signal is. If that's important, you could fabricate a meter driver circuit that allows the meter to sit on zero with no input.

With the Spectra case open I would receive RFI audio on the portable radio when both the Spectra and the hand held are on the same frequency. Does the Spectra put out that much RFI with the covers off? Is the .69 volts that I am seeing with no incoming signal due to RFI from the Spectra?
Apparently it does. Take note of all the internal shielding, and the tight fit of the covers. It's there to keep things in, or keep things out, or more likely, both.

The Spectra speaker does not have any hiss or noise coming from it, it sounds normal. Needless to say, none of the test leads that I am using have shielding so I figured that was issue. If I run a piece of RG174 to the case and install a plug for an external meter will I still see that .69 volts on the meter? Additionally, will this reduce the quality of my received signal?
What you'll want to do is decouple any leads that come out of the radio. Everything that comes into and out of the radio (except RF) is decoupled at multiple places in the radio... on the connectors between boards, on the ribbon connectors on the PA assembly, etc. Look at all the internal connectors and you will see a network of chip capacitors surrounding each connector.

The Spectra does such a good job on the receive side, I don’t want to deteriorate the quality of the receiver.
There may be some function that you don't use that appears on the accessory connector that's already properly decoupled. Disconnect it on the board at the appropriate location, and route the RSSI signal to that point, and bring it to the outside world using existing bypassing.
 

Doug_G

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Awesome timing!

zz0468,

Your timing is awesome. Yesterday I was able to plug in a 600ohm to 600ohm audio transformer and hooked up. It works great. My donor Spectra hooked up played nicely with my Ramsey DDF-1 without any smoke escaping from the case.

I just received my LM3914 IC's the other day and I plan to build a combo analog/LED S-meter. Your comments make perfect sense. The S-meter schematics that I have show pots at the input and the output sides of the circuit which will allow me to set the S-meter scale.

I just need a little more spare time to sit down and build projects. Next week looks pretty interesting at work which is going to limit my spare time. After the first of the month I plan to spend some time building my combo S-meter in addition to a simple LED S-meter. Once done, I will be able to compare signals rather than just find their direction.

Added to my list of building projects is a tower mounted antenna for the Doppler. Once I have a base set up here at home I need to figure out how to set up a Doppler to record signals and the direction while unattended. Our interference is very brief and varies from time to time.

It's just a matter of timing and I know I'm going to find the problem. Good thing is, I'm learning by doing.

Thanks for the help! Your input is greatly appreciated.
 
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